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Re: [PATCH RFC 2/3] pipe: protect pipe_max_size access with a mutex

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  Re: [PATCH RFC 2/3] pipe: protect pipe_max_size access with a  mutex Mikulas Patocka <mpatocka@redhat.com> - 2017-09-15 01:10 +0200
    Re: [PATCH RFC 2/3] pipe: protect pipe_max_size access with a mutex Joe Lawrence <joe.lawrence@redhat.com> - 2017-09-15 16:10 +0200
      Re: [PATCH RFC 2/3] pipe: protect pipe_max_size access with a  mutex Mikulas Patocka <mpatocka@redhat.com> - 2017-09-19 10:00 +0200
        Re: [PATCH RFC 2/3] pipe: protect pipe_max_size access with a mutex Joe Lawrence <joe.lawrence@redhat.com> - 2017-09-19 23:40 +0200
          Re: [PATCH RFC 2/3] pipe: protect pipe_max_size access with a  mutex Mikulas Patocka <mpatocka@redhat.com> - 2017-09-21 12:10 +0200
    Re: [PATCH RFC 2/3] pipe: protect pipe_max_size access with a mutex Joe Lawrence <joe.lawrence@redhat.com> - 2017-09-19 23:50 +0200
      Re: [PATCH RFC 2/3] pipe: protect pipe_max_size access with a  mutex Mikulas Patocka <mpatocka@redhat.com> - 2017-09-25 12:50 +0200

#1732639 — Re: [PATCH RFC 2/3] pipe: protect pipe_max_size access with a mutex

I think this mutex is too heavy - if multiple processes simultaneously 
create a pipe, the mutex would cause performance degradation.

You can call do_proc_dointvec with a custom callback "conv" function that 
does the rounding of the pipe size value.

Mikulas

On Tue, 5 Sep 2017, Joe Lawrence wrote:

> pipe_max_size is assigned directly via procfs sysctl:
> 
>   static struct ctl_table fs_table[] = {
>           ...
>           {
>                   .procname       = "pipe-max-size",
>                   .data           = &pipe_max_size,
>                   .maxlen         = sizeof(int),
>                   .mode           = 0644,
>                   .proc_handler   = &pipe_proc_fn,
>                   .extra1         = &pipe_min_size,
>           },
>           ...
> 
>   int pipe_proc_fn(struct ctl_table *table, int write, void __user *buf,
>                    size_t *lenp, loff_t *ppos)
>   {
>           ...
>           ret = proc_dointvec_minmax(table, write, buf, lenp, ppos)
>           ...
> 
> and then later rounded in-place a few statements later:
> 
>           ...
>           pipe_max_size = round_pipe_size(pipe_max_size);
>           ...
> 
> This leaves a window of time between initial assignment and rounding
> that may be visible to other threads.  (For example, one thread sets a
> non-rounded value to pipe_max_size while another reads its value.)
> 
> Similar reads of pipe_max_size are potentially racey:
> 
>   pipe.c :: alloc_pipe_info()
>   pipe.c :: pipe_set_size()
> 
> Protect them and the procfs sysctl assignment with a mutex.
> 
> Reported-by: Mikulas Patocka <mpatocka@redhat.com>
> Signed-off-by: Joe Lawrence <joe.lawrence@redhat.com>
> ---
>  fs/pipe.c | 28 ++++++++++++++++++++++++----
>  1 file changed, 24 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/pipe.c b/fs/pipe.c
> index fa28910b3c59..33bb11b0d78e 100644
> --- a/fs/pipe.c
> +++ b/fs/pipe.c
> @@ -35,6 +35,11 @@
>  unsigned int pipe_max_size = 1048576;
>  
>  /*
> + * Provide mutual exclusion around access to pipe_max_size
> + */
> +static DEFINE_MUTEX(pipe_max_mutex);
> +
> +/*
>   * Minimum pipe size, as required by POSIX
>   */
>  unsigned int pipe_min_size = PAGE_SIZE;
> @@ -623,13 +628,18 @@ struct pipe_inode_info *alloc_pipe_info(void)
>  	unsigned long pipe_bufs = PIPE_DEF_BUFFERS;
>  	struct user_struct *user = get_current_user();
>  	unsigned long user_bufs;
> +	unsigned int max_size;
>  
>  	pipe = kzalloc(sizeof(struct pipe_inode_info), GFP_KERNEL_ACCOUNT);
>  	if (pipe == NULL)
>  		goto out_free_uid;
>  
> -	if (pipe_bufs * PAGE_SIZE > pipe_max_size && !capable(CAP_SYS_RESOURCE))
> -		pipe_bufs = pipe_max_size >> PAGE_SHIFT;
> +	mutex_lock(&pipe_max_mutex);
> +	max_size = pipe_max_size;
> +	mutex_unlock(&pipe_max_mutex);
> +
> +	if (pipe_bufs * PAGE_SIZE > max_size && !capable(CAP_SYS_RESOURCE))
> +		pipe_bufs = max_size >> PAGE_SHIFT;
>  
>  	user_bufs = account_pipe_buffers(user, 0, pipe_bufs);
>  
> @@ -1039,6 +1049,7 @@ static long pipe_set_size(struct pipe_inode_info *pipe, unsigned long arg)
>  	struct pipe_buffer *bufs;
>  	unsigned int size, nr_pages;
>  	unsigned long user_bufs;
> +	unsigned int max_size;
>  	long ret = 0;
>  
>  	size = round_pipe_size(arg);
> @@ -1056,8 +1067,11 @@ static long pipe_set_size(struct pipe_inode_info *pipe, unsigned long arg)
>  	 * Decreasing the pipe capacity is always permitted, even
>  	 * if the user is currently over a limit.
>  	 */
> +	mutex_lock(&pipe_max_mutex);
> +	max_size = pipe_max_size;
> +	mutex_unlock(&pipe_max_mutex);
>  	if (nr_pages > pipe->buffers &&
> -			size > pipe_max_size && !capable(CAP_SYS_RESOURCE))
> +			size > max_size && !capable(CAP_SYS_RESOURCE))
>  		return -EPERM;
>  
>  	user_bufs = account_pipe_buffers(pipe->user, pipe->buffers, nr_pages);
> @@ -1131,18 +1145,24 @@ int pipe_proc_fn(struct ctl_table *table, int write, void __user *buf,
>  	unsigned int rounded_pipe_max_size;
>  	int ret;
>  
> +	mutex_lock(&pipe_max_mutex);
>  	orig_pipe_max_size = pipe_max_size;
>  	ret = proc_dointvec_minmax(table, write, buf, lenp, ppos);
> -	if (ret < 0 || !write)
> +	if (ret < 0 || !write) {
> +		mutex_unlock(&pipe_max_mutex);
>  		return ret;
> +	}
>  
>  	rounded_pipe_max_size = round_pipe_size(pipe_max_size);
>  	if (rounded_pipe_max_size == 0) {
>  		pipe_max_size = orig_pipe_max_size;
> +		mutex_unlock(&pipe_max_mutex);
>  		return -EINVAL;
>  	}
>  
>  	pipe_max_size = rounded_pipe_max_size;
> +	mutex_unlock(&pipe_max_mutex);
> +
>  	return ret;
>  }
>  
> -- 
> 1.8.3.1
> 

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#1732894 — Re: [PATCH RFC 2/3] pipe: protect pipe_max_size access with a mutex

On 09/14/2017 07:09 PM, Mikulas Patocka wrote:
> On Tue, 5 Sep 2017, Joe Lawrence wrote:
>> pipe_max_size is assigned directly via procfs sysctl:
>>
>>   static struct ctl_table fs_table[] = {
>>           ...
>>           {
>>                   .procname       = "pipe-max-size",
>>                   .data           = &pipe_max_size,
>>                   .maxlen         = sizeof(int),
>>                   .mode           = 0644,
>>                   .proc_handler   = &pipe_proc_fn,
>>                   .extra1         = &pipe_min_size,
>>           },
>>           ...
>>
>>   int pipe_proc_fn(struct ctl_table *table, int write, void __user *buf,
>>                    size_t *lenp, loff_t *ppos)
>>   {
>>           ...
>>           ret = proc_dointvec_minmax(table, write, buf, lenp, ppos)
>>           ...
>>
>> and then later rounded in-place a few statements later:
>>
>>           ...
>>           pipe_max_size = round_pipe_size(pipe_max_size);
>>           ...
>>
>> This leaves a window of time between initial assignment and rounding
>> that may be visible to other threads.  (For example, one thread sets a
>> non-rounded value to pipe_max_size while another reads its value.)
>>
>> Similar reads of pipe_max_size are potentially racey:
>>
>>   pipe.c :: alloc_pipe_info()
>>   pipe.c :: pipe_set_size()
>>
>> Protect them and the procfs sysctl assignment with a mutex.
>>
>> Reported-by: Mikulas Patocka <mpatocka@redhat.com>
>> Signed-off-by: Joe Lawrence <joe.lawrence@redhat.com>
>> ---
>>  fs/pipe.c | 28 ++++++++++++++++++++++++----
>>  1 file changed, 24 insertions(+), 4 deletions(-)
>>
>> diff --git a/fs/pipe.c b/fs/pipe.c
>> index fa28910b3c59..33bb11b0d78e 100644
>> --- a/fs/pipe.c
>> +++ b/fs/pipe.c
>> @@ -35,6 +35,11 @@
>>  unsigned int pipe_max_size = 1048576;
>>  
>>  /*
>> + * Provide mutual exclusion around access to pipe_max_size
>> + */
>> +static DEFINE_MUTEX(pipe_max_mutex);
>> +
>> +/*
>>   * Minimum pipe size, as required by POSIX
>>   */
>>  unsigned int pipe_min_size = PAGE_SIZE;
>> @@ -623,13 +628,18 @@ struct pipe_inode_info *alloc_pipe_info(void)
>>  	unsigned long pipe_bufs = PIPE_DEF_BUFFERS;
>>  	struct user_struct *user = get_current_user();
>>  	unsigned long user_bufs;
>> +	unsigned int max_size;
>>  
>>  	pipe = kzalloc(sizeof(struct pipe_inode_info), GFP_KERNEL_ACCOUNT);
>>  	if (pipe == NULL)
>>  		goto out_free_uid;
>>  
>> -	if (pipe_bufs * PAGE_SIZE > pipe_max_size && !capable(CAP_SYS_RESOURCE))
>> -		pipe_bufs = pipe_max_size >> PAGE_SHIFT;
>> +	mutex_lock(&pipe_max_mutex);
>> +	max_size = pipe_max_size;
>> +	mutex_unlock(&pipe_max_mutex);
>> +
>> +	if (pipe_bufs * PAGE_SIZE > max_size && !capable(CAP_SYS_RESOURCE))
>> +		pipe_bufs = max_size >> PAGE_SHIFT;
>>  
>>  	user_bufs = account_pipe_buffers(user, 0, pipe_bufs);
>>  
>> @@ -1039,6 +1049,7 @@ static long pipe_set_size(struct pipe_inode_info *pipe, unsigned long arg)
>>  	struct pipe_buffer *bufs;
>>  	unsigned int size, nr_pages;
>>  	unsigned long user_bufs;
>> +	unsigned int max_size;
>>  	long ret = 0;
>>  
>>  	size = round_pipe_size(arg);
>> @@ -1056,8 +1067,11 @@ static long pipe_set_size(struct pipe_inode_info *pipe, unsigned long arg)
>>  	 * Decreasing the pipe capacity is always permitted, even
>>  	 * if the user is currently over a limit.
>>  	 */
>> +	mutex_lock(&pipe_max_mutex);
>> +	max_size = pipe_max_size;
>> +	mutex_unlock(&pipe_max_mutex);
>>  	if (nr_pages > pipe->buffers &&
>> -			size > pipe_max_size && !capable(CAP_SYS_RESOURCE))
>> +			size > max_size && !capable(CAP_SYS_RESOURCE))
>>  		return -EPERM;
>>  
>>  	user_bufs = account_pipe_buffers(pipe->user, pipe->buffers, nr_pages);
>> @@ -1131,18 +1145,24 @@ int pipe_proc_fn(struct ctl_table *table, int write, void __user *buf,
>>  	unsigned int rounded_pipe_max_size;
>>  	int ret;
>>  
>> +	mutex_lock(&pipe_max_mutex);
>>  	orig_pipe_max_size = pipe_max_size;
>>  	ret = proc_dointvec_minmax(table, write, buf, lenp, ppos);
>> -	if (ret < 0 || !write)
>> +	if (ret < 0 || !write) {
>> +		mutex_unlock(&pipe_max_mutex);
>>  		return ret;
>> +	}
>>  
>>  	rounded_pipe_max_size = round_pipe_size(pipe_max_size);
>>  	if (rounded_pipe_max_size == 0) {
>>  		pipe_max_size = orig_pipe_max_size;
>> +		mutex_unlock(&pipe_max_mutex);
>>  		return -EINVAL;
>>  	}
>>  
>>  	pipe_max_size = rounded_pipe_max_size;
>> +	mutex_unlock(&pipe_max_mutex);
>> +
>>  	return ret;
>>  }
>>  
>> -- 
>> 1.8.3.1
>>
> I think this mutex is too heavy - if multiple processes simultaneously
> create a pipe, the mutex would cause performance degradation.
>
> You can call do_proc_dointvec with a custom callback "conv" function that
> does the rounding of the pipe size value.
>
> Mikulas
>

Hi Mikulas,

I'm not strong when it comes to memory barriers, but one of the
side-effects of using the mutex is that pipe_set_size() and
alloc_pipe_info() should have a consistent view of pipe_max_size.

If I remove the mutex (and assume that I implement a custom
do_proc_dointvec "conv" callback), is it safe for these routines to
directly use pipe_max_size as they had done before?

If not, is it safe to alias through a temporary stack variable (ie,
could the compiler re-read pipe_max_size multiple times in the function)?

Would READ_ONCE() help in any way?

The mutex covered up some confusion on my part here.

OTOH, since pipe_max_size is read-only for pipe_set_size() and
alloc_pipe_info() and only updated occasionally by pipe_proc_fn(), would
rw_semaphore or RCU be a fit here?

Regards,

-- Joe

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#1734722

On Fri, 15 Sep 2017, Joe Lawrence wrote:

> On 09/14/2017 07:09 PM, Mikulas Patocka wrote:
> > On Tue, 5 Sep 2017, Joe Lawrence wrote:
> >> pipe_max_size is assigned directly via procfs sysctl:
> >>
> >>   static struct ctl_table fs_table[] = {
> >>           ...
> >>           {
> >>                   .procname       = "pipe-max-size",
> >>                   .data           = &pipe_max_size,
> >>                   .maxlen         = sizeof(int),
> >>                   .mode           = 0644,
> >>                   .proc_handler   = &pipe_proc_fn,
> >>                   .extra1         = &pipe_min_size,
> >>           },
> >>           ...
> >>
> >>   int pipe_proc_fn(struct ctl_table *table, int write, void __user *buf,
> >>                    size_t *lenp, loff_t *ppos)
> >>   {
> >>           ...
> >>           ret = proc_dointvec_minmax(table, write, buf, lenp, ppos)
> >>           ...
> >>
> >> and then later rounded in-place a few statements later:
> >>
> >>           ...
> >>           pipe_max_size = round_pipe_size(pipe_max_size);
> >>           ...
> >>
> >> This leaves a window of time between initial assignment and rounding
> >> that may be visible to other threads.  (For example, one thread sets a
> >> non-rounded value to pipe_max_size while another reads its value.)
> >>
> >> Similar reads of pipe_max_size are potentially racey:
> >>
> >>   pipe.c :: alloc_pipe_info()
> >>   pipe.c :: pipe_set_size()
> >>
> >> Protect them and the procfs sysctl assignment with a mutex.
> >>
> >> Reported-by: Mikulas Patocka <mpatocka@redhat.com>
> >> Signed-off-by: Joe Lawrence <joe.lawrence@redhat.com>
> >> ---
> >>  fs/pipe.c | 28 ++++++++++++++++++++++++----
> >>  1 file changed, 24 insertions(+), 4 deletions(-)
> >>
> >> diff --git a/fs/pipe.c b/fs/pipe.c
> >> index fa28910b3c59..33bb11b0d78e 100644
> >> --- a/fs/pipe.c
> >> +++ b/fs/pipe.c
> >> @@ -35,6 +35,11 @@
> >>  unsigned int pipe_max_size = 1048576;
> >>  
> >>  /*
> >> + * Provide mutual exclusion around access to pipe_max_size
> >> + */
> >> +static DEFINE_MUTEX(pipe_max_mutex);
> >> +
> >> +/*
> >>   * Minimum pipe size, as required by POSIX
> >>   */
> >>  unsigned int pipe_min_size = PAGE_SIZE;
> >> @@ -623,13 +628,18 @@ struct pipe_inode_info *alloc_pipe_info(void)
> >>  	unsigned long pipe_bufs = PIPE_DEF_BUFFERS;
> >>  	struct user_struct *user = get_current_user();
> >>  	unsigned long user_bufs;
> >> +	unsigned int max_size;
> >>  
> >>  	pipe = kzalloc(sizeof(struct pipe_inode_info), GFP_KERNEL_ACCOUNT);
> >>  	if (pipe == NULL)
> >>  		goto out_free_uid;
> >>  
> >> -	if (pipe_bufs * PAGE_SIZE > pipe_max_size && !capable(CAP_SYS_RESOURCE))
> >> -		pipe_bufs = pipe_max_size >> PAGE_SHIFT;
> >> +	mutex_lock(&pipe_max_mutex);
> >> +	max_size = pipe_max_size;
> >> +	mutex_unlock(&pipe_max_mutex);
> >> +
> >> +	if (pipe_bufs * PAGE_SIZE > max_size && !capable(CAP_SYS_RESOURCE))
> >> +		pipe_bufs = max_size >> PAGE_SHIFT;
> >>  
> >>  	user_bufs = account_pipe_buffers(user, 0, pipe_bufs);
> >>  
> >> @@ -1039,6 +1049,7 @@ static long pipe_set_size(struct pipe_inode_info *pipe, unsigned long arg)
> >>  	struct pipe_buffer *bufs;
> >>  	unsigned int size, nr_pages;
> >>  	unsigned long user_bufs;
> >> +	unsigned int max_size;
> >>  	long ret = 0;
> >>  
> >>  	size = round_pipe_size(arg);
> >> @@ -1056,8 +1067,11 @@ static long pipe_set_size(struct pipe_inode_info *pipe, unsigned long arg)
> >>  	 * Decreasing the pipe capacity is always permitted, even
> >>  	 * if the user is currently over a limit.
> >>  	 */
> >> +	mutex_lock(&pipe_max_mutex);
> >> +	max_size = pipe_max_size;
> >> +	mutex_unlock(&pipe_max_mutex);
> >>  	if (nr_pages > pipe->buffers &&
> >> -			size > pipe_max_size && !capable(CAP_SYS_RESOURCE))
> >> +			size > max_size && !capable(CAP_SYS_RESOURCE))
> >>  		return -EPERM;
> >>  
> >>  	user_bufs = account_pipe_buffers(pipe->user, pipe->buffers, nr_pages);
> >> @@ -1131,18 +1145,24 @@ int pipe_proc_fn(struct ctl_table *table, int write, void __user *buf,
> >>  	unsigned int rounded_pipe_max_size;
> >>  	int ret;
> >>  
> >> +	mutex_lock(&pipe_max_mutex);
> >>  	orig_pipe_max_size = pipe_max_size;
> >>  	ret = proc_dointvec_minmax(table, write, buf, lenp, ppos);
> >> -	if (ret < 0 || !write)
> >> +	if (ret < 0 || !write) {
> >> +		mutex_unlock(&pipe_max_mutex);
> >>  		return ret;
> >> +	}
> >>  
> >>  	rounded_pipe_max_size = round_pipe_size(pipe_max_size);
> >>  	if (rounded_pipe_max_size == 0) {
> >>  		pipe_max_size = orig_pipe_max_size;
> >> +		mutex_unlock(&pipe_max_mutex);
> >>  		return -EINVAL;
> >>  	}
> >>  
> >>  	pipe_max_size = rounded_pipe_max_size;
> >> +	mutex_unlock(&pipe_max_mutex);
> >> +
> >>  	return ret;
> >>  }
> >>  
> >> -- 
> >> 1.8.3.1
> >>
> > I think this mutex is too heavy - if multiple processes simultaneously
> > create a pipe, the mutex would cause performance degradation.
> >
> > You can call do_proc_dointvec with a custom callback "conv" function that
> > does the rounding of the pipe size value.
> >
> > Mikulas
> >
> 
> Hi Mikulas,
> 
> I'm not strong when it comes to memory barriers, but one of the
> side-effects of using the mutex is that pipe_set_size() and
> alloc_pipe_info() should have a consistent view of pipe_max_size.
> 
> If I remove the mutex (and assume that I implement a custom
> do_proc_dointvec "conv" callback), is it safe for these routines to
> directly use pipe_max_size as they had done before?
>
> If not, is it safe to alias through a temporary stack variable (ie,
> could the compiler re-read pipe_max_size multiple times in the function)?
>
> Would READ_ONCE() help in any way?

Theoretically re-reading the variable is possible and you should use 
ACCESS_ONCE or READ_ONCE+WRITE_ONCE on that variable.

In practice, ACCESS_ONCE/READ_ONCE/WRITE_ONCE is missing at a lot of 
kernel variables that could be modified asynchronously and no one is 
complaining about it and no one is making any systematic effort to fix it.

That re-reading happens (I have some test code that makes the gcc 
optimizer re-read a variable), but it happens very rarely.

Another theoretical problem is that when reading or writing a variable 
without ACCESS_ONCE, the compiler could read and write the variable using 
multiple smaller memory accesses. But in practice, it happens only on some 
non-common architectures.

> The mutex covered up some confusion on my part here.
> 
> OTOH, since pipe_max_size is read-only for pipe_set_size() and
> alloc_pipe_info() and only updated occasionally by pipe_proc_fn(), would
> rw_semaphore or RCU be a fit here?

RW semaphore causes cache-line ping-pong between CPUs, it slows down the 
kernel just like a normal spinlock or mutex.

RCU would be useless here (i.e. you don't want to allocate memory and 
atomically assign it with rcu_assign_pointer).

> Regards,
> 
> -- Joe

Mikulas

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#1735272 — Re: [PATCH RFC 2/3] pipe: protect pipe_max_size access with a mutex

On 09/19/2017 03:53 AM, Mikulas Patocka wrote:
> On Fri, 15 Sep 2017, Joe Lawrence wrote:
> [ ... snip ... ]
>> Hi Mikulas,
>>
>> I'm not strong when it comes to memory barriers, but one of the
>> side-effects of using the mutex is that pipe_set_size() and
>> alloc_pipe_info() should have a consistent view of pipe_max_size.
>>
>> If I remove the mutex (and assume that I implement a custom
>> do_proc_dointvec "conv" callback), is it safe for these routines to
>> directly use pipe_max_size as they had done before?
>>
>> If not, is it safe to alias through a temporary stack variable (ie,
>> could the compiler re-read pipe_max_size multiple times in the function)?
>>
>> Would READ_ONCE() help in any way?
> 
> Theoretically re-reading the variable is possible and you should use 
> ACCESS_ONCE or READ_ONCE+WRITE_ONCE on that variable.
> 
> In practice, ACCESS_ONCE/READ_ONCE/WRITE_ONCE is missing at a lot of 
> kernel variables that could be modified asynchronously and no one is 
> complaining about it and no one is making any systematic effort to fix it.
> 
> That re-reading happens (I have some test code that makes the gcc 
> optimizer re-read a variable), but it happens very rarely.

This would be interesting to look at if you are willing to share (can
send offlist).

> Another theoretical problem is that when reading or writing a variable 
> without ACCESS_ONCE, the compiler could read and write the variable using 
> multiple smaller memory accesses. But in practice, it happens only on some 
> non-common architectures.

Smaller access than word size?

>> The mutex covered up some confusion on my part here.
>>
>> OTOH, since pipe_max_size is read-only for pipe_set_size() and
>> alloc_pipe_info() and only updated occasionally by pipe_proc_fn(), would
>> rw_semaphore or RCU be a fit here?
> 
> RW semaphore causes cache-line ping-pong between CPUs, it slows down the 
> kernel just like a normal spinlock or mutex.

Ah right.

> RCU would be useless here (i.e. you don't want to allocate memory and 
> atomically assign it with rcu_assign_pointer).

And good point here.

Thanks for the explanations, they confirm and expand what I as already
thinking in this space.

--- Joe

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#1736516

On Tue, 19 Sep 2017, Joe Lawrence wrote:

> On 09/19/2017 03:53 AM, Mikulas Patocka wrote:
> > On Fri, 15 Sep 2017, Joe Lawrence wrote:
> > [ ... snip ... ]
> >> Hi Mikulas,
> >>
> >> I'm not strong when it comes to memory barriers, but one of the
> >> side-effects of using the mutex is that pipe_set_size() and
> >> alloc_pipe_info() should have a consistent view of pipe_max_size.
> >>
> >> If I remove the mutex (and assume that I implement a custom
> >> do_proc_dointvec "conv" callback), is it safe for these routines to
> >> directly use pipe_max_size as they had done before?
> >>
> >> If not, is it safe to alias through a temporary stack variable (ie,
> >> could the compiler re-read pipe_max_size multiple times in the function)?
> >>
> >> Would READ_ONCE() help in any way?
> > 
> > Theoretically re-reading the variable is possible and you should use 
> > ACCESS_ONCE or READ_ONCE+WRITE_ONCE on that variable.
> > 
> > In practice, ACCESS_ONCE/READ_ONCE/WRITE_ONCE is missing at a lot of 
> > kernel variables that could be modified asynchronously and no one is 
> > complaining about it and no one is making any systematic effort to fix it.
> > 
> > That re-reading happens (I have some test code that makes the gcc 
> > optimizer re-read a variable), but it happens very rarely.
> 
> This would be interesting to look at if you are willing to share (can
> send offlist).

struct s {
        unsigned a, b, c, d;
};

unsigned fn(struct s *s)
{
        unsigned a = s->a;
        s->b = a;
        asm("nop":::"ebx","ecx","edx","esi","edi","ebp");
        s->c = a;
        return s->d;
}

This piece of code makes gcc read the variable s->a twice (although it is 
read only once in the source code). Compile it with -m32 -O2. The result 
is this:

00000000 <fn>:
   0:   55                      push   %ebp
   1:   57                      push   %edi
   2:   56                      push   %esi
   3:   53                      push   %ebx
   4:   8b 44 24 14             mov    0x14(%esp),%eax
   8:   8b 10                   mov    (%eax),%edx   <--- 1st load of s->a
   a:   89 50 04                mov    %edx,0x4(%eax)
   d:   90                      nop
   e:   8b 08                   mov    (%eax),%ecx   <--- 2nd load of s->a
  10:   89 48 08                mov    %ecx,0x8(%eax)
  13:   8b 40 0c                mov    0xc(%eax),%eax
  16:   5b                      pop    %ebx
  17:   5e                      pop    %esi
  18:   5f                      pop    %edi
  19:   5d                      pop    %ebp
  1a:   c3                      ret


> > Another theoretical problem is that when reading or writing a variable 
> > without ACCESS_ONCE, the compiler could read and write the variable using 
> > multiple smaller memory accesses. But in practice, it happens only on some 
> > non-common architectures.
> 
> Smaller access than word size?

Yes. The file Documentation/memory-barriers.txt says:
 (*) For aligned memory locations whose size allows them to be accessed
     with a single memory-reference instruction, prevents "load tearing"
     and "store tearing," in which a single large access is replaced by
     multiple smaller accesses.  For example, given an architecture having
     16-bit store instructions with 7-bit immediate fields, the compiler
     might be tempted to use two 16-bit store-immediate instructions to
     implement the following 32-bit store:

        p = 0x00010002;

     Please note that GCC really does use this sort of optimization,
     which is not surprising given that it would likely take more
     than two instructions to build the constant and then store it.

But it doesn't say on which architecture gcc does it.

Mikulas

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#1735281 — Re: [PATCH RFC 2/3] pipe: protect pipe_max_size access with a mutex

On 09/14/2017 07:09 PM, Mikulas Patocka wrote:
> On Tue, 5 Sep 2017, Joe Lawrence wrote:
> 
>> pipe_max_size is assigned directly via procfs sysctl:
>>
>>   static struct ctl_table fs_table[] = {
>>           ...
>>           {
>>                   .procname       = "pipe-max-size",
>>                   .data           = &pipe_max_size,
>>                   .maxlen         = sizeof(int),
>>                   .mode           = 0644,
>>                   .proc_handler   = &pipe_proc_fn,
>>                   .extra1         = &pipe_min_size,
>>           },
>>           ...
>>
>>   int pipe_proc_fn(struct ctl_table *table, int write, void __user *buf,
>>                    size_t *lenp, loff_t *ppos)
>>   {
>>           ...
>>           ret = proc_dointvec_minmax(table, write, buf, lenp, ppos)
>>           ...
>>
>> and then later rounded in-place a few statements later:
>>
>>           ...
>>           pipe_max_size = round_pipe_size(pipe_max_size);
>>           ...
>>
>> This leaves a window of time between initial assignment and rounding
>> that may be visible to other threads.  (For example, one thread sets a
>> non-rounded value to pipe_max_size while another reads its value.)
>>
>> Similar reads of pipe_max_size are potentially racey:
>>
>>   pipe.c :: alloc_pipe_info()
>>   pipe.c :: pipe_set_size()
>>
>> Protect them and the procfs sysctl assignment with a mutex.
>>
>> Reported-by: Mikulas Patocka <mpatocka@redhat.com>
>> Signed-off-by: Joe Lawrence <joe.lawrence@redhat.com>
>> ---
>>  fs/pipe.c | 28 ++++++++++++++++++++++++----
>>  1 file changed, 24 insertions(+), 4 deletions(-)
>>
>> diff --git a/fs/pipe.c b/fs/pipe.c
>> index fa28910b3c59..33bb11b0d78e 100644
>> --- a/fs/pipe.c
>> +++ b/fs/pipe.c
>> @@ -35,6 +35,11 @@
>>  unsigned int pipe_max_size = 1048576;
>>  
>>  /*
>> + * Provide mutual exclusion around access to pipe_max_size
>> + */
>> +static DEFINE_MUTEX(pipe_max_mutex);
>> +
>> +/*
>>   * Minimum pipe size, as required by POSIX
>>   */
>>  unsigned int pipe_min_size = PAGE_SIZE;
>> @@ -623,13 +628,18 @@ struct pipe_inode_info *alloc_pipe_info(void)
>>  	unsigned long pipe_bufs = PIPE_DEF_BUFFERS;
>>  	struct user_struct *user = get_current_user();
>>  	unsigned long user_bufs;
>> +	unsigned int max_size;
>>  
>>  	pipe = kzalloc(sizeof(struct pipe_inode_info), GFP_KERNEL_ACCOUNT);
>>  	if (pipe == NULL)
>>  		goto out_free_uid;
>>  
>> -	if (pipe_bufs * PAGE_SIZE > pipe_max_size && !capable(CAP_SYS_RESOURCE))
>> -		pipe_bufs = pipe_max_size >> PAGE_SHIFT;
>> +	mutex_lock(&pipe_max_mutex);
>> +	max_size = pipe_max_size;
>> +	mutex_unlock(&pipe_max_mutex);
>> +
>> +	if (pipe_bufs * PAGE_SIZE > max_size && !capable(CAP_SYS_RESOURCE))
>> +		pipe_bufs = max_size >> PAGE_SHIFT;
>>  
>>  	user_bufs = account_pipe_buffers(user, 0, pipe_bufs);
>>  
>> @@ -1039,6 +1049,7 @@ static long pipe_set_size(struct pipe_inode_info *pipe, unsigned long arg)
>>  	struct pipe_buffer *bufs;
>>  	unsigned int size, nr_pages;
>>  	unsigned long user_bufs;
>> +	unsigned int max_size;
>>  	long ret = 0;
>>  
>>  	size = round_pipe_size(arg);
>> @@ -1056,8 +1067,11 @@ static long pipe_set_size(struct pipe_inode_info *pipe, unsigned long arg)
>>  	 * Decreasing the pipe capacity is always permitted, even
>>  	 * if the user is currently over a limit.
>>  	 */
>> +	mutex_lock(&pipe_max_mutex);
>> +	max_size = pipe_max_size;
>> +	mutex_unlock(&pipe_max_mutex);
>>  	if (nr_pages > pipe->buffers &&
>> -			size > pipe_max_size && !capable(CAP_SYS_RESOURCE))
>> +			size > max_size && !capable(CAP_SYS_RESOURCE))
>>  		return -EPERM;
>>  
>>  	user_bufs = account_pipe_buffers(pipe->user, pipe->buffers, nr_pages);
>> @@ -1131,18 +1145,24 @@ int pipe_proc_fn(struct ctl_table *table, int write, void __user *buf,
>>  	unsigned int rounded_pipe_max_size;
>>  	int ret;
>>  
>> +	mutex_lock(&pipe_max_mutex);
>>  	orig_pipe_max_size = pipe_max_size;
>>  	ret = proc_dointvec_minmax(table, write, buf, lenp, ppos);
>> -	if (ret < 0 || !write)
>> +	if (ret < 0 || !write) {
>> +		mutex_unlock(&pipe_max_mutex);
>>  		return ret;
>> +	}
>>  
>>  	rounded_pipe_max_size = round_pipe_size(pipe_max_size);
>>  	if (rounded_pipe_max_size == 0) {
>>  		pipe_max_size = orig_pipe_max_size;
>> +		mutex_unlock(&pipe_max_mutex);
>>  		return -EINVAL;
>>  	}
>>  
>>  	pipe_max_size = rounded_pipe_max_size;
>> +	mutex_unlock(&pipe_max_mutex);
>> +
>>  	return ret;
>>  }
>>  
>> -- 
>> 1.8.3.1
>>
> I think this mutex is too heavy - if multiple processes simultaneously
> create a pipe, the mutex would cause performance degradation.
>
> You can call do_proc_dointvec with a custom callback "conv" function
> that does the rounding of the pipe size value.

Okay, that would consolidate the fiddling with pipe_max_size into a
single place / assignment.

Implementation wise, are you suggesting something like the following
(work in progress).  round_pipe_size() would still be used by
fs/pipe.c::pipe_set_size() so it would need to be visible to both source
files.  The proc_do... function would be pipe-max-size specific as it's
basically:

 - proc_douintvec_minmax, but
    - without the "max" check
    - with an added PAGE_SIZE floor
    - PAGE_SIZE increment

and exported for pipe.c to call.

Plumbing in the opposite direction (export do_proc_douintvec() and stick
the new conv implementation in fs/pipe.c) might be a little easier, but
seems like the same ick-factor in the end.

Regards,

-- Joe

-->8-- -->8-- -->8-- -->8--

diff --git a/fs/pipe.c b/fs/pipe.c
index 8cbc97d97753..4db3cd2d139c 100644
--- a/fs/pipe.c
+++ b/fs/pipe.c
@@ -1019,7 +1019,7 @@ static int fifo_open(struct inode *inode, struct
file *filp)
  * Currently we rely on the pipe array holding a power-of-2 number
  * of pages. Returns 0 on error.
  */
-static inline unsigned int round_pipe_size(unsigned int size)
+unsigned int round_pipe_size(unsigned int size)
 {
 	unsigned long nr_pages;

@@ -1130,19 +1130,7 @@ static long pipe_set_size(struct pipe_inode_info
*pipe, unsigned long arg)
 int pipe_proc_fn(struct ctl_table *table, int write, void __user *buf,
 		 size_t *lenp, loff_t *ppos)
 {
-	unsigned int rounded_pipe_max_size;
-	int ret;
-
-	ret = proc_douintvec_minmax(table, write, buf, lenp, ppos);
-	if (ret < 0 || !write)
-		return ret;
-
-	rounded_pipe_max_size = round_pipe_size(pipe_max_size);
-	if (rounded_pipe_max_size == 0)
-		return -EINVAL;
-
-	pipe_max_size = rounded_pipe_max_size;
-	return ret;
+	return proc_dopipe_max_size(table, write, buf, lenp, ppos);
 }

 /*
diff --git a/include/linux/pipe_fs_i.h b/include/linux/pipe_fs_i.h
index e7497c9dde7f..485cf7a7aa8f 100644
--- a/include/linux/pipe_fs_i.h
+++ b/include/linux/pipe_fs_i.h
@@ -190,5 +190,6 @@ static inline int pipe_buf_steal(struct
pipe_inode_info *pipe,
 struct pipe_inode_info *get_pipe_info(struct file *file);

 int create_pipe_files(struct file **, int);
+unsigned int round_pipe_size(unsigned int size);

 #endif
diff --git a/include/linux/sysctl.h b/include/linux/sysctl.h
index 1d4dba490fb6..ba24ca72800c 100644
--- a/include/linux/sysctl.h
+++ b/include/linux/sysctl.h
@@ -50,6 +50,9 @@ extern int proc_dointvec_minmax(struct ctl_table *, int,
 extern int proc_douintvec_minmax(struct ctl_table *table, int write,
 				 void __user *buffer, size_t *lenp,
 				 loff_t *ppos);
+extern int proc_dopipe_max_size(struct ctl_table *table, int write,
+				void __user *buffer, size_t *lenp,
+				loff_t *ppos);
 extern int proc_dointvec_jiffies(struct ctl_table *, int,
 				 void __user *, size_t *, loff_t *);
 extern int proc_dointvec_userhz_jiffies(struct ctl_table *, int,
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index c976719bf37a..7a2913c5546e 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -67,6 +67,7 @@
 #include <linux/kexec.h>
 #include <linux/bpf.h>
 #include <linux/mount.h>
+#include <linux/pipe_fs_i.h>

 #include <linux/uaccess.h>
 #include <asm/processor.h>
@@ -2631,6 +2632,47 @@ int proc_douintvec_minmax(struct ctl_table
*table, int write,
 				 do_proc_douintvec_minmax_conv, &param);
 }

+struct do_proc_dopipe_max_size_conv_param {
+	unsigned int *min;
+};
+
+static int do_proc_dopipe_max_size_conv(unsigned long *lvalp,
+					unsigned int *valp,
+					int write, void *data)
+{
+	struct do_proc_dopipe_max_size_conv_param *param = data;
+
+	if (write) {
+		unsigned int val = round_pipe_size(*lvalp);
+
+		if (val == 0)
+			return -EINVAL;
+
+		if (param->min && *param->min > val)
+			return -ERANGE;
+
+		if (*lvalp > UINT_MAX)
+			return -EINVAL;
+
+		*valp = val;
+	} else {
+		unsigned int val = *valp;
+		*lvalp = (unsigned long) val;
+	}
+
+	return 0;
+}
+
+int proc_dopipe_max_size(struct ctl_table *table, int write,
+			 void __user *buffer, size_t *lenp, loff_t *ppos)
+{
+	struct do_proc_dopipe_max_size_conv_param param = {
+		.min = (unsigned int *) table->extra1,
+	};
+	return do_proc_douintvec(table, write, buffer, lenp, ppos,
+				 do_proc_dopipe_max_size_conv, &param);
+}
+
 static void validate_coredump_safety(void)
 {
 #ifdef CONFIG_COREDUMP
@@ -3179,6 +3221,7 @@ int proc_doulongvec_ms_jiffies_minmax(struct
ctl_table *table, int write,
 EXPORT_SYMBOL(proc_dointvec_jiffies);
 EXPORT_SYMBOL(proc_dointvec_minmax);
 EXPORT_SYMBOL_GPL(proc_douintvec_minmax);
+EXPORT_SYMBOL_GPL(proc_dopipe_max_size);
 EXPORT_SYMBOL(proc_dointvec_userhz_jiffies);
 EXPORT_SYMBOL(proc_dointvec_ms_jiffies);
 EXPORT_SYMBOL(proc_dostring);
-- 
1.8.3.1

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#1738917

On Tue, 19 Sep 2017, Joe Lawrence wrote:

> On 09/14/2017 07:09 PM, Mikulas Patocka wrote:
> >
> > I think this mutex is too heavy - if multiple processes simultaneously
> > create a pipe, the mutex would cause performance degradation.
> >
> > You can call do_proc_dointvec with a custom callback "conv" function
> > that does the rounding of the pipe size value.
> 
> Okay, that would consolidate the fiddling with pipe_max_size into a
> single place / assignment.
> 
> Implementation wise, are you suggesting something like the following
> (work in progress).  round_pipe_size() would still be used by
> fs/pipe.c::pipe_set_size() so it would need to be visible to both source
> files.  The proc_do... function would be pipe-max-size specific as it's
> basically:
> 
>  - proc_douintvec_minmax, but
>     - without the "max" check
>     - with an added PAGE_SIZE floor
>     - PAGE_SIZE increment
> 
> and exported for pipe.c to call.
> 
> Plumbing in the opposite direction (export do_proc_douintvec() and stick
> the new conv implementation in fs/pipe.c) might be a little easier, but
> seems like the same ick-factor in the end.
> 
> Regards,
> 
> -- Joe

Yes. I think this is correct.

Reviewed-by: Mikulas Patocka <mpatocka@redhat.com>

> -->8-- -->8-- -->8-- -->8--
> 
> diff --git a/fs/pipe.c b/fs/pipe.c
> index 8cbc97d97753..4db3cd2d139c 100644
> --- a/fs/pipe.c
> +++ b/fs/pipe.c
> @@ -1019,7 +1019,7 @@ static int fifo_open(struct inode *inode, struct
> file *filp)
>   * Currently we rely on the pipe array holding a power-of-2 number
>   * of pages. Returns 0 on error.
>   */
> -static inline unsigned int round_pipe_size(unsigned int size)
> +unsigned int round_pipe_size(unsigned int size)
>  {
>  	unsigned long nr_pages;
> 
> @@ -1130,19 +1130,7 @@ static long pipe_set_size(struct pipe_inode_info
> *pipe, unsigned long arg)
>  int pipe_proc_fn(struct ctl_table *table, int write, void __user *buf,
>  		 size_t *lenp, loff_t *ppos)
>  {
> -	unsigned int rounded_pipe_max_size;
> -	int ret;
> -
> -	ret = proc_douintvec_minmax(table, write, buf, lenp, ppos);
> -	if (ret < 0 || !write)
> -		return ret;
> -
> -	rounded_pipe_max_size = round_pipe_size(pipe_max_size);
> -	if (rounded_pipe_max_size == 0)
> -		return -EINVAL;
> -
> -	pipe_max_size = rounded_pipe_max_size;
> -	return ret;
> +	return proc_dopipe_max_size(table, write, buf, lenp, ppos);
>  }
> 
>  /*
> diff --git a/include/linux/pipe_fs_i.h b/include/linux/pipe_fs_i.h
> index e7497c9dde7f..485cf7a7aa8f 100644
> --- a/include/linux/pipe_fs_i.h
> +++ b/include/linux/pipe_fs_i.h
> @@ -190,5 +190,6 @@ static inline int pipe_buf_steal(struct
> pipe_inode_info *pipe,
>  struct pipe_inode_info *get_pipe_info(struct file *file);
> 
>  int create_pipe_files(struct file **, int);
> +unsigned int round_pipe_size(unsigned int size);
> 
>  #endif
> diff --git a/include/linux/sysctl.h b/include/linux/sysctl.h
> index 1d4dba490fb6..ba24ca72800c 100644
> --- a/include/linux/sysctl.h
> +++ b/include/linux/sysctl.h
> @@ -50,6 +50,9 @@ extern int proc_dointvec_minmax(struct ctl_table *, int,
>  extern int proc_douintvec_minmax(struct ctl_table *table, int write,
>  				 void __user *buffer, size_t *lenp,
>  				 loff_t *ppos);
> +extern int proc_dopipe_max_size(struct ctl_table *table, int write,
> +				void __user *buffer, size_t *lenp,
> +				loff_t *ppos);
>  extern int proc_dointvec_jiffies(struct ctl_table *, int,
>  				 void __user *, size_t *, loff_t *);
>  extern int proc_dointvec_userhz_jiffies(struct ctl_table *, int,
> diff --git a/kernel/sysctl.c b/kernel/sysctl.c
> index c976719bf37a..7a2913c5546e 100644
> --- a/kernel/sysctl.c
> +++ b/kernel/sysctl.c
> @@ -67,6 +67,7 @@
>  #include <linux/kexec.h>
>  #include <linux/bpf.h>
>  #include <linux/mount.h>
> +#include <linux/pipe_fs_i.h>
> 
>  #include <linux/uaccess.h>
>  #include <asm/processor.h>
> @@ -2631,6 +2632,47 @@ int proc_douintvec_minmax(struct ctl_table
> *table, int write,
>  				 do_proc_douintvec_minmax_conv, &param);
>  }
> 
> +struct do_proc_dopipe_max_size_conv_param {
> +	unsigned int *min;
> +};
> +
> +static int do_proc_dopipe_max_size_conv(unsigned long *lvalp,
> +					unsigned int *valp,
> +					int write, void *data)
> +{
> +	struct do_proc_dopipe_max_size_conv_param *param = data;
> +
> +	if (write) {
> +		unsigned int val = round_pipe_size(*lvalp);
> +
> +		if (val == 0)
> +			return -EINVAL;
> +
> +		if (param->min && *param->min > val)
> +			return -ERANGE;
> +
> +		if (*lvalp > UINT_MAX)
> +			return -EINVAL;
> +
> +		*valp = val;
> +	} else {
> +		unsigned int val = *valp;
> +		*lvalp = (unsigned long) val;
> +	}
> +
> +	return 0;
> +}
> +
> +int proc_dopipe_max_size(struct ctl_table *table, int write,
> +			 void __user *buffer, size_t *lenp, loff_t *ppos)
> +{
> +	struct do_proc_dopipe_max_size_conv_param param = {
> +		.min = (unsigned int *) table->extra1,
> +	};
> +	return do_proc_douintvec(table, write, buffer, lenp, ppos,
> +				 do_proc_dopipe_max_size_conv, &param);
> +}
> +
>  static void validate_coredump_safety(void)
>  {
>  #ifdef CONFIG_COREDUMP
> @@ -3179,6 +3221,7 @@ int proc_doulongvec_ms_jiffies_minmax(struct
> ctl_table *table, int write,
>  EXPORT_SYMBOL(proc_dointvec_jiffies);
>  EXPORT_SYMBOL(proc_dointvec_minmax);
>  EXPORT_SYMBOL_GPL(proc_douintvec_minmax);
> +EXPORT_SYMBOL_GPL(proc_dopipe_max_size);
>  EXPORT_SYMBOL(proc_dointvec_userhz_jiffies);
>  EXPORT_SYMBOL(proc_dointvec_ms_jiffies);
>  EXPORT_SYMBOL(proc_dostring);
> -- 
> 1.8.3.1
> 

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