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[PATCH v2] fs: clear file privilege bits when mmap writing

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  [PATCH v2] fs: clear file privilege bits when mmap writing Kees Cook <keescook@chromium.org> - 2015-12-03 01:10 +0100
    Re: [PATCH v2] fs: clear file privilege bits when mmap writing Andrew Morton <akpm@linux-foundation.org> - 2015-12-03 01:20 +0100
      Re: [PATCH v2] fs: clear file privilege bits when mmap writing Kees Cook <keescook@chromium.org> - 2015-12-03 17:10 +0100
      Re: [PATCH v2] fs: clear file privilege bits when mmap writing Kees Cook <keescook@chromium.org> - 2015-12-03 19:20 +0100
    Re: [PATCH v2] clear file privilege bits when mmap writing yalin wang <yalin.wang2010@gmail.com> - 2015-12-04 02:50 +0100
      Re: [PATCH v2] clear file privilege bits when mmap writing Kees Cook <keescook@chromium.org> - 2015-12-07 23:50 +0100
        Re: [PATCH v2] clear file privilege bits when mmap writing Kees Cook <keescook@chromium.org> - 2015-12-08 01:50 +0100
          Re: [PATCH v2] clear file privilege bits when mmap writing Jan Kara <jack@suse.cz> - 2015-12-09 09:30 +0100
            Re: [PATCH v2] clear file privilege bits when mmap writing Kees Cook <keescook@chromium.org> - 2015-12-10 00:00 +0100

#1282608 — [PATCH v2] fs: clear file privilege bits when mmap writing

Normally, when a user can modify a file that has setuid or setgid bits,
those bits are cleared when they are not the file owner or a member
of the group. This is enforced when using write and truncate but not
when writing to a shared mmap on the file. This could allow the file
writer to gain privileges by changing a binary without losing the
setuid/setgid/caps bits.

Changing the bits requires holding inode->i_mutex, so it cannot be done
during the page fault (due to mmap_sem being held during the fault).
Instead, clear the bits if PROT_WRITE is being used at mmap time.

Signed-off-by: Kees Cook <keescook@chromium.org>
Cc: stable@vger.kernel.org
---
v2:
 - move check from page fault to mmap open
---
 mm/mmap.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/mm/mmap.c b/mm/mmap.c
index 2ce04a649f6b..a27735aabc73 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -1340,6 +1340,17 @@ unsigned long do_mmap(struct file *file, unsigned long addr,
 			if (locks_verify_locked(file))
 				return -EAGAIN;
 
+			/*
+			 * If we must remove privs, we do it here since
+			 * doing it during page COW is expensive and
+			 * cannot hold inode->i_mutex.
+			 */
+			if (prot & PROT_WRITE && !IS_NOSEC(inode)) {
+				mutex_lock(&inode->i_mutex);
+				file_remove_privs(file);
+				mutex_unlock(&inode->i_mutex);
+			}
+
 			vm_flags |= VM_SHARED | VM_MAYSHARE;
 			if (!(file->f_mode & FMODE_WRITE))
 				vm_flags &= ~(VM_MAYWRITE | VM_SHARED);
-- 
1.9.1


-- 
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Chrome OS & Brillo Security
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#1282622

On Wed, 2 Dec 2015 16:03:42 -0800 Kees Cook <keescook@chromium.org> wrote:

> Normally, when a user can modify a file that has setuid or setgid bits,
> those bits are cleared when they are not the file owner or a member
> of the group. This is enforced when using write and truncate but not
> when writing to a shared mmap on the file. This could allow the file
> writer to gain privileges by changing a binary without losing the
> setuid/setgid/caps bits.
> 
> Changing the bits requires holding inode->i_mutex, so it cannot be done
> during the page fault (due to mmap_sem being held during the fault).
> Instead, clear the bits if PROT_WRITE is being used at mmap time.
> 
> ...
>
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -1340,6 +1340,17 @@ unsigned long do_mmap(struct file *file, unsigned long addr,
>  			if (locks_verify_locked(file))
>  				return -EAGAIN;
>  
> +			/*
> +			 * If we must remove privs, we do it here since
> +			 * doing it during page COW is expensive and
> +			 * cannot hold inode->i_mutex.
> +			 */
> +			if (prot & PROT_WRITE && !IS_NOSEC(inode)) {
> +				mutex_lock(&inode->i_mutex);
> +				file_remove_privs(file);
> +				mutex_unlock(&inode->i_mutex);
> +			}
> +

Still ignoring the file_remove_privs() return value.  If this is
deliberate then a description of the reasons should be included?

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#1283156

On Wed, Dec 2, 2015 at 4:18 PM, Andrew Morton <akpm@linux-foundation.org> wrote:
> On Wed, 2 Dec 2015 16:03:42 -0800 Kees Cook <keescook@chromium.org> wrote:
>
>> Normally, when a user can modify a file that has setuid or setgid bits,
>> those bits are cleared when they are not the file owner or a member
>> of the group. This is enforced when using write and truncate but not
>> when writing to a shared mmap on the file. This could allow the file
>> writer to gain privileges by changing a binary without losing the
>> setuid/setgid/caps bits.
>>
>> Changing the bits requires holding inode->i_mutex, so it cannot be done
>> during the page fault (due to mmap_sem being held during the fault).
>> Instead, clear the bits if PROT_WRITE is being used at mmap time.
>>
>> ...
>>
>> --- a/mm/mmap.c
>> +++ b/mm/mmap.c
>> @@ -1340,6 +1340,17 @@ unsigned long do_mmap(struct file *file, unsigned long addr,
>>                       if (locks_verify_locked(file))
>>                               return -EAGAIN;
>>
>> +                     /*
>> +                      * If we must remove privs, we do it here since
>> +                      * doing it during page COW is expensive and
>> +                      * cannot hold inode->i_mutex.
>> +                      */
>> +                     if (prot & PROT_WRITE && !IS_NOSEC(inode)) {
>> +                             mutex_lock(&inode->i_mutex);
>> +                             file_remove_privs(file);
>> +                             mutex_unlock(&inode->i_mutex);
>> +                     }
>> +
>
> Still ignoring the file_remove_privs() return value.  If this is
> deliberate then a description of the reasons should be included?

Argh, yes, sorry. I will send a v3.

-Kees

-- 
Kees Cook
Chrome OS & Brillo Security
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#1283239

On Wed, Dec 2, 2015 at 4:18 PM, Andrew Morton <akpm@linux-foundation.org> wrote:
> On Wed, 2 Dec 2015 16:03:42 -0800 Kees Cook <keescook@chromium.org> wrote:
>
>> Normally, when a user can modify a file that has setuid or setgid bits,
>> those bits are cleared when they are not the file owner or a member
>> of the group. This is enforced when using write and truncate but not
>> when writing to a shared mmap on the file. This could allow the file
>> writer to gain privileges by changing a binary without losing the
>> setuid/setgid/caps bits.
>>
>> Changing the bits requires holding inode->i_mutex, so it cannot be done
>> during the page fault (due to mmap_sem being held during the fault).
>> Instead, clear the bits if PROT_WRITE is being used at mmap time.
>>
>> ...
>>
>> --- a/mm/mmap.c
>> +++ b/mm/mmap.c
>> @@ -1340,6 +1340,17 @@ unsigned long do_mmap(struct file *file, unsigned long addr,
>>                       if (locks_verify_locked(file))
>>                               return -EAGAIN;
>>
>> +                     /*
>> +                      * If we must remove privs, we do it here since
>> +                      * doing it during page COW is expensive and
>> +                      * cannot hold inode->i_mutex.
>> +                      */
>> +                     if (prot & PROT_WRITE && !IS_NOSEC(inode)) {
>> +                             mutex_lock(&inode->i_mutex);
>> +                             file_remove_privs(file);
>> +                             mutex_unlock(&inode->i_mutex);
>> +                     }
>> +
>
> Still ignoring the file_remove_privs() return value.  If this is
> deliberate then a description of the reasons should be included?

Actually, there is a bigger problem:
https://lists.01.org/pipermail/lkp/2015-December/003185.html

[   37.741286] trinity-c0/742 is trying to acquire lock:
[   37.741982]  (&sb->s_type->i_mutex_key#8){+.+.+.}, at: [<811c3b34>]
do_mmap+0x544/0x670
[   37.752562]
[   37.752562] but task is already holding lock:
[   37.753442]  (&mm->mmap_sem){++++++}, at: [<811c3d70>]
SyS_remap_file_pages+0xe0/0x350

Jan, any thoughts on avoiding this?

-Kees

-- 
Kees Cook
Chrome OS & Brillo Security
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#1283536 — Re: [PATCH v2] clear file privilege bits when mmap writing

> On Dec 2, 2015, at 16:03, Kees Cook <keescook@chromium.org> wrote:
> 
> Normally, when a user can modify a file that has setuid or setgid bits,
> those bits are cleared when they are not the file owner or a member
> of the group. This is enforced when using write and truncate but not
> when writing to a shared mmap on the file. This could allow the file
> writer to gain privileges by changing a binary without losing the
> setuid/setgid/caps bits.
> 
> Changing the bits requires holding inode->i_mutex, so it cannot be done
> during the page fault (due to mmap_sem being held during the fault).
> Instead, clear the bits if PROT_WRITE is being used at mmap time.
> 
> Signed-off-by: Kees Cook <keescook@chromium.org>
> Cc: stable@vger.kernel.org
> —

is this means mprotect() sys call also need add this check?
mprotect() can change to PROT_WRITE, then it can write to a 
read only map again , also a secure hole here .

Thanks

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#1286073 — Re: [PATCH v2] clear file privilege bits when mmap writing

On Thu, Dec 3, 2015 at 5:45 PM, yalin wang <yalin.wang2010@gmail.com> wrote:
>
>> On Dec 2, 2015, at 16:03, Kees Cook <keescook@chromium.org> wrote:
>>
>> Normally, when a user can modify a file that has setuid or setgid bits,
>> those bits are cleared when they are not the file owner or a member
>> of the group. This is enforced when using write and truncate but not
>> when writing to a shared mmap on the file. This could allow the file
>> writer to gain privileges by changing a binary without losing the
>> setuid/setgid/caps bits.
>>
>> Changing the bits requires holding inode->i_mutex, so it cannot be done
>> during the page fault (due to mmap_sem being held during the fault).
>> Instead, clear the bits if PROT_WRITE is being used at mmap time.
>>
>> Signed-off-by: Kees Cook <keescook@chromium.org>
>> Cc: stable@vger.kernel.org
>> —
>
> is this means mprotect() sys call also need add this check?
> mprotect() can change to PROT_WRITE, then it can write to a
> read only map again , also a secure hole here .

Yes, good point. This needs to be added. I will send a new patch. Thanks!

-Kees

-- 
Kees Cook
Chrome OS & Brillo Security
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#1286131 — Re: [PATCH v2] clear file privilege bits when mmap writing

On Mon, Dec 7, 2015 at 2:42 PM, Kees Cook <keescook@chromium.org> wrote:
> On Thu, Dec 3, 2015 at 5:45 PM, yalin wang <yalin.wang2010@gmail.com> wrote:
>>
>>> On Dec 2, 2015, at 16:03, Kees Cook <keescook@chromium.org> wrote:
>>>
>>> Normally, when a user can modify a file that has setuid or setgid bits,
>>> those bits are cleared when they are not the file owner or a member
>>> of the group. This is enforced when using write and truncate but not
>>> when writing to a shared mmap on the file. This could allow the file
>>> writer to gain privileges by changing a binary without losing the
>>> setuid/setgid/caps bits.
>>>
>>> Changing the bits requires holding inode->i_mutex, so it cannot be done
>>> during the page fault (due to mmap_sem being held during the fault).
>>> Instead, clear the bits if PROT_WRITE is being used at mmap time.
>>>
>>> Signed-off-by: Kees Cook <keescook@chromium.org>
>>> Cc: stable@vger.kernel.org
>>> —
>>
>> is this means mprotect() sys call also need add this check?
>> mprotect() can change to PROT_WRITE, then it can write to a
>> read only map again , also a secure hole here .
>
> Yes, good point. This needs to be added. I will send a new patch. Thanks!

This continues to look worse and worse.

So... to check this at mprotect time, I have to know it's MAP_SHARED,
but that's in the vma_flags, which I can only see after holding
mmap_sem.

The best I can think of now is to strip the bits at munmap time, since
you can't execute an mmapped file until it closes.

Jan, thoughts on this?

-Kees

-- 
Kees Cook
Chrome OS & Brillo Security
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#1287186 — Re: [PATCH v2] clear file privilege bits when mmap writing

On Mon 07-12-15 16:40:14, Kees Cook wrote:
> On Mon, Dec 7, 2015 at 2:42 PM, Kees Cook <keescook@chromium.org> wrote:
> > On Thu, Dec 3, 2015 at 5:45 PM, yalin wang <yalin.wang2010@gmail.com> wrote:
> >>
> >>> On Dec 2, 2015, at 16:03, Kees Cook <keescook@chromium.org> wrote:
> >>>
> >>> Normally, when a user can modify a file that has setuid or setgid bits,
> >>> those bits are cleared when they are not the file owner or a member
> >>> of the group. This is enforced when using write and truncate but not
> >>> when writing to a shared mmap on the file. This could allow the file
> >>> writer to gain privileges by changing a binary without losing the
> >>> setuid/setgid/caps bits.
> >>>
> >>> Changing the bits requires holding inode->i_mutex, so it cannot be done
> >>> during the page fault (due to mmap_sem being held during the fault).
> >>> Instead, clear the bits if PROT_WRITE is being used at mmap time.
> >>>
> >>> Signed-off-by: Kees Cook <keescook@chromium.org>
> >>> Cc: stable@vger.kernel.org
> >>> —
> >>
> >> is this means mprotect() sys call also need add this check?
> >> mprotect() can change to PROT_WRITE, then it can write to a
> >> read only map again , also a secure hole here .
> >
> > Yes, good point. This needs to be added. I will send a new patch. Thanks!
> 
> This continues to look worse and worse.
> 
> So... to check this at mprotect time, I have to know it's MAP_SHARED,
> but that's in the vma_flags, which I can only see after holding
> mmap_sem.
> 
> The best I can think of now is to strip the bits at munmap time, since
> you can't execute an mmapped file until it closes.
> 
> Jan, thoughts on this?

Umm, so we actually refuse to execute a file while someone has it open for
writing (deny_write_access() in do_open_execat()). So dropping the suid /
sgid bits when closing file for writing could be plausible. Grabbing
i_mutex from __fput() context is safe (it gets called from task_work
context when returning to userspace).

That way we could actually remove the checks done for each write. To avoid
unexpected removal of suid/sgid bits when someone just opens & closes the
file, we could mark the file as needing suid/sgid treatment by a flag in
inode->i_flags when file gets written to or mmaped and then check for this
in __fput().

I've added Al Viro to CC just in case he is aware of some issues with
this...

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR
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#1287974 — Re: [PATCH v2] clear file privilege bits when mmap writing

On Wed, Dec 9, 2015 at 12:26 AM, Jan Kara <jack@suse.cz> wrote:
> On Mon 07-12-15 16:40:14, Kees Cook wrote:
>> On Mon, Dec 7, 2015 at 2:42 PM, Kees Cook <keescook@chromium.org> wrote:
>> > On Thu, Dec 3, 2015 at 5:45 PM, yalin wang <yalin.wang2010@gmail.com> wrote:
>> >>
>> >>> On Dec 2, 2015, at 16:03, Kees Cook <keescook@chromium.org> wrote:
>> >>>
>> >>> Normally, when a user can modify a file that has setuid or setgid bits,
>> >>> those bits are cleared when they are not the file owner or a member
>> >>> of the group. This is enforced when using write and truncate but not
>> >>> when writing to a shared mmap on the file. This could allow the file
>> >>> writer to gain privileges by changing a binary without losing the
>> >>> setuid/setgid/caps bits.
>> >>>
>> >>> Changing the bits requires holding inode->i_mutex, so it cannot be done
>> >>> during the page fault (due to mmap_sem being held during the fault).
>> >>> Instead, clear the bits if PROT_WRITE is being used at mmap time.
>> >>>
>> >>> Signed-off-by: Kees Cook <keescook@chromium.org>
>> >>> Cc: stable@vger.kernel.org
>> >>> —
>> >>
>> >> is this means mprotect() sys call also need add this check?
>> >> mprotect() can change to PROT_WRITE, then it can write to a
>> >> read only map again , also a secure hole here .
>> >
>> > Yes, good point. This needs to be added. I will send a new patch. Thanks!
>>
>> This continues to look worse and worse.
>>
>> So... to check this at mprotect time, I have to know it's MAP_SHARED,
>> but that's in the vma_flags, which I can only see after holding
>> mmap_sem.
>>
>> The best I can think of now is to strip the bits at munmap time, since
>> you can't execute an mmapped file until it closes.
>>
>> Jan, thoughts on this?
>
> Umm, so we actually refuse to execute a file while someone has it open for
> writing (deny_write_access() in do_open_execat()). So dropping the suid /
> sgid bits when closing file for writing could be plausible. Grabbing
> i_mutex from __fput() context is safe (it gets called from task_work
> context when returning to userspace).
>
> That way we could actually remove the checks done for each write. To avoid
> unexpected removal of suid/sgid bits when someone just opens & closes the
> file, we could mark the file as needing suid/sgid treatment by a flag in
> inode->i_flags when file gets written to or mmaped and then check for this
> in __fput().

Yeah, this is ultimately where I ended up for the v4 (and fixed up in
v5). I added the flag to file, though, not inode. Sending v5 now...

-Kees

>
> I've added Al Viro to CC just in case he is aware of some issues with
> this...
>
>                                                                 Honza
> --
> Jan Kara <jack@suse.com>
> SUSE Labs, CR



-- 
Kees Cook
Chrome OS & Brillo Security
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