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Re: dm ioctl: Access user-land memory through safe functions.

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  Re: dm ioctl: Access user-land memory through safe functions. Mike Snitzer <snitzer@redhat.com> - 2016-01-05 21:20 +0100
    Re: dm ioctl: Access user-land memory through safe functions. Mike Snitzer <snitzer@redhat.com> - 2016-01-05 22:20 +0100
      Re: dm ioctl: Access user-land memory through safe functions. Scotty Bauer <sbauer@eng.utah.edu> - 2016-01-07 02:30 +0100
        Re: dm ioctl: Access user-land memory through safe functions. Mike Snitzer <snitzer@redhat.com> - 2016-01-07 03:10 +0100

#1302049 — Re: dm ioctl: Access user-land memory through safe functions.

On Tue, Dec 08 2015 at  1:26pm -0500,
Scotty Bauer <sbauer@eng.utah.edu> wrote:

> 
> On 12/01/2015 11:11 AM, Scotty wrote:
> > 
> > 0001-dm-ioctl-Access-user-land-memory-through-safe-functi.patch
> > 
> > 
> > From b26adf880eba03ac6f2b1dd87426bb96fd2a0282 Mon Sep 17 00:00:00 2001
> > From: Scotty Bauer <sbauer@eng.utah.edu>
> > Date: Tue, 1 Dec 2015 10:52:46 -0700
> > Subject: [PATCH] dm ioctl: Access user-land memory through safe functions.
> > 
> > This patch fixes a user-land dereference. Now we use
> > the safe copy_from_user to access the memory.
> > 
> > Signed-off-by: Scotty Bauer <sbauer@eng.utah.edu>
> > ---
> >  drivers/md/dm-ioctl.c | 8 ++++++--
> >  1 file changed, 6 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/md/dm-ioctl.c b/drivers/md/dm-ioctl.c
> > index 80a4395..39a9d1a 100644
> > --- a/drivers/md/dm-ioctl.c
> > +++ b/drivers/md/dm-ioctl.c
> > @@ -1642,9 +1642,13 @@ static ioctl_fn lookup_ioctl(unsigned int cmd, int *ioctl_flags)
> >  static int check_version(unsigned int cmd, struct dm_ioctl __user *user)
> >  {
> >  	uint32_t version[3];
> > +	uint32_t __user *version_ptr;
> >  	int r = 0;
> >  
> > -	if (copy_from_user(version, user->version, sizeof(version)))
> > +	if (copy_from_user(&version_ptr, &user->version, sizeof(version_ptr)))
> > +		return -EFAULT;
> > +
> > +	if (copy_from_user(version, version_ptr, sizeof(version)))
> >  		return -EFAULT;
> >  
> >  	if ((DM_VERSION_MAJOR != version[0]) ||
> > @@ -1663,7 +1667,7 @@ static int check_version(unsigned int cmd, struct dm_ioctl __user *user)
> >  	version[0] = DM_VERSION_MAJOR;
> >  	version[1] = DM_VERSION_MINOR;
> >  	version[2] = DM_VERSION_PATCHLEVEL;
> > -	if (copy_to_user(user->version, version, sizeof(version)))
> > +	if (copy_to_user(version_ptr, version, sizeof(version)))
> >  		return -EFAULT;
> >  
> >  	return r;
> > -- 
> 
> 
> Friendly ping, is anyone interested in this?

The passed @user argument is flagged via __user so it can be
deferenced directly.  It does look like directly deferencing
user->version is wrong.

But even if such indirect access is needed (because __user flag is only
applicable to @user arg, not the contained version member) we could more
easily just do something like this no?:

  uint32_t __user *versionp = (uint32_t __user *)user->version;
  ...
  if (copy_from_user(version, versionp, sizeof(version)))
     return -EFAULT;

I've staged the following, thanks:
https://git.kernel.org/cgit/linux/kernel/git/device-mapper/linux-dm.git/commit/?h=dm-4.5&id=bffc9e237a0c3176712bcd93fc6a184a61e0df26
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#1302200

On Tue, Jan 05 2016 at  3:16pm -0500,
Mike Snitzer <snitzer@redhat.com> wrote:

> On Tue, Dec 08 2015 at  1:26pm -0500,
> Scotty Bauer <sbauer@eng.utah.edu> wrote:
> 
> > Friendly ping, is anyone interested in this?
> 
> The passed @user argument is flagged via __user so it can be
> deferenced directly.  It does look like directly deferencing
> user->version is wrong.
> 
> But even if such indirect access is needed (because __user flag is only
> applicable to @user arg, not the contained version member) we could more
> easily just do something like this no?:
> 
>   uint32_t __user *versionp = (uint32_t __user *)user->version;
>   ...
>   if (copy_from_user(version, versionp, sizeof(version)))
>      return -EFAULT;
> 
> I've staged the following, thanks:
> https://git.kernel.org/cgit/linux/kernel/git/device-mapper/linux-dm.git/commit/?h=dm-4.5&id=bffc9e237a0c3176712bcd93fc6a184a61e0df26

Alasdair helped me understand that we do need your original fix.
I've staged it for 4.5 (and stable@) here:
https://git.kernel.org/cgit/linux/kernel/git/device-mapper/linux-dm.git/commit/?h=dm-4.5&id=ead3db62bf10fe143bec99e7b7ff370d7a6d23ef

Thanks again,
Mike
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#1303223

[Multipart message — attachments visible in raw view] — view raw

On 01/05/2016 02:13 PM, Mike Snitzer wrote:
> On Tue, Jan 05 2016 at  3:16pm -0500,
> Mike Snitzer <snitzer@redhat.com> wrote:
> 
>> On Tue, Dec 08 2015 at  1:26pm -0500,
>> Scotty Bauer <sbauer@eng.utah.edu> wrote:
>>
>>> Friendly ping, is anyone interested in this?
>>
>> The passed @user argument is flagged via __user so it can be
>> deferenced directly.  It does look like directly deferencing
>> user->version is wrong.
>>
>> But even if such indirect access is needed (because __user flag is only
>> applicable to @user arg, not the contained version member) we could more
>> easily just do something like this no?:
>>
>>   uint32_t __user *versionp = (uint32_t __user *)user->version;
>>   ...
>>   if (copy_from_user(version, versionp, sizeof(version)))
>>      return -EFAULT;
>>
>> I've staged the following, thanks:
>> https://git.kernel.org/cgit/linux/kernel/git/device-mapper/linux-dm.git/commit/?h=dm-4.5&id=bffc9e237a0c3176712bcd93fc6a184a61e0df26
> 
> Alasdair helped me understand that we do need your original fix.
> I've staged it for 4.5 (and stable@) here:
> https://git.kernel.org/cgit/linux/kernel/git/device-mapper/linux-dm.git/commit/?h=dm-4.5&id=ead3db62bf10fe143bec99e7b7ff370d7a6d23ef
> 
> Thanks again,
> Mike
> --

This broke linux-next because I'm dumb and didn't test it. I thought it was a trivial enough of a patch that I wouldn't screw it up, but I did.

I incorrectly assumed that user->version was essentially a pointer in userland, not a flat chunk of memory. Ie it was a pointer to some malloc'd region, not an inlined version[3].

I thought it was this:
struct dm_ioctl {

uint32_t *version;
...
}

It is really this:

struct dm_ioctl {

uint32_t version[3];

}

I was trying to get the values out of *version, which would have been a pointer, but instead what the code ended up doing was actually getting 8 bytes of the version (think 4,3,1) out and trying to access that version as a memory address, oops.

It turns out that the original code is correct and doesn't actually touch user memory without a copy_from_user().  Gcc is smart enough to see that version[3] is inlined, and it can emit code which simply takes the userland pointer (struct dm_ioctl __user user), and calculates on offset based on the pointer, thus no actual user dereference occurs. Had the struct looked like the first example I believe the patch would work.

I'm wondering now if we should switch the code a bit to make it less ambiguous, so someone like me doesn't come along again thinking the code dereferences userland memory and waste everyones time.

I've attached a patch based off linux-next-20150616 which reverts my broken code but adds an & to the front of user->version so it looks like the code is doing the right thing.

If I should be basing my patch off something other than linux-next let me know and I'll rewrite it, or we can just revert the old patch and ignore this one.

Thanks and very sorry for the confusion and breakage.

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#1303238

On Wed, Jan 06 2016 at  8:22pm -0500,
Scotty Bauer <sbauer@eng.utah.edu> wrote:

> 
> 
> On 01/05/2016 02:13 PM, Mike Snitzer wrote:
> > On Tue, Jan 05 2016 at  3:16pm -0500,
> > Mike Snitzer <snitzer@redhat.com> wrote:
> > 
> >> On Tue, Dec 08 2015 at  1:26pm -0500,
> >> Scotty Bauer <sbauer@eng.utah.edu> wrote:
> >>
> >>> Friendly ping, is anyone interested in this?
> >>
> >> The passed @user argument is flagged via __user so it can be
> >> deferenced directly.  It does look like directly deferencing
> >> user->version is wrong.
> >>
> >> But even if such indirect access is needed (because __user flag is only
> >> applicable to @user arg, not the contained version member) we could more
> >> easily just do something like this no?:
> >>
> >>   uint32_t __user *versionp = (uint32_t __user *)user->version;
> >>   ...
> >>   if (copy_from_user(version, versionp, sizeof(version)))
> >>      return -EFAULT;
> >>
> >> I've staged the following, thanks:
> >> https://git.kernel.org/cgit/linux/kernel/git/device-mapper/linux-dm.git/commit/?h=dm-4.5&id=bffc9e237a0c3176712bcd93fc6a184a61e0df26
> > 
> > Alasdair helped me understand that we do need your original fix.
> > I've staged it for 4.5 (and stable@) here:
> > https://git.kernel.org/cgit/linux/kernel/git/device-mapper/linux-dm.git/commit/?h=dm-4.5&id=ead3db62bf10fe143bec99e7b7ff370d7a6d23ef
> > 
> > Thanks again,
> > Mike
> > --
> 
> This broke linux-next because I'm dumb and didn't test it. I thought it was a trivial enough of a patch that I wouldn't screw it up, but I did.
> 
> I incorrectly assumed that user->version was essentially a pointer in userland, not a flat chunk of memory. Ie it was a pointer to some malloc'd region, not an inlined version[3].
> 
> I thought it was this:
> struct dm_ioctl {
> 
> uint32_t *version;
> ...
> }
> 
> It is really this:
> 
> struct dm_ioctl {
> 
> uint32_t version[3];
> 
> }
> 
> I was trying to get the values out of *version, which would have been a pointer, but instead what the code ended up doing was actually getting 8 bytes of the version (think 4,3,1) out and trying to access that version as a memory address, oops.
> 
> It turns out that the original code is correct and doesn't actually touch user memory without a copy_from_user().  Gcc is smart enough to see that version[3] is inlined, and it can emit code which simply takes the userland pointer (struct dm_ioctl __user user), and calculates on offset based on the pointer, thus no actual user dereference occurs. Had the struct looked like the first example I believe the patch would work.
> 
> I'm wondering now if we should switch the code a bit to make it less ambiguous, so someone like me doesn't come along again thinking the code dereferences userland memory and waste everyones time.
> 
> I've attached a patch based off linux-next-20150616 which reverts my broken code but adds an & to the front of user->version so it looks like the code is doing the right thing.
> 
> If I should be basing my patch off something other than linux-next let me know and I'll rewrite it, or we can just revert the old patch and ignore this one.
> 
> Thanks and very sorry for the confusion and breakage.

You're fine, no worries.

But I've just dropped the offending original commit from linux-next and
it obviously won't be included in 4.5

I'll revisit whether we need to bother with the extra & change you're
suggesting while coming to terms with why I was able to be lulled into
thinking your original patch was correct ;)
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