Re: [Linux-kernel-mentees] [PATCH] drm/amdgpu: Prevent kernel-infoleak in amdgpu_info_ioctl()

[Christian KÃnig]

Am 31.07.20 um 09:10 schrieb Greg Kroah-Hartman:
> On Fri, Jul 31, 2020 at 08:57:53AM +0200, Christian KÃnig wrote:
> > Am 31.07.20 um 08:53 schrieb Greg Kroah-Hartman:
> > > On Thu, Jul 30, 2020 at 05:09:07PM -0400, Luben Tuikov wrote:
> > > > On 2020-07-29 9:49 a.m., Alex Deucher wrote:
> > > > > On Wed, Jul 29, 2020 at 4:11 AM Christian KÃnig
> > > > > <ckoenig.leichtzumerken@xxxxxxxxx> wrote:
> > > > > > Am 28.07.20 um 21:29 schrieb Peilin Ye:
> > > > > > > Compiler leaves a 4-byte hole near the end of `dev_info`, causing
> > > > > > > amdgpu_info_ioctl() to copy uninitialized kernel stack memory to userspace
> > > > > > > when `size` is greater than 356.
> > > > > > >
> > > > > > > In 2015 we tried to fix this issue by doing `= {};` on `dev_info`, which
> > > > > > > unfortunately does not initialize that 4-byte hole. Fix it by using
> > > > > > > memset() instead.
> > > > > > >
> > > > > > > Cc: stable@xxxxxxxxxxxxxxx
> > > > > > > Fixes: c193fa91b918 ("drm/amdgpu: information leak in amdgpu_info_ioctl()")
> > > > > > > Fixes: d38ceaf99ed0 ("drm/amdgpu: add core driver (v4)")
> > > > > > > Suggested-by: Dan Carpenter <dan.carpenter@xxxxxxxxxx>
> > > > > > > Signed-off-by: Peilin Ye <yepeilin.cs@xxxxxxxxx>
> > > > > > Reviewed-by: Christian KÃnig <christian.koenig@xxxxxxx>
> > > > > >
> > > > > > I can't count how many of those we have fixed over the years.
> > > > > >
> > > > > > At some point we should probably document that using "= {}" or "= { 0 }"
> > > > > > in the kernel is a really bad idea and should be avoided.
> > > > > Moreover, it seems like different compilers seem to behave relatively
> > > > > differently with these and we often get reports of warnings with these
> > > > > on clang.  When in doubt, memset.
> > > > There are quite a few of those under drivers/gpu/drm, for "amd/", "scheduler/"
> > > > drm*.c files,
> > > >
> > > > $find . \( -regex "./drm.*\.c" -or -regex "./amd/.*\.c" -or -regex "./scheduler/.*\.c" \) -exec egrep -n -- " *= *{ *(|NULL|0) *}" \{\} \+ | wc -l
> > > > 374
> > > > $_
> > > >
> > > > Out of which only 16 are of the non-ISO C variety, "= {}",
> > > >
> > > > $find . \( -regex "./drm.*\.c" -or -regex "./amd/.*\.c" -or -regex "./scheduler/.*\.c" \) -exec egrep -n -- " *= *{ *}" \{\} \+ | wc -l
> > > > 16
> > > > $_
> > > >
> > > > Perhaps the latter are the more pressing ones, since it is a C++ initializer and not a ISO C one.
> > > It only matters when we care copying the data to userspace, if it all
> > > stays in the kernel, all is fine.
> > Well only as long as you don't try to compute a CRC32, MD5 or any
> > fingerprint for a hash from the bytes from the structure.
> >
> > Then it fails horrible and you wonder why the code doesn't works as
> > expected.
> True, but the number of times I have ever needed to do that to a
> structure for a driver is, um, never...
>
> If a structure ever needs to have that happen to it, I would sure hope
> the developer was aware of padding fields, otherwise, well, someone
> needs to take away their C language certification :)

Well it is very likely that stack allocated structures have the same 
values in the padding bytes most of the time. So the problem is very 
subtle and hard to detect.

We've seen enough problems with that over the last ~10 years that I'm 
clearly in favor of adding something to checkpatch.pl to spill out a 
warning if "= { }" is used for zero initialization.

Alternatively some of the people who know gcc/clang better than I do 
could come up with a warning that you shouldn't cast a structure with 
uninitialized padding to void* or u8*.

I mean KASAN is already doing a great job detecting that kind of stuff, 
but for this you still need to hit the offending code path.

Thanks,
Christian.

> thanks,
>
> greg k-h