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

[Christian KÃnig]

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.

Regards,
Christian.

> thanks,
>
> greg k-h