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

[Arnd Bergmann <arnd@arndb.de>]

On Thu, Jul 30, 2020 at 11:09 PM Luben Tuikov <luben.tuikov@amd.com> wrote:
> On 2020-07-29 9:49 a.m., Alex Deucher wrote:
> > On Wed, Jul 29, 2020 at 4:11 AM Christian König
> > <ckoenig.leichtzumerken@gmail.com> 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@vger.kernel.org
> >>> Fixes: c193fa91b918 ("drm/amdgpu: information leak in amdgpu_info_ioctl()")
> >>> Fixes: d38ceaf99ed0 ("drm/amdgpu: add core driver (v4)")
> >>> Suggested-by: Dan Carpenter <dan.carpenter@oracle.com>
> >>> Signed-off-by: Peilin Ye <yepeilin.cs@gmail.com>
> >>
> >> Reviewed-by: Christian König <christian.koenig@amd.com>
> >>
> >> 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
> $_

That is an unrelated issue, those were introduced to deal with older compilers
that do not accept '{0}' as an initializer for an aggregate whose
first member is
another aggregate. Generally speaking, '= { }' is better to use in the
kernel than
'= { 0 }' because all supported compilers interpret that the same way for all
structures.

     Arnd