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

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Fri, Jul 31, 2020 at 08:57:53AM +0200, Christian König 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 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
> > > $_
> > > 
> > > 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 :)

thanks,

greg k-h