[PATCH] drm: Restore the NULL check for drm_gem_object_put()

[Emil Velikov]

On Wed, 20 May 2020 at 15:24, Chris Wilson <chris at chris-wilson.co.uk> wrote:
>
> Some users want to pass NULL to drm_gem_object_put(), but those using
> __drm_gem_object_put() did not. Compromise, have both and let the
> compiler sort it out.
>
> drm_gem_fb_destroy() calls drm_gem_object_put() with NULL obj causing:
> [   11.584209] BUG: kernel NULL pointer dereference, address: 0000000000000000
> [   11.584213] #PF: supervisor write access in kernel mode
> [   11.584215] #PF: error_code(0x0002) - not-present page
> [   11.584216] PGD 0 P4D 0
> [   11.584220] Oops: 0002 [#1] SMP NOPTI
> [   11.584223] CPU: 7 PID: 1571 Comm: gnome-shell Tainted: G            E     5.7.0-rc1-1-default+ #27
> [   11.584225] Hardware name: Micro-Star International Co., Ltd. MS-7A31/X370 XPOWER GAMING TITANIUM (MS-7A31), BIOS 1.MR 12/03/2019
> [   11.584237] RIP: 0010:drm_gem_fb_destroy+0x28/0x70 [drm_kms_helper]
> <snip>
> [   11.584256] Call Trace:
> [   11.584279]  drm_mode_rmfb+0x189/0x1c0 [drm]
> [   11.584299]  ? drm_mode_rmfb+0x1c0/0x1c0 [drm]
> [   11.584314]  drm_ioctl_kernel+0xaa/0xf0 [drm]
> [   11.584329]  drm_ioctl+0x1ff/0x3b0 [drm]
> [   11.584347]  ? drm_mode_rmfb+0x1c0/0x1c0 [drm]
> [   11.584421]  amdgpu_drm_ioctl+0x49/0x80 [amdgpu]
> [   11.584427]  ksys_ioctl+0x87/0xc0
> [   11.584430]  __x64_sys_ioctl+0x16/0x20
> [   11.584434]  do_syscall_64+0x5f/0x240
> [   11.584438]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
> [   11.584440] RIP: 0033:0x7f0ef80f7227
>
> Reported-by: Nirmoy Das <nirmoy.das at amd.com>
> Fixes: b5d250744ccc ("drm/gem: fold drm_gem_object_put_unlocked and __drm_gem_object_put()")
> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> Cc: Nirmoy Das <nirmoy.das at amd.com>
> Cc: Emil Velikov <emil.velikov at collabora.com>
> Cc: Christian König <christian.koenig at amd.com>.
> Acked-by: Nirmoy Das <nirmoy.das at amd.com>

Reviewed-by: Emil Velikov <emil.velikov at collabora.com>

I'm half way through checking the callers and I've noticed a handful of bugs.
Will send the series in due time, although your patch is a perfect
intermediate solution.

Thank you
Emil