Re: [PATCH 1/1] drm: check for NULL pointer in drm_gem_object_put

From: Chris Wilson <chris@chris-wilson.co.uk>
To: Christian König <christian.koenig@amd.com>,
	Nirmoy Das <nirmoy.aiemd@gmail.com>,
	dri-devel@lists.freedesktop.org
Cc: Nirmoy Das <nirmoy.das@amd.com>, Emil Velikov <emil.l.velikov@gmail.com>
Subject: Re: [PATCH 1/1] drm: check for NULL pointer in drm_gem_object_put
Date: Wed, 20 May 2020 13:49:17 +0100	[thread overview]
Message-ID: <158997895740.8153.16875812670855148581@build.alporthouse.com> (raw)
In-Reply-To: <96a01388-7d91-56b5-4cd4-16052aba7a60@amd.com>

Quoting Christian König (2020-05-20 13:19:42)
> Am 20.05.20 um 14:14 schrieb Nirmoy Das:
> > 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
> >
> > Signed-off-by: Nirmoy Das <nirmoy.das@amd.com>
> 
> Fixes: .... missing here. Apart from that Reviewed-by: Christian König 
> <christian.koenig@amd.com>.
> 
> Please resend with the tag added, then I'm going to push this to 
> drm-misc-next asap.
> 
> Christian.
> 
> > ---
> >   include/drm/drm_gem.h | 3 +++
> >   1 file changed, 3 insertions(+)
> >
> > diff --git a/include/drm/drm_gem.h b/include/drm/drm_gem.h
> > index 52173abdf500..a13510346a9b 100644
> > --- a/include/drm/drm_gem.h
> > +++ b/include/drm/drm_gem.h
> > @@ -372,6 +372,9 @@ static inline void drm_gem_object_get(struct drm_gem_object *obj)
> >   static inline void
> >   drm_gem_object_put(struct drm_gem_object *obj)
> >   {
> > +     if (!obj)
> > +             return;
> > +

This adds several thousand NULL checks where there were previously none.
I doubt the compiler eliminates them all.

I'd suggest reverting
b5d250744ccc ("drm/gem: fold drm_gem_object_put_unlocked and __drm_gem_object_put()")
-Chris
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