[PATCH] drm: drm_bufs: Error out if 'dev->agp' is a null pointer

[Zheyu Ma]

On Tue, Mar 22, 2022 at 10:27 PM Daniel Vetter <daniel at ffwll.ch> wrote:
>
> On Mon, Mar 21, 2022 at 09:02:47PM +0800, Zheyu Ma wrote:
> > On Thu, Mar 17, 2022 at 6:49 PM Daniel Vetter <daniel at ffwll.ch> wrote:
> > >
> > > On Fri, Mar 11, 2022 at 07:23:02AM +0000, Zheyu Ma wrote:
> > > > The user program can control the 'drm_buf_desc::flags' via ioctl system
> > > > call and enter the function drm_legacy_addbufs_agp(). If the driver
> > > > doesn't initialize the agp resources, the driver will cause a null
> > > > pointer dereference.
> > > >
> > > > The following log reveals it:
> > > >     general protection fault, probably for non-canonical address
> > > >     0xdffffc000000000f: 0000 [#1] PREEMPT SMP KASAN PTI
> > > >     KASAN: null-ptr-deref in range [0x0000000000000078-0x000000000000007f]
> > > >     Call Trace:
> > > >      <TASK>
> > > >      drm_ioctl_kernel+0x342/0x450 drivers/gpu/drm/drm_ioctl.c:785
> > > >      drm_ioctl+0x592/0x940 drivers/gpu/drm/drm_ioctl.c:885
> > > >      vfs_ioctl fs/ioctl.c:51 [inline]
> > > >      __do_sys_ioctl fs/ioctl.c:874 [inline]
> > > >      __se_sys_ioctl+0xaa/0xf0 fs/ioctl.c:860
> > > >      do_syscall_x64 arch/x86/entry/common.c:50 [inline]
> > > >      do_syscall_64+0x43/0x90 arch/x86/entry/common.c:80
> > > >      entry_SYSCALL_64_after_hwframe+0x44/0xae
> > > >
> > > > Fix this bug by adding a check.
> > > >
> > > > Signed-off-by: Zheyu Ma <zheyuma97 at gmail.com>
> > >
> > > You can only hit this if you enabled a DRIVER_LEGACY drm driver, which
> > > opens you up to tons of other CVEs and issues. What's your .config?
> >
> > Yes, I enable the DRM_LEGACY option in the config.
> > I think you mean this is not a normal configuration file? Do you have
> > a recommended configuration option for when I want to test the GPU
> > driver?
>
> Yeah DRM_LEGACY gives you all kinds of horrible and known-broken
> interfaces. Don't enable that :-)
>
> You have a bunch of other drivers enable which require DRM_LEGACY, so
> those will disable too. Anything else I think would be an upstream bug and
> we'd need to adjust Kconfig (or fix the code).

Thanks for your explanation! I will pay attention next time.

Regards,
Zheyu Ma