NULL ptr dereference in i915_gem_alloc_object()

[Daniel Vetter]

On Sun, Jan 19, 2014 at 12:36:20PM +0100, Daniel Vetter wrote:
> On Sun, Jan 19, 2014 at 2:28 AM, Linus Torvalds
> <torvalds at linux-foundation.org> wrote:
> > Testing running out of file descriptors shows a NULL pointer
> > dereference in i915_gem_alloc_object() because base.filp ends up being
> > NULL. So the line
> >
> >     mapping = file_inode(obj->base.filp)->i_mapping;
> >
> > will cause an oops. The call chain is
> >
> > SyS_ioctl ->
> >   do_vfs_ioctl ->
> >     drm_ioctl ->
> >       i915_gem_create_ioctl ->
> >         i915_gem_create ->
> >           i915_gem_alloc_object
> >
> > Now, some functions do test "base.filp" for NULL (see for example
> > i915_gem_pread_ioctl()) so clearly people know that the filp may not
> > exist. But that path does not.
> 
> This is unrelated since only shmem backed objects (should) have
> base.filp set, but other gem objects (backed by stolen mem or from
> dma-buf) don't have that. A bunch of gem ioclts aren't supported with
> these special objects (like pread/pwrite) and so we check for that and
> bail out.
> 
> > Comments? Patches even?
> 
> If we fail to allocate the shmem file drm_gem_object_init should fail
> and i915_gem_alloc_object should bail out early. Apparently that
> doesn't work as it should. On a quick check it's not the lack of
> _OR_NULL in drm_gem_object_init so I'm not really sure what's going on
> here. I'll try to reproduce this here meanwhile.
> 
> Adding dri-devel since other gem drivers should be equally affected.

This took much longer than I'll ever dare to admit, but I think I've
stitched a testcase together for this and pushed it to our i915 test repo:

http://cgit.freedesktop.org/xorg/app/intel-gpu-tools/commit/?id=70082e3677711bc01330ba3483fbdce5779f915d

But I couldn't blow up my system here with that. Same on stable kernels,
so this isn't something we've fixed by accident in drm-intel-next.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch