If the radix tree underlying the IDR happens to be full and we attempt
to remove an id which is larger than any id in the IDR, we will call
__radix_tree_delete() with an uninitialised 'slot' pointer, at which
point anything could happen.  This was easiest to hit with a single
entry at id 0 and attempting to remove a non-0 id, but it could have
happened with 64 entries and attempting to remove an id >= 64.

Roman said:

  The syzcaller test boils down to opening /dev/kvm, creating an
  eventfd, and calling a couple of KVM ioctls. None of this requires
  superuser. And the result is dereferencing an uninitialized pointer
  which is likely a crash. The specific path caught by syzbot is via
  KVM_HYPERV_EVENTD ioctl which is new in 4.17. But I guess there are
  other user-triggerable paths, so cc:stable is probably justified.

Matthew added:

  We have around 250 calls to idr_remove() in the kernel today. Many of
  them pass an ID which is embedded in the object they're removing, so
  they're safe. Picking a few likely candidates:

  drivers/firewire/core-cdev.c looks unsafe; the ID comes from an ioctl.
  drivers/gpu/drm/amd/amdgpu/amdgpu_ctx.c is similar
  drivers/atm/nicstar.c could be taken down by a handcrafted packet

Link: http://lkml.kernel.org/r/20180518175025.GD6361@bombadil.infradead.org
Fixes: 0a835c4f

 ("Reimplement IDR and IDA using the radix tree")
Reported-by:  <syzbot+35666cba7f0a337e2e79@syzkaller.appspotmail.com>
Debugged-by: Roman Kagan <rkagan@virtuozzo.com>
Signed-off-by: Matthew Wilcox <mawilcox@microsoft.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>