dm ioctl: don't let data_start run past the buffer

struct dm_ioctl has seven bytes of trailing padding, so the smallest
buffer userspace is allowed to ask for, offsetof(struct dm_ioctl, data),
is seven bytes shorter than sizeof(struct dm_ioctl).

get_result_buffer() sets data_start to sizeof(struct dm_ioctl) without
looking at how big the buffer actually is. Ask for the minimum and
data_start ends up seven bytes past the end. Callers add it to
data_size, and ctl_ioctl() copies that much back to userspace:

  BUG: KASAN: slab-out-of-bounds in _copy_to_user+0xad/0xd0
  Read of size 312 at addr ffff8880169e4c00 by task repro-dm/140
   ctl_ioctl+0x5e3/0xcb0
   dm_ctl_ioctl+0x25/0x40
  allocated 305-byte region [ffff8880169e4c00, ffff8880169e4d31)

A DM_LIST_DEVICES with data_size set to 305 is enough to hit it, and the
seven bytes that leak are whatever sat next to the allocation.

Clamp data_start to the buffer size.

Note that this is not a security bug because the out-of-bounds read can
only be triggered by root.

Reported-by: syzbot+48d935cf48a4a76be346@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=48d935cf48a4a76be346
Fixes: 76c072b48e39 ("dm ioctl: move compat code")
Cc: stable@vger.kernel.org
Signed-off-by: Palla Raghunath <raghunathpalla.0209@gmail.com>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>

0 files changed, 0 insertions, 0 deletions