• Calvin Owens's avatar
    sg: Fix double-free when drives detach during SG_IO · 79cfd634
    Calvin Owens authored
    commit f3951a37 upstream.
    
    In sg_common_write(), we free the block request and return -ENODEV if
    the device is detached in the middle of the SG_IO ioctl().
    
    Unfortunately, sg_finish_rem_req() also tries to free srp->rq, so we
    end up freeing rq->cmd in the already free rq object, and then free
    the object itself out from under the current user.
    
    This ends up corrupting random memory via the list_head on the rq
    object. The most common crash trace I saw is this:
    
      ------------[ cut here ]------------
      kernel BUG at block/blk-core.c:1420!
      Call Trace:
      [<ffffffff81281eab>] blk_put_request+0x5b/0x80
      [<ffffffffa0069e5b>] sg_finish_rem_req+0x6b/0x120 [sg]
      [<ffffffffa006bcb9>] sg_common_write.isra.14+0x459/0x5a0 [sg]
      [<ffffffff8125b328>] ? selinux_file_alloc_security+0x48/0x70
      [<ffffffffa006bf95>] sg_new_write.isra.17+0x195/0x2d0 [sg]
      [<ffffffffa006cef4>] sg_ioctl+0x644/0xdb0 [sg]
      [<ffffffff81170f80>] do_vfs_ioctl+0x90/0x520
      [<ffffffff81258967>] ? file_has_perm+0x97/0xb0
      [<ffffffff811714a1>] SyS_ioctl+0x91/0xb0
      [<ffffffff81602afb>] tracesys+0xdd/0xe2
        RIP [<ffffffff81281e04>] __blk_put_request+0x154/0x1a0
    
    The solution is straightforward: just set srp->rq to NULL in the
    failure branch so that sg_finish_rem_req() doesn't attempt to re-free
    it.
    
    Additionally, since sg_rq_end_io() will never be called on the object
    when this happens, we need to free memory backing ->cmd if it isn't
    embedded in the object itself.
    
    KASAN was extremely helpful in finding the root cause of this bug.
    Signed-off-by: default avatarCalvin Owens <calvinowens@fb.com>
    Acked-by: default avatarDouglas Gilbert <dgilbert@interlog.com>
    Signed-off-by: default avatarMartin K. Petersen <martin.petersen@oracle.com>
    [bwh: Backported to 3.16:
     - sg_finish_rem_req() would not free srp->rq->cmd so don't do it here either
     - Adjust context]
    Signed-off-by: default avatarBen Hutchings <ben@decadent.org.uk>
    79cfd634
sg.c 69.9 KB