sg: Fix double-free when drives detach during SG_IO

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: Calvin Owens <calvinowens@fb.com>
Acked-by: Douglas Gilbert <dgilbert@interlog.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

diff --git a/drivers/scsi/sg.c b/drivers/scsi/sg.c
index 9d7b7db75e4b96b6fbb33bf24b91c205add07d79..503ab8b46c0b4e8a73d7e826ba1aaf3be1abd6b0 100644 (file)
--- a/drivers/scsi/sg.c
+++ b/drivers/scsi/sg.c
@@ -787,8 +787,14 @@ sg_common_write(Sg_fd * sfp, Sg_request * srp,
                return k;       /* probably out of space --> ENOMEM */
        }
        if (atomic_read(&sdp->detaching)) {
-               if (srp->bio)
+               if (srp->bio) {
+                       if (srp->rq->cmd != srp->rq->__cmd)
+                               kfree(srp->rq->cmd);
+
                        blk_end_request_all(srp->rq, -EIO);
+                       srp->rq = NULL;
+               }
+
                sg_finish_rem_req(srp);
                return -ENODEV;
        }