Patch "sg: Fix double-free when drives detach during SG_IO" has been added to the 3.18-stable tree

This is a note to let you know that I've just added the patch titled

sg: Fix double-free when drives detach during SG_IO

to the 3.18-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
sg-fix-double-free-when-drives-detach-during-sg_io.patch
and it can be found in the queue-3.18 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Calvin Owens <[email protected]>
Date: Fri, 30 Oct 2015 16:57:00 -0700
Subject: sg: Fix double-free when drives detach during SG_IO

From: Calvin Owens <[email protected]>

commit f3951a3709ff50990bf3e188c27d346792103432 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.

---
 drivers/scsi/sg.c |    8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

@@ -791,8 +791,14 @@ sg_common_write(Sg_fd * sfp, Sg_request
 		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;
 	}

Patches currently in stable-queue which might be from [email protected] are

queue-3.18/sg-fix-double-free-when-drives-detach-during-sg_io.patch