[PATCH] ublk: fix use-after-free in ublk_cancel_cmd()

[Ming Lei <tom.leiming@gmail.com>]

When ublk_reset_ch_dev() clears io->cmd via ublk_queue_reinit()
concurrently with ublk_cancel_cmd(), ublk_cancel_cmd() can read a
stale pointer and pass it to io_uring_cmd_done(), causing a
use-after-free.

Fix by synchronizing the two paths with ubq->cancel_lock:

- ublk_cancel_cmd(): read and clear io->cmd under cancel_lock,
  then call io_uring_cmd_done() on the saved local copy outside
  the lock.

- ublk_reset_ch_dev(): hold cancel_lock across ublk_queue_reinit()
  so that io->cmd and io->flags are cleared atomically with respect
  to ublk_cancel_cmd().

Fixes: 216c8f5ef0f2 ("ublk: replace monitor with cancelable uring_cmd")
Signed-off-by: Ming Lei <tom.leiming@gmail.com>
---
 drivers/block/ublk_drv.c | 20 +++++++++++++++-----
 1 file changed, 15 insertions(+), 5 deletions(-)

diff --git a/drivers/block/ublk_drv.c b/drivers/block/ublk_drv.c
index 57ec900f0ce0..6d13f1481de0 100644
--- a/drivers/block/ublk_drv.c
+++ b/drivers/block/ublk_drv.c
@@ -2411,8 +2411,14 @@ static void ublk_reset_ch_dev(struct ublk_device *ub)
 {
 	int i;
 
-	for (i = 0; i < ub->dev_info.nr_hw_queues; i++)
-		ublk_queue_reinit(ub, ublk_get_queue(ub, i));
+	for (i = 0; i < ub->dev_info.nr_hw_queues; i++) {
+		struct ublk_queue *ubq = ublk_get_queue(ub, i);
+
+		/* Sync with ublk_cancel_cmd() */
+		spin_lock(&ubq->cancel_lock);
+		ublk_queue_reinit(ub, ubq);
+		spin_unlock(&ubq->cancel_lock);
+	}
 
 	/* set to NULL, otherwise new tasks cannot mmap io_cmd_buf */
 	ub->mm = NULL;
@@ -2753,6 +2759,7 @@ static void ublk_cancel_cmd(struct ublk_queue *ubq, unsigned tag,
 {
 	struct ublk_io *io = &ubq->ios[tag];
 	struct ublk_device *ub = ubq->dev;
+	struct io_uring_cmd *cmd = NULL;
 	struct request *req;
 	bool done;
 
@@ -2775,12 +2782,15 @@ static void ublk_cancel_cmd(struct ublk_queue *ubq, unsigned tag,
 
 	spin_lock(&ubq->cancel_lock);
 	done = !!(io->flags & UBLK_IO_FLAG_CANCELED);
-	if (!done)
+	if (!done) {
 		io->flags |= UBLK_IO_FLAG_CANCELED;
+		cmd = io->cmd;
+		io->cmd = NULL;
+	}
 	spin_unlock(&ubq->cancel_lock);
 
-	if (!done)
-		io_uring_cmd_done(io->cmd, UBLK_IO_RES_ABORT, issue_flags);
+	if (!done && cmd)
+		io_uring_cmd_done(cmd, UBLK_IO_RES_ABORT, issue_flags);
 }
 
 /*
-- 
2.53.0