[PATCH 5.10/5.15] io_uring: avoid null-ptr-deref in io_arm_poll_handler

[Fedor Pchelkin]

No upstream commit exists for this commit.

The issue was introduced with backporting upstream commit c16bda37594f
("io_uring/poll: allow some retries for poll triggering spuriously").

Memory allocation can possibly fail causing invalid pointer be
dereferenced just before comparing it to NULL value.

Move the pointer check in proper place (upstream has the similar location
of the check). In case the request has REQ_F_POLLED flag up, apoll can't
be NULL so no need to check there.

Found by Linux Verification Center (linuxtesting.org) with Syzkaller.

Signed-off-by: Fedor Pchelkin <pchelkin@xxxxxxxxx>
---
 io_uring/io_uring.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c
index 445afda927f4..fd799567fc23 100644
--- a/io_uring/io_uring.c
+++ b/io_uring/io_uring.c
@@ -5792,10 +5792,10 @@ static int io_arm_poll_handler(struct io_kiocb *req)
 		}
 	} else {
 		apoll = kmalloc(sizeof(*apoll), GFP_ATOMIC);
+		if (unlikely(!apoll))
+			return IO_APOLL_ABORTED;
 		apoll->poll.retries = APOLL_MAX_RETRY;
 	}
-	if (unlikely(!apoll))
-		return IO_APOLL_ABORTED;
 	apoll->double_poll = NULL;
 	req->apoll = apoll;
 	req->flags |= REQ_F_POLLED;
-- 
2.34.1