[PATCH 5.10 17/63] io_uring: dont assume mm is constant across submits

[Greg Kroah-Hartman]

From: Jens Axboe <axboe@xxxxxxxxx>

commit 77788775c7132a8d93c6930ab1bd84fc743c7cb7 upstream.

If we COW the identity, we assume that ->mm never changes. But this
isn't true of multiple processes end up sharing the ring. Hence treat
id->mm like like any other process compontent when it comes to the
identity mapping. This is pretty trivial, just moving the existing grab
into io_grab_identity(), and including a check for the match.

Cc: stable@xxxxxxxxxxxxxxx # 5.10
Fixes: 1e6fa5216a0e ("io_uring: COW io_identity on mismatch")
Reported-by: Christian Brauner <christian.brauner@xxxxxxxxxx>:
Tested-by: Christian Brauner <christian.brauner@xxxxxxxxxx>:
Signed-off-by: Jens Axboe <axboe@xxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 fs/io_uring.c |   14 +++++++-------
 1 file changed, 7 insertions(+), 7 deletions(-)

--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -1369,6 +1369,13 @@ static bool io_grab_identity(struct io_k
 		spin_unlock_irq(&ctx->inflight_lock);
 		req->work.flags |= IO_WQ_WORK_FILES;
 	}
+	if (!(req->work.flags & IO_WQ_WORK_MM) &&
+	    (def->work_flags & IO_WQ_WORK_MM)) {
+		if (id->mm != current->mm)
+			return false;
+		mmgrab(id->mm);
+		req->work.flags |= IO_WQ_WORK_MM;
+	}
 
 	return true;
 }
@@ -1393,13 +1400,6 @@ static void io_prep_async_work(struct io
 			req->work.flags |= IO_WQ_WORK_UNBOUND;
 	}
 
-	/* ->mm can never change on us */
-	if (!(req->work.flags & IO_WQ_WORK_MM) &&
-	    (def->work_flags & IO_WQ_WORK_MM)) {
-		mmgrab(id->mm);
-		req->work.flags |= IO_WQ_WORK_MM;
-	}
-
 	/* if we fail grabbing identity, we must COW, regrab, and retry */
 	if (io_grab_identity(req))
 		return;