io_uring: don't assume mm is constant across submits

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@vger.kernel.org # 5.10
Fixes: 1e6fa521 ("io_uring: COW io_identity on mismatch")
Reported-by: Christian Brauner <christian.brauner@ubuntu.com>:
Tested-by: Christian Brauner <christian.brauner@ubuntu.com>:
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -1501,6 +1501,13 @@ static bool io_grab_identity(struct io_kiocb *req)
 		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;
 }
@@ -1525,13 +1532,6 @@ static void io_prep_async_work(struct io_kiocb *req)
 			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;