io_uring: fix not locked access to fixed buf table

commit 05b538c upstream.

We can look inside the fixed buffer table only while holding
->uring_lock, however in some cases we don't do the right async prep for
IORING_OP_{WRITE,READ}_FIXED ending up with NULL req->imu forcing making
an io-wq worker to try to resolve the fixed buffer without proper
locking.

Move req->imu setup into early req init paths, i.e. io_prep_rw(), which
is called unconditionally for rw requests and under uring_lock.

Fixes: 634d00d ("io_uring: add full-fledged dynamic buffers support")
Signed-off-by: Pavel Begunkov <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
(cherry picked from commit ea512d540a55a75058ee4b78abfd1d499b229d1d)
Signed-off-by: Jack Vogel <[email protected]>

Author: isilence

fs/io_uring.c

@@ -2932,15 +2932,24 @@ static int io_prep_rw(struct io_kiocb *req, const struct io_uring_sqe *sqe,
 		kiocb->ki_complete = io_complete_rw;
 	}
 
+	/* used for fixed read/write too - just read unconditionally */
+	req->buf_index = READ_ONCE(sqe->buf_index);
+	req->imu = NULL;
+
 	if (req->opcode == IORING_OP_READ_FIXED ||
 	    req->opcode == IORING_OP_WRITE_FIXED) {
-		req->imu = NULL;
+		struct io_ring_ctx *ctx = req->ctx;
+		u16 index;
+
+		if (unlikely(req->buf_index >= ctx->nr_user_bufs))
+			return -EFAULT;
+		index = array_index_nospec(req->buf_index, ctx->nr_user_bufs);
+		req->imu = ctx->user_bufs[index];
 		io_req_set_rsrc_node(req);
 	}
 
 	req->rw.addr = READ_ONCE(sqe->addr);
 	req->rw.len = READ_ONCE(sqe->len);
-	req->buf_index = READ_ONCE(sqe->buf_index);
 	return 0;
 }
 
@@ -3066,18 +3075,9 @@ static int __io_import_fixed(struct io_kiocb *req, int rw, struct iov_iter *iter
 
 static int io_import_fixed(struct io_kiocb *req, int rw, struct iov_iter *iter)
 {
-	struct io_ring_ctx *ctx = req->ctx;
-	struct io_mapped_ubuf *imu = req->imu;
-	u16 index, buf_index = req->buf_index;
-
-	if (likely(!imu)) {
-		if (unlikely(buf_index >= ctx->nr_user_bufs))
-			return -EFAULT;
-		index = array_index_nospec(buf_index, ctx->nr_user_bufs);
-		imu = READ_ONCE(ctx->user_bufs[index]);
-		req->imu = imu;
-	}
-	return __io_import_fixed(req, rw, iter, imu);
+	if (WARN_ON_ONCE(!req->imu))
+		return -EFAULT;
+	return __io_import_fixed(req, rw, iter, req->imu);
 }
 
 static void io_ring_submit_unlock(struct io_ring_ctx *ctx, bool needs_lock)