io_uring: fix not locked access to fixed buf table

commit 05b538c1765f8d14a71ccf5f85258dcbeaf189f7 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: 634d00df5e1cf ("io_uring: add full-fledged dynamic buffers support")
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/fs/io_uring.c b/fs/io_uring.c
index be21765..a8470a9 100644 (file)
--- a/fs/io_uring.c
+++ b/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)