io_uring: fix not locked access to fixed buf table

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>

diff --git a/fs/io_uring.c b/fs/io_uring.c
index be05f375a77646ced8713c81d675cea0c3af5683..fd8a1ffe6a1a2025cf67d476dacdb538f64b921f 100644 (file)
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -3636,6 +3636,20 @@ static int io_prep_rw(struct io_kiocb *req, const struct io_uring_sqe *sqe)
        int ret;
 
        kiocb->ki_pos = READ_ONCE(sqe->off);
+       /* used for fixed read/write too - just read unconditionally */
+       req->buf_index = READ_ONCE(sqe->buf_index);
+
+       if (req->opcode == IORING_OP_READ_FIXED ||
+           req->opcode == IORING_OP_WRITE_FIXED) {
+               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, ctx, 0);
+       }
 
        ioprio = READ_ONCE(sqe->ioprio);
        if (ioprio) {
@@ -3648,12 +3662,9 @@ static int io_prep_rw(struct io_kiocb *req, const struct io_uring_sqe *sqe)
                kiocb->ki_ioprio = get_current_ioprio();
        }
 
-       req->imu = NULL;
        req->rw.addr = READ_ONCE(sqe->addr);
        req->rw.len = READ_ONCE(sqe->len);
        req->rw.flags = READ_ONCE(sqe->rw_flags);
-       /* used for fixed read/write too - just read unconditionally */
-       req->buf_index = READ_ONCE(sqe->buf_index);
        return 0;
 }
 
@@ -3785,20 +3796,9 @@ 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,
                           unsigned int issue_flags)
 {
-       struct io_mapped_ubuf *imu = req->imu;
-       u16 index, buf_index = req->buf_index;
-
-       if (likely(!imu)) {
-               struct io_ring_ctx *ctx = req->ctx;
-
-               if (unlikely(buf_index >= ctx->nr_user_bufs))
-                       return -EFAULT;
-               io_req_set_rsrc_node(req, ctx, issue_flags);
-               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 int io_buffer_add_list(struct io_ring_ctx *ctx,