io_files_update_with_index_alloc() installs a fixed file via
io_fixed_fd_install(), which drops ->uring_lock when the request runs on
io-wq, then put_user()s the allocated slot back to userspace.  If that
faults, the error path calls __io_close_fixed() by slot index after
re-taking the lock.

The slot can be closed and reused between the install and that cleanup:
another fixed CLOSE plus open/accept/pipe/socket, or a FILES_UPDATE, can
put an unrelated file into the same slot.  The error path then removes
that file instead of the one this request installed, ending an unrelated
file's fixed-table reference early.  This is a correctness/DoS issue, not
a use-after-free: fixed-file requests hold an io_rsrc_node reference, so
the node and file stay alive until they are done.

Hold ->uring_lock across __io_fixed_fd_install() and put_user(), and do
the failure cleanup with io_fixed_fd_remove() under the same lock.  This
is the same class as the io_pipe_fixed() fix; faulting while holding
->uring_lock is fine, the registration paths already do it.

Fixes: a7c41b4687f5 ("io_uring: let IORING_OP_FILES_UPDATE support choosing fixed file slots")
Signed-off-by: Yuansheng Yang <0xbeac@gmail.com>
---
Resending with corrected Signed-off-by and without the CVE reference (being invalidated); no code changes.

 io_uring/rsrc.c | 24 +++++++++++++++++++-----
 1 file changed, 19 insertions(+), 5 deletions(-)

diff --git a/io_uring/rsrc.c b/io_uring/rsrc.c
index e0345c3e76..6c85eb50f0 100644
--- a/io_uring/rsrc.c
+++ b/io_uring/rsrc.c
@@ -570,13 +570,14 @@ int io_files_update_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe)
 static int io_files_update_with_index_alloc(struct io_kiocb *req,
 					    unsigned int issue_flags)
 {
+	struct io_ring_ctx *ctx = req->ctx;
 	struct io_rsrc_update *up = io_kiocb_to_cmd(req, struct io_rsrc_update);
 	__s32 __user *fds = u64_to_user_ptr(up->arg);
 	unsigned int done;
 	struct file *file;
 	int ret, fd;
 
-	if (!req->ctx->file_table.data.nr)
+	if (!ctx->file_table.data.nr)
 		return -ENXIO;
 
 	for (done = 0; done < up->nr_args; done++) {
@@ -590,15 +591,28 @@ static int io_files_update_with_index_alloc(struct io_kiocb *req,
 			ret = -EBADF;
 			break;
 		}
-		ret = io_fixed_fd_install(req, issue_flags, file,
-					  IORING_FILE_INDEX_ALLOC);
-		if (ret < 0)
+		/*
+		 * io_fixed_fd_install() drops ->uring_lock when running on
+		 * io-wq, leaving a window where the slot can be closed and
+		 * reused before put_user() faults.  Keep the lock and undo
+		 * the install under it.  Faulting under ->uring_lock is fine,
+		 * registration paths already do that.
+		 */
+		io_ring_submit_lock(ctx, issue_flags);
+		ret = __io_fixed_fd_install(ctx, file,
+					    IORING_FILE_INDEX_ALLOC);
+		if (ret < 0) {
+			io_ring_submit_unlock(ctx, issue_flags);
+			fput(file);
 			break;
+		}
 		if (put_user(ret, &fds[done])) {
-			__io_close_fixed(req->ctx, issue_flags, ret);
+			io_fixed_fd_remove(ctx, ret);
+			io_ring_submit_unlock(ctx, issue_flags);
 			ret = -EFAULT;
 			break;
 		}
+		io_ring_submit_unlock(ctx, issue_flags);
 	}
 
 	if (done)
-- 
2.43.0