[PATCH 7.3.y 9/9] io_uring/rsrc: hold uring_lock across index-alloc put_user

[Jens Axboe <axboe@kernel.dk>]

From: Yuansheng Yang <0xbeac@gmail.com>

Commit d7c7037253f6935af243570635d71b4920b5a4ea upstream.

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 in a3d7ee11289f. 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>
Link: https://patch.msgid.link/20261104154655.81876-1-0xbeac@gmail.com
[axboe: various touch-ups]
Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 io_uring/rsrc.c | 21 ++++++++++++++++-----
 1 file changed, 16 insertions(+), 5 deletions(-)

diff --git a/io_uring/rsrc.c b/io_uring/rsrc.c
index d69baa47c7e7..c1ab760c4591 100644
--- a/io_uring/rsrc.c
+++ b/io_uring/rsrc.c
@@ -521,11 +521,12 @@ static int io_files_update_with_index_alloc(struct io_kiocb *req,
 {
 	struct io_rsrc_update *up = io_kiocb_to_cmd(req, struct io_rsrc_update);
 	__s32 __user *fds = u64_to_user_ptr(up->arg);
+	struct io_ring_ctx *ctx = req->ctx;
 	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++) {
@@ -539,15 +540,25 @@ 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)
+		/*
+		 * Hold the lock across the put_user(), if it's dropped the
+		 * slot can be closed and reused before the error path runs.
+		 */
+		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)
-- 
2.56.0