[PATCH] io_uring/nop: fix file reference leak with IOSQE_FIXED_FILE

[Vasileios Almpanis <vasilisalmpanis@gmail.com>]

NOP file-acquisition support choses between a fixed (registered) file and
a normal fget()'d file based on its own IORING_NOP_FIXED_FILE flag in
sqe->nop_flags. However, a request's REQ_F_FIXED_FILE is set
independently from the generic IOSQE_FIXED_FILE sqe flag during request
init, before the issue handler runs.

If a NOP is submitted with IOSQE_FIXED_FILE set (so REQ_F_FIXED_FILE is
set) but without IORING_NOP_FIXED_FILE, io_nop() takes the normal path
and grabs a real reference via io_file_get_normal(). On completion,
io_put_file() only drops the reference when REQ_F_FIXED_FILE is clear,
so the fget()'d file is never released and leaks:

  BUG: memory leak
  unreferenced object 0xffff88800f42c240 (size 176):
    kmem_cache_alloc_noprof+0x358/0x440
    alloc_empty_file+0x57/0x180
    path_openat+0x44/0x1e50
    do_file_open+0x121/0x200
    do_sys_openat2+0xa7/0x150
    __x64_sys_openat+0x82/0xf0

Decide between fixed and normal file acquisition from REQ_F_FIXED_FILE,
the same way io_assign_file() does for every other opcode, and fold
IORING_NOP_FIXED_FILE into REQ_F_FIXED_FILE at prep time.

Cc: stable@vger.kernel.org
Fixes: a85f31052bce ("io_uring/nop: add support for testing registered files and buffers")
Reported-by: syzbot+2cd473471e77bda12b0e@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?id=879092631b98f73a28ea405adacfa5bb34a14a25
Signed-off-by: Vasileios Almpanis <vasilisalmpanis@gmail.com>
---
 io_uring/nop.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/io_uring/nop.c b/io_uring/nop.c
index 91ae0b2e7e55..60ab19604b36 100644
--- a/io_uring/nop.c
+++ b/io_uring/nop.c
@@ -40,6 +40,8 @@ int io_nop_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe)
 		nop->fd = READ_ONCE(sqe->fd);
 	else
 		nop->fd = -1;
+	if (nop->flags & IORING_NOP_FIXED_FILE)
+		req->flags |= REQ_F_FIXED_FILE;
 	if (nop->flags & IORING_NOP_FIXED_BUFFER)
 		req->buf_index = READ_ONCE(sqe->buf_index);
 	if (nop->flags & IORING_NOP_CQE32) {
@@ -59,12 +61,10 @@ int io_nop(struct io_kiocb *req, unsigned int issue_flags)
 	int ret = nop->result;
 
 	if (nop->flags & IORING_NOP_FILE) {
-		if (nop->flags & IORING_NOP_FIXED_FILE) {
+		if (req->flags & REQ_F_FIXED_FILE)
 			req->file = io_file_get_fixed(req, nop->fd, issue_flags);
-			req->flags |= REQ_F_FIXED_FILE;
-		} else {
+		else
 			req->file = io_file_get_normal(req, nop->fd);
-		}
 		if (!req->file) {
 			ret = -EBADF;
 			goto done;
-- 
2.47.3