[PATCH 5/5] io_uring: fix use after free

Pavel Begunkov
2020-06-29T20:43:50Z

After __io_free_req() put a ctx ref, it should assumed that the ctx may
already be gone. However, it can be accessed to put back fallback req.
Free req first and then put a req.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
---
fs/io_uring.c | 7 +++++--
1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/fs/io_uring.c b/fs/io_uring.c
index 40996f25822e..013f31e35e78 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -1526,12 +1526,15 @@ static void io_dismantle_req(struct io_kiocb *req)

static void __io_free_req(struct io_kiocb *req)
{
+ struct io_ring_ctx *ctx;
+
io_dismantle_req(req);
- percpu_ref_put(&req->ctx->refs);
+ ctx = req->ctx;
if (likely(!io_is_fallback_req(req)))
kmem_cache_free(req_cachep, req);
else
- clear_bit_unlock(0, (unsigned long *) &req->ctx->fallback_req);
+ clear_bit_unlock(0, (unsigned long *) &ctx->fallback_req);
+ percpu_ref_put(&ctx->refs);
}

static bool io_link_cancel_timeout(struct io_kiocb *req)
--
2.24.0