Re: KASAN: use-after-free Read in io_async_task_func

[Jens Axboe]

On 8/11/20 12:47 AM, syzbot wrote:
> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    fc80c51f Merge tag 'kbuild-v5.9' of git://git.kernel.org/p..
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=17601ab2900000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=d48472fcc2f68903
> dashboard link: https://syzkaller.appspot.com/bug?extid=9b260fc33297966f5a8e
> compiler:       clang version 10.0.0 (https://github.com/llvm/llvm-project/ 
> c2443155a0fb245c8f17f2c1c72b6ea391e86e81)
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=174272b2900000
> 
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+9b260fc33297966f5...@syzkaller.appspotmail.com

I think the below should fix it, for this use case and potentially others.
If the ring is closed and the task_work bound req is holding the last
reference, we need to hold a ctx reference around it.

diff --git a/fs/io_uring.c b/fs/io_uring.c
index 5488698189da..cc4bb16ff570 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -1821,8 +1821,10 @@ static void __io_req_task_submit(struct io_kiocb *req)
 static void io_req_task_submit(struct callback_head *cb)
 {
        struct io_kiocb *req = container_of(cb, struct io_kiocb, task_work);
+       struct io_ring_ctx *ctx = req->ctx;
 
        __io_req_task_submit(req);
+       percpu_ref_put(&ctx->refs);
 }
 
 static void io_req_task_queue(struct io_kiocb *req)
@@ -1830,6 +1832,7 @@ static void io_req_task_queue(struct io_kiocb *req)
        int ret;
 
        init_task_work(&req->task_work, io_req_task_submit);
+       percpu_ref_get(&req->ctx->refs);
 
        ret = io_req_task_work_add(req, &req->task_work);
        if (unlikely(ret)) {
@@ -2318,6 +2321,8 @@ static void io_rw_resubmit(struct callback_head *cb)
                refcount_inc(&req->refs);
                io_queue_async_work(req);
        }
+
+       percpu_ref_put(&ctx->refs);
 }
 #endif
 
@@ -2330,6 +2335,8 @@ static bool io_rw_reissue(struct io_kiocb *req, long res)
                return false;
 
        init_task_work(&req->task_work, io_rw_resubmit);
+       percpu_ref_get(&req->ctx->refs);
+
        ret = io_req_task_work_add(req, &req->task_work);
        if (!ret)
                return true;
@@ -3033,6 +3040,8 @@ static int io_async_buf_func(struct wait_queue_entry 
*wait, unsigned mode,
        list_del_init(&wait->entry);
 
        init_task_work(&req->task_work, io_req_task_submit);
+       percpu_ref_get(&req->ctx->refs);
+
        /* submit ref gets dropped, acquire a new one */
        refcount_inc(&req->refs);
        ret = io_req_task_work_add(req, &req->task_work);
@@ -4565,6 +4574,8 @@ static int __io_async_wake(struct io_kiocb *req, struct 
io_poll_iocb *poll,
 
        req->result = mask;
        init_task_work(&req->task_work, func);
+       percpu_ref_get(&req->ctx->refs);
+
        /*
         * If this fails, then the task is exiting. When a task exits, the
         * work gets canceled, so just cancel this request as well instead
@@ -4652,11 +4663,13 @@ static void io_poll_task_handler(struct io_kiocb *req, 
struct io_kiocb **nxt)
 static void io_poll_task_func(struct callback_head *cb)
 {
        struct io_kiocb *req = container_of(cb, struct io_kiocb, task_work);
+       struct io_ring_ctx *ctx = req->ctx;
        struct io_kiocb *nxt = NULL;
 
        io_poll_task_handler(req, &nxt);
        if (nxt)
                __io_req_task_submit(nxt);
+       percpu_ref_put(&ctx->refs);
 }
 
 static int io_poll_double_wake(struct wait_queue_entry *wait, unsigned mode,
@@ -4752,6 +4765,7 @@ static void io_async_task_func(struct callback_head *cb)
 
        if (io_poll_rewait(req, &apoll->poll)) {
                spin_unlock_irq(&ctx->completion_lock);
+               percpu_ref_put(&ctx->refs);
                return;
        }
 

-- 
Jens Axboe