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Subject: Re: KASAN: use-after-free Read in io_async_task_func
To:     syzbot <syzbot+9b260fc33297966f5a8e@syzkaller.appspotmail.com>,
        io-uring@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        linux-kernel@vger.kernel.org, syzkaller-bugs@googlegroups.com,
        viro@zeniv.linux.org.uk
References: <0000000000002753ac05ac9471f4@google.com>
From:   Jens Axboe <axboe@kernel.dk>
Message-ID: <4acc639d-ec4a-e630-3a0b-33c4f675b41d@kernel.dk>
Date:   Tue, 11 Aug 2020 07:52:55 -0600
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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+9b260fc33297966f5a8e@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