linux-kernel - Re: BUG: unable to handle kernel paging request in io_wq_cancel

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Date:   Wed, 30 Oct 2019 08:41:46 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     syzbot <syzbot+221cc24572a2fed23b6b@...kaller.appspotmail.com>,
        akpm@...ux-foundation.org, dan.j.williams@...el.com,
        dhowells@...hat.com, gregkh@...uxfoundation.org,
        hannes@...xchg.org, joel@...lfernandes.org,
        linux-block@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, mchehab+samsung@...nel.org,
        mingo@...hat.com, patrick.bellasi@....com, rgb@...hat.com,
        rostedt@...dmis.org, syzkaller-bugs@...glegroups.com,
        viro@...iv.linux.org.uk, yamada.masahiro@...ionext.com
Subject: Re: BUG: unable to handle kernel paging request in io_wq_cancel_all

On 10/30/19 1:44 AM, syzbot wrote:
> syzbot has bisected this bug to:
> 
> commit ef0524d3654628ead811f328af0a4a2953a8310f
> Author: Jens Axboe <axboe@...nel.dk>
> Date:   Thu Oct 24 13:25:42 2019 +0000
> 
>       io_uring: replace workqueue usage with io-wq
> 
> bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=16acf5d0e00000
> start commit:   c57cf383 Add linux-next specific files for 20191029
> git tree:       linux-next
> final crash:    https://syzkaller.appspot.com/x/report.txt?x=15acf5d0e00000
> console output: https://syzkaller.appspot.com/x/log.txt?x=11acf5d0e00000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=cb86688f30db053d
> dashboard link: https://syzkaller.appspot.com/bug?extid=221cc24572a2fed23b6b
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=168671d4e00000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=140f4898e00000
> 
> Reported-by: syzbot+221cc24572a2fed23b6b@...kaller.appspotmail.com
> Fixes: ef0524d36546 ("io_uring: replace workqueue usage with io-wq")

Good catch, it's a case of NULL vs ERR_PTR() confusion. I'll fold in
the below fix.


diff --git a/fs/io_uring.c b/fs/io_uring.c
index af1937d66aee..76d653085987 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -3534,8 +3534,9 @@ static int io_sq_offload_start(struct io_ring_ctx *ctx,
 	/* Do QD, or 4 * CPUS, whatever is smallest */
 	concurrency = min(ctx->sq_entries, 4 * num_online_cpus());
 	ctx->io_wq = io_wq_create(concurrency, ctx->sqo_mm);
-	if (!ctx->io_wq) {
-		ret = -ENOMEM;
+	if (IS_ERR(ctx->io_wq)) {
+		ret = PTR_ERR(ctx->io_wq);
+		ctx->io_wq = NULL;
 		goto err;
 	}
 

-- 
Jens Axboe