linux-kernel - Re: [PATCH] list: Fix another data-race around ep->rdllist.

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Date:   Sat, 26 Mar 2022 12:32:05 -0400
From:   Soheil Hassas Yeganeh <soheil@...gle.com>
To:     Kuniyuki Iwashima <kuniyu@...zon.co.jp>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Kuniyuki Iwashima <kuni1840@...il.com>,
        linux-fsdevel@...r.kernel.org,
        linux-kernel <linux-kernel@...r.kernel.org>,
        syzbot+19480160ef25c9ffa29d@...kaller.appspotmail.com,
        Davidlohr Bueso <dave@...olabs.net>,
        Sridhar Samudrala <sridhar.samudrala@...el.com>,
        Alexander Duyck <alexander.h.duyck@...el.com>
Subject: Re: [PATCH] list: Fix another data-race around ep->rdllist.

On Sat, Mar 26, 2022 at 2:36 AM Kuniyuki Iwashima <kuniyu@...zon.co.jp> wrote:
>
> syzbot had reported another race around ep->rdllist.  ep_poll() calls
> list_empty_careful() locklessly to check if the list is empty or not
> by testing rdllist->prev == rdllist->next.
>
> When the list does not have any nodes, the next and prev arguments of
> __list_add() is the same head pointer.  Thus the write to head->prev
> there is racy with lockless list_empty_careful() and needs WRITE_ONCE()
> to avoid store-tearing.
>
> Note that the reader side is already fixed in the patch [0].
>
> [0]: https://lore.kernel.org/mm-commits/20220326031647.DD24EC004DD@smtp.kernel.org/
>
> BUG: KCSAN: data-race in do_epoll_ctl / do_epoll_wait
>
> write to 0xffff888103e43058 of 8 bytes by task 1799 on cpu 0:
>  __list_add include/linux/list.h:72 [inline]
>  list_add_tail include/linux/list.h:102 [inline]
>  ep_insert fs/eventpoll.c:1542 [inline]
>  do_epoll_ctl+0x1331/0x1880 fs/eventpoll.c:2141
>  __do_sys_epoll_ctl fs/eventpoll.c:2192 [inline]
>  __se_sys_epoll_ctl fs/eventpoll.c:2183 [inline]
>  __x64_sys_epoll_ctl+0xc2/0xf0 fs/eventpoll.c:2183
>  do_syscall_x64 arch/x86/entry/common.c:50 [inline]
>  do_syscall_64+0x44/0xd0 arch/x86/entry/common.c:80
>  entry_SYSCALL_64_after_hwframe+0x44/0xae
>
> read to 0xffff888103e43058 of 8 bytes by task 1802 on cpu 1:
>  list_empty_careful include/linux/list.h:329 [inline]
>  ep_events_available fs/eventpoll.c:381 [inline]
>  ep_poll fs/eventpoll.c:1797 [inline]
>  do_epoll_wait+0x279/0xf40 fs/eventpoll.c:2234
>  do_epoll_pwait fs/eventpoll.c:2268 [inline]
>  __do_sys_epoll_pwait fs/eventpoll.c:2281 [inline]
>  __se_sys_epoll_pwait+0x12b/0x240 fs/eventpoll.c:2275
>  __x64_sys_epoll_pwait+0x74/0x80 fs/eventpoll.c:2275
>  do_syscall_x64 arch/x86/entry/common.c:50 [inline]
>  do_syscall_64+0x44/0xd0 arch/x86/entry/common.c:80
>  entry_SYSCALL_64_after_hwframe+0x44/0xae
>
> value changed: 0xffff888103e43050 -> 0xffff88812d515498
>
> Reported by Kernel Concurrency Sanitizer on:
> CPU: 1 PID: 1802 Comm: syz-fuzzer Not tainted 5.17.0-rc8-syzkaller-00003-g56e337f2cf13-dirty #0
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
>
> Fixes: e59d3c64cba6 ("epoll: eliminate unnecessary lock for zero timeout")
> Fixes: c5a282e9635e ("fs/epoll: reduce the scope of wq lock in epoll_wait()")
> Fixes: bf3b9f6372c4 ("epoll: Add busy poll support to epoll with socket fds.")
> Reported-by: syzbot+19480160ef25c9ffa29d@...kaller.appspotmail.com
> Signed-off-by: Kuniyuki Iwashima <kuniyu@...zon.co.jp>
> ---
> CC: Soheil Hassas Yeganeh <soheil@...gle.com>
> CC: Davidlohr Bueso <dave@...olabs.net>
> CC: Sridhar Samudrala <sridhar.samudrala@...el.com>
> CC: Alexander Duyck <alexander.h.duyck@...el.com>

Acked-by: Soheil Hassas Yeganeh <soheil@...gle.com>

Thank you for the fix!

> ---
>  include/linux/list.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/include/linux/list.h b/include/linux/list.h
> index dd6c2041d..2eaadc84a 100644
> --- a/include/linux/list.h
> +++ b/include/linux/list.h
> @@ -69,10 +69,10 @@ static inline void __list_add(struct list_head *new,
>         if (!__list_add_valid(new, prev, next))
>                 return;
>
> -       next->prev = new;
>         new->next = next;
>         new->prev = prev;
>         WRITE_ONCE(prev->next, new);
> +       WRITE_ONCE(next->prev, new);
>  }
>
>  /**
> --
> 2.30.2
>