From: Ryusuke Konishi <konishi.ryusuke-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
To: Shigeru Yoshida <syoshida-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
Cc: linux-nilfs-u79uwXL29TY76Z2rM5mHXA@public.gmane.org,
linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org,
syzkaller-bugs-/JYPxA39Uh5TLH3MbocFFw@public.gmane.org,
syzbot+f816fa82f8783f7a02bb-Pl5Pbv+GP7P466ipTTIvnc23WoclnBCfAL8bYrjMMd8@public.gmane.org
Subject: Re: [PATCH] nilfs: Avoid use-after-free caused by nilfs->ns_writer
Date: Fri, 4 Nov 2022 03:56:19 +0900 [thread overview]
Message-ID: <CAKFNMonawKi4b4G1d3CfJ6-+NLFOqVkHzNOO_5DocAJxGnkz7A@mail.gmail.com> (raw)
In-Reply-To: <20221103141759.1836312-1-syoshida-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
On Thu, Nov 3, 2022 at 11:18 PM Shigeru Yoshida wrote:
>
> syzbot reported use-after-free in nilfs_segctor_sync() [1].
>
> The use-after-free occurs with nilfs->ns_writer. The scenario for the
> issue is as follows:
>
> Task1 Task2
> ----------------------------------------------------------------------
> nilfs_construct_segment
> nilfs_segctor_sync
> init_wait
> init_waitqueue_entry
> add_wait_queue
> schedule
> nilfs_detach_log_writer
> nilfs_segctor_destroy
> kfree
> finish_wait
> _raw_spin_lock_irqsave
> __raw_spin_lock_irqsave
> do_raw_spin_lock
> debug_spin_lock_before <-- use-after-free
>
> While Task1 is sleeping, nilfs->ns_writer is freed by Task2. After
> Task1 waked up, Task1 accesses nilfs->ns_writer which is already
> freed.
>
> This patch fixes the issue by taking nilfs->ns_segctor_sem in
> nilfs_construct_segment() so that nilfs->ns_segctor_sem cannot be
> freed while nilfs_segctor_sync() is sleeping.
>
> Link: https://syzkaller.appspot.com/bug?id=79a4c002e960419ca173d55e863bd09e8112df8b [1]
> Reported-by: syzbot+f816fa82f8783f7a02bb-Pl5Pbv+GP7P466ipTTIvnc23WoclnBCfAL8bYrjMMd8@public.gmane.org
> Signed-off-by: Shigeru Yoshida <syoshida-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
> ---
> fs/nilfs2/segment.c | 14 +++++++++++---
> 1 file changed, 11 insertions(+), 3 deletions(-)
>
> diff --git a/fs/nilfs2/segment.c b/fs/nilfs2/segment.c
> index b4cebad21b48..d4f10d82664d 100644
> --- a/fs/nilfs2/segment.c
> +++ b/fs/nilfs2/segment.c
> @@ -2239,16 +2239,24 @@ static void nilfs_segctor_wakeup(struct nilfs_sc_info *sci, int err)
> int nilfs_construct_segment(struct super_block *sb)
> {
> struct the_nilfs *nilfs = sb->s_fs_info;
> - struct nilfs_sc_info *sci = nilfs->ns_writer;
> + struct nilfs_sc_info *sci;
> struct nilfs_transaction_info *ti;
> + int ret;
>
> - if (!sci)
> + down_write(&nilfs->ns_segctor_sem);
> + sci = nilfs->ns_writer;
> + if (!sci) {
> + up_write(&nilfs->ns_segctor_sem);
> return -EROFS;
> + }
>
> /* A call inside transactions causes a deadlock. */
> BUG_ON((ti = current->journal_info) && ti->ti_magic == NILFS_TI_MAGIC);
>
> - return nilfs_segctor_sync(sci);
> + ret = nilfs_segctor_sync(sci);
> + up_write(&nilfs->ns_segctor_sem);
> +
> + return ret;
> }
>
> /**
> --
> 2.37.3
>
Thank you for your help Yoshida-san.
Your analysis of the bug cause is quite correct.
However, this patch caused deadlock for tests with fsync() or mkcp
command (i.e. nilfs_ioctl_sync).
They both call nilfs_construct_segment().
The approach implemented in this patch may not avoid the deadlock regression.
I'd like to consider other approaches.
My current thoughts are either to stop detaching ns_writer on remount
which is the root cause of the UAF races, or to introduce a refcount
on struct nilfs_sc_info.
Regards,
Ryusuke Konishi
prev parent reply other threads:[~2022-11-03 18:56 UTC|newest]
Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-11-03 14:17 [PATCH] nilfs: Avoid use-after-free caused by nilfs->ns_writer Shigeru Yoshida
[not found] ` <20221103141759.1836312-1-syoshida-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
2022-11-03 18:56 ` Ryusuke Konishi [this message]
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