linux-kernel - Re: [PATCH v2] acct: fix possible deadlock in acct_pin

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Date:   Thu, 4 Apr 2019 22:33:14 +0300
From:   Amir Goldstein <amir73il@...il.com>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     Miklos Szeredi <miklos@...redi.hu>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>
Subject: Re: [PATCH v2] acct: fix possible deadlock in acct_pin_kill

On Thu, Apr 4, 2019 at 10:05 PM Al Viro <viro@...iv.linux.org.uk> wrote:
>
...
> OK, so...  My first reaction had been complete BS.  However, the
> same goes for your analysis - it's not an ordering problem at all.
> What happens is that we are replacing file->path.mnt with a clone
> and we want the write count contribution (file is opened for write)
> to be transferred.  That's it.  We do *NOT* want any kind of
> freeze protection for the duration of switchover.
>
> IOW, the solution is to switch to __mnt_{want,drop}_write() for that
> switchover; we don't want to mess with freeze protection at all.
>
> Signed-off-by: Al Viro <viro@...iv.linux.org.uk>

Cool. That works for me.
Thanks for setting this straight.

You may add:
Tested-by: Amir Goldstein <amir73il@...il.com>
Reported-by: syzbot+2a73a6ea9507b7112141@...kaller.appspotmail.com

Thanks,
Amir.

> ---
> diff --git a/fs/internal.h b/fs/internal.h
> index 6a8b71643af4..2e7362837a6e 100644
> --- a/fs/internal.h
> +++ b/fs/internal.h
> @@ -89,9 +89,7 @@ extern int sb_prepare_remount_readonly(struct super_block *);
>
>  extern void __init mnt_init(void);
>
> -extern int __mnt_want_write(struct vfsmount *);
>  extern int __mnt_want_write_file(struct file *);
> -extern void __mnt_drop_write(struct vfsmount *);
>  extern void __mnt_drop_write_file(struct file *);
>
>  /*
> diff --git a/include/linux/mount.h b/include/linux/mount.h
> index 9197ddbf35fb..bf8cc4108b8f 100644
> --- a/include/linux/mount.h
> +++ b/include/linux/mount.h
> @@ -87,6 +87,8 @@ extern bool mnt_may_suid(struct vfsmount *mnt);
>
>  struct path;
>  extern struct vfsmount *clone_private_mount(const struct path *path);
> +extern int __mnt_want_write(struct vfsmount *);
> +extern void __mnt_drop_write(struct vfsmount *);
>
>  struct file_system_type;
>  extern struct vfsmount *fc_mount(struct fs_context *fc);
> diff --git a/kernel/acct.c b/kernel/acct.c
> index addf7732fb56..81f9831a7859 100644
> --- a/kernel/acct.c
> +++ b/kernel/acct.c
> @@ -227,7 +227,7 @@ static int acct_on(struct filename *pathname)
>                 filp_close(file, NULL);
>                 return PTR_ERR(internal);
>         }
> -       err = mnt_want_write(internal);
> +       err = __mnt_want_write(internal);
>         if (err) {
>                 mntput(internal);
>                 kfree(acct);
> @@ -252,7 +252,7 @@ static int acct_on(struct filename *pathname)
>         old = xchg(&ns->bacct, &acct->pin);
>         mutex_unlock(&acct->lock);
>         pin_kill(old);
> -       mnt_drop_write(mnt);
> +       __mnt_drop_write(mnt);
>         mntput(mnt);
>         return 0;
>  }