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

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Date:   Thu, 11 Apr 2019 22:10:06 +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 10:33 PM Amir Goldstein <amir73il@...il.com> wrote:
>
> 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 *);
> >

Al,

One minor nit.

If you place these function declarations by their definition order
above their wrappers, it would be nicer + patch should apply
cleanly to stable v4.4+. As it is, it applies with a minor conflict.

Just a suggestion.

Thanks,
Amir.