Re: [PATCH 2/3] namei: implement AT_THIS_ROOT chroot-like path resolution

[Aleksa Sarai <cyphar@cyphar.com>]

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On 2018-10-05, Jann Horn <jannh@google.com> wrote:
> > What if we took rename_lock (call it nd->r_seq) at the start of the
> > resolution, and then only tried the __d_path-style check
> >
> >   if (read_seqretry(&rename_lock, nd->r_seq) ||
> >       read_seqretry(&mount_lock, nd->m_seq))
> >           /* do the __d_path lookup. */
> >
> > That way you would only hit the slow path if there were concurrent
> > renames or mounts *and* you are doing a path resolution with
> > AT_THIS_ROOT or AT_BENEATH. I've attached a modified patch that does
> > this (and after some testing it also appears to work).
> 
> Yeah, I think that might do the job.

*phew* I was all out of other ideas. :P

> > ---
> >  fs/namei.c | 49 ++++++++++++++++++++++++++++++++++++++++++++++---
> >  1 file changed, 46 insertions(+), 3 deletions(-)
> >
> > diff --git a/fs/namei.c b/fs/namei.c
> > index 6f995e6de6b1..12c9be175cb4 100644
> > --- a/fs/namei.c
> > +++ b/fs/namei.c
> > @@ -493,7 +493,7 @@ struct nameidata {
> >         struct path     root;
> >         struct inode    *inode; /* path.dentry.d_inode */
> >         unsigned int    flags;
> > -       unsigned        seq, m_seq;
> > +       unsigned        seq, m_seq, r_seq;
> >         int             last_type;
> >         unsigned        depth;
> >         int             total_link_count;
> > @@ -1375,6 +1375,27 @@ static int follow_dotdot_rcu(struct nameidata *nd)
> >                                 return -EXDEV;
> >                         break;
> >                 }
> > +               if (unlikely((nd->flags & (LOOKUP_BENEATH | LOOKUP_CHROOT)) &&
> > +                            (read_seqretry(&rename_lock, nd->r_seq) ||
> > +                             read_seqretry(&mount_lock, nd->m_seq)))) {
> > +                       char *pathbuf, *pathptr;
> > +
> > +                       nd->r_seq = read_seqbegin(&rename_lock);
> > +                       /* Cannot take m_seq here. */
> > +
> > +                       pathbuf = kmalloc(PATH_MAX, GFP_ATOMIC);
> > +                       if (!pathbuf)
> > +                               return -ECHILD;
> > +                       pathptr = __d_path(&nd->path, &nd->root, pathbuf, PATH_MAX);
> > +                       kfree(pathbuf);
> 
> You're doing this check before actually looking up the parent, right?
> So as long as I don't trigger the "path_equal(&nd->path, &nd->root)"
> check that you do for O_BENEATH, escaping up by one level is possible,
> right? You should probably move this check so that it happens after
> following "..".

Yup, you're right. I'll do that.

> (Also: I assume that you're going to get rid of that memory allocation
> in a future version.)

Sure. Would you prefer adding some scratch space in nameidata, or that I
change __d_path so it accepts NULL as the buffer (and thus it doesn't
actually do any string operations)?

> >                 if (nd->path.dentry != nd->path.mnt->mnt_root) {
> >                         int ret = path_parent_directory(&nd->path);
> >                         if (ret)
> > @@ -2269,6 +2311,9 @@ static const char *path_init(struct nameidata *nd, unsigned flags)
> >         nd->last_type = LAST_ROOT; /* if there are only slashes... */
> >         nd->flags = flags | LOOKUP_JUMPED | LOOKUP_PARENT;
> >         nd->depth = 0;
> > +       nd->m_seq = read_seqbegin(&mount_lock);
> > +       nd->r_seq = read_seqbegin(&rename_lock);
> 
> This means that now, attempting to perform a lookup while something is
> holding the rename_lock will spin on the lock. I don't know whether
> that's a problem in practice though. Does anyone on this thread know
> whether this is problematic?

I could make it so that we only take &rename_lock
  if (nd->flags & (FOLLOW_BENEATH | FOLLOW_CHROOT)),
since it's not used outside of that path.

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

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