Re: [PATCH RFC 0/1] mount: universally disallow mounting over symlinks

Aleksa Sarai
2020-01-01T14:44:34Z

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On 2020-01-01, Al Viro <viro@zeniv.linux.org.uk> wrote:
> On Wed, Jan 01, 2020 at 12:54:46AM +0000, Al Viro wrote:
> > Note, BTW, that lookup_last() (aka walk_component()) does just
> > that - we only hit step_into() on LAST_NORM. The same goes
> > for do_last(). mountpoint_last() not doing the same is _not_
> > intentional - it's definitely a bug.
> >
> > Consider your testcase; link points to . here. So the only
> > thing you could expect from trying to follow it would be
> > the directory 'link' lives in. And you don't have it
> > when you reach the fscker via /proc/self/fd/3; what happens
> > instead is nd->path set to ./link (by nd_jump_link()) *AND*
> > step_into() called, pushing the same ./link onto stack.
> > It violates all kinds of assumptions made by fs/namei.c -
> > when pushing a symlink onto stack nd->path is expected to
> > contain the base directory for resolving it.
> >
> > I'm fairly sure that this is the cause of at least some
> > of the insanity you've caught; there always could be
> > something else, of course, but this hole needs to be
> > closed in any case.
>
> ... and with removal of now unused local variable, that's
>
> mountpoint_last(): fix the treatment of LAST_BIND
>
> step_into() should be attempted only in LAST_NORM
> case, when we have the parent directory (in nd->path).
> We get away with that for LAST_DOT and LOST_DOTDOT,
> since those can't be symlinks, making step_init() and
> equivalent of path_to_nameidata() - we do a bit of
> useless work, but that's it. For LAST_BIND (i.e.
> the case when we'd just followed a procfs-style
> symlink) we really can't go there - result might
> be a symlink and we really can't attempt following
> it.
>
> lookup_last() and do_last() do handle that properly;
> mountpoint_last() should do the same.
>
> Cc: stable@vger.kernel.org
> Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

Thanks, this fixes the issue for me (and also fixes another reproducer I
found -- mounting a symlink on top of itself then trying to umount it).

Reported-by: Aleksa Sarai <cyphar@cyphar.com>
Tested-by: Aleksa Sarai <cyphar@cyphar.com>

As for the original topic of bind-mounting symlinks -- given this is a
supported feature, would you be okay with me sending an updated
O_EMPTYPATH series?

> ---
> diff --git a/fs/namei.c b/fs/namei.c
> index d6c91d1e88cb..13f9f973722b 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -2643,7 +2643,6 @@ EXPORT_SYMBOL(user_path_at_empty);
> static int
> mountpoint_last(struct nameidata *nd)
> {
> - int error = 0;
> struct dentry *dir = nd->path.dentry;
> struct path path;
>
> @@ -2656,10 +2655,7 @@ mountpoint_last(struct nameidata *nd)
> nd->flags &= ~LOOKUP_PARENT;
>
> if (unlikely(nd->last_type != LAST_NORM)) {
> - error = handle_dots(nd, nd->last_type);
> - if (error)
> - return error;
> - path.dentry = dget(nd->path.dentry);
> + return handle_dots(nd, nd->last_type);
> } else {
> path.dentry = d_lookup(dir, &nd->last);
> if (!path.dentry) {

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

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