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

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Date:   Thu, 2 Jan 2020 01:44:07 +1100
From:   Aleksa Sarai <cyphar@...har.com>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     David Howells <dhowells@...hat.com>,
        Eric Biederman <ebiederm@...ssion.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        stable@...r.kernel.org,
        Christian Brauner <christian.brauner@...ntu.com>,
        Serge Hallyn <serge@...lyn.com>, dev@...ncontainers.org,
        containers@...ts.linux-foundation.org, linux-api@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC 0/1] mount: universally disallow mounting over
 symlinks

On 2020-01-01, Al Viro <viro@...iv.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@...r.kernel.org
> Signed-off-by: Al Viro <viro@...iv.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@...har.com>
Tested-by: Aleksa Sarai <cyphar@...har.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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