Re: [PATCH v9 01/10] namei: obey trailing magic-link DAC permissions

From: Al Viro <viro@zeniv.linux.org.uk>
To: Aleksa Sarai <cyphar@cyphar.com>
Cc: Jeff Layton <jlayton@kernel.org>,
	"J. Bruce Fields" <bfields@fieldses.org>,
	Arnd Bergmann <arnd@arndb.de>,
	David Howells <dhowells@redhat.com>,
	Shuah Khan <shuah@kernel.org>,
	Shuah Khan <skhan@linuxfoundation.org>,
	Andy Lutomirski <luto@kernel.org>,
	Christian Brauner <christian@brauner.io>,
	Eric Biederman <ebiederm@xmission.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Alexei Starovoitov <ast@kernel.org>,
	Kees Cook <keescook@chromium.org>, Jann Horn <jannh@google.com>,
	Tycho Andersen <tycho@tycho.ws>,
	David Drysdale <drysdale@google.com>,
	Chanho Min <chanho.min@lge.com>, Oleg Nesterov <oleg@redhat.com>,
	Aleksa Sarai <asarai@suse.de>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	containers@lists.linux-foundation.org,
	linux-alpha@vger.kernel.org
Subject: Re: [PATCH v9 01/10] namei: obey trailing magic-link DAC permissions
Date: Fri, 12 Jul 2019 04:14:54 +0000	[thread overview]
Message-ID: <20190712041454.GG17978@ZenIV.linux.org.uk> (raw)
In-Reply-To: <20190706145737.5299-2-cyphar@cyphar.com>

On Sun, Jul 07, 2019 at 12:57:28AM +1000, Aleksa Sarai wrote:

> @@ -514,7 +516,14 @@ static void set_nameidata(struct nameidata *p, int dfd, struct filename *name)
>  	p->stack = p->internal;
>  	p->dfd = dfd;
>  	p->name = name;
> -	p->total_link_count = old ? old->total_link_count : 0;
> +	p->total_link_count = 0;
> +	p->acc_mode = 0;
> +	p->opath_mask = FMODE_PATH_READ | FMODE_PATH_WRITE;
> +	if (old) {
> +		p->total_link_count = old->total_link_count;
> +		p->acc_mode = old->acc_mode;
> +		p->opath_mask = old->opath_mask;
> +	}

Huh?  Could somebody explain why traversals of NFS4 referrals should inherit
->acc_mode and ->opath_mask?

>  static __always_inline
> -const char *get_link(struct nameidata *nd)
> +const char *get_link(struct nameidata *nd, bool trailing)
>  {
>  	struct saved *last = nd->stack + nd->depth - 1;
>  	struct dentry *dentry = last->link.dentry;
> @@ -1081,6 +1134,44 @@ const char *get_link(struct nameidata *nd)
>  		} else {
>  			res = get(dentry, inode, &last->done);
>  		}
> +		/* If we just jumped it was because of a magic-link. */
> +		if (unlikely(nd->flags & LOOKUP_JUMPED)) {

That's not quite guaranteed (it is possible to bind a symlink on top
of a regular file, and you will get LOOKUP_JUMPED on the entry into
trailing_symlink() when looking the result up).  Moreover, why bother
with LOOKUP_JUMPED here?  See that
	nd->last_type = LAST_BIND;
several lines prior?  That's precisely to be able to recognize those
suckers.

And _that_ would've avoided another piece of ugliness - your LOOKUP_JUMPED
kludge forces you to handle that cra^Wsclero^Wvaluable security hardening
in get_link(), instead of trailing_symlink() where you apparently want
it to be.  Simply because nd_jump_root() done later in get_link() will set
LOOKUP_JUMPED for absolute symlinks, confusing your test.

Moreover, I'm not sure that trailing_symlink() is the right place for
that either - I would be rather tempted to fold do_o_path() into
path_openat(), inline path_lookupat() there (as in
        s = path_init(nd, flags);

        while (!(error = link_path_walk(s, nd))
                && ((error = lookup_last(nd)) > 0)) {
                s = trailing_symlink(nd);
        }
        if (!error)
                error = complete_walk(nd);
        if (!error && nd->flags & LOOKUP_DIRECTORY)
                if (!d_can_lookup(nd->path.dentry))
                        error = -ENOTDIR;
        if (!error) {
                audit_inode(nd->name, nd->path.dentry, 0);
                error = vfs_open(&nd->path, file);
        }
        terminate_walk(nd);
- we don't need LOOKUP_DOWN there) and then we only care about the
two callers of trailing_symlink() that are in path_openat().  Which
is where you have your ->acc_mode and ->opath_mask without the need
to dump them into nameidata.  Or to bring that mess into the
things like stat(2) et.al. - it simply doesn't belong there.

In any case, this "bool trailing" is completely wrong; whether that
check belongs in trailing_symlink() or (some of) its callers, putting
it into get_link() is a mistake, forced by kludgy check for procfs-style
symlinks.