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

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Date:   Fri, 12 Jul 2019 05:14:54 +0100
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Aleksa Sarai <cyphar@...har.com>
Cc:     Jeff Layton <jlayton@...nel.org>,
        "J. Bruce Fields" <bfields@...ldses.org>,
        Arnd Bergmann <arnd@...db.de>,
        David Howells <dhowells@...hat.com>,
        Shuah Khan <shuah@...nel.org>,
        Shuah Khan <skhan@...uxfoundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Christian Brauner <christian@...uner.io>,
        Eric Biederman <ebiederm@...ssion.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Jann Horn <jannh@...gle.com>, Tycho Andersen <tycho@...ho.ws>,
        David Drysdale <drysdale@...gle.com>,
        Chanho Min <chanho.min@....com>,
        Oleg Nesterov <oleg@...hat.com>, Aleksa Sarai <asarai@...e.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        containers@...ts.linux-foundation.org, linux-alpha@...r.kernel.org,
        linux-api@...r.kernel.org, linux-arch@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-fsdevel@...r.kernel.org, linux-ia64@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
        linux-m68k@...ts.linux-m68k.org, linux-mips@...r.kernel.org,
        linux-parisc@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        linux-s390@...r.kernel.org, linux-sh@...r.kernel.org,
        linux-xtensa@...ux-xtensa.org, sparclinux@...r.kernel.org
Subject: Re: [PATCH v9 01/10] namei: obey trailing magic-link DAC permissions

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.