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

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Message-ID: <20190712122017.xkowq2cjreylpotm@yavin>
Date:   Fri, 12 Jul 2019 22:20:17 +1000
From:   Aleksa Sarai <cyphar@...har.com>
To:     Al Viro <viro@...iv.linux.org.uk>
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 2019-07-12, Al Viro <viro@...iv.linux.org.uk> wrote:
> 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?

I'll be honest -- I don't understand what set_nameidata() did so I just
did what I thought would be an obvious change (to just copy the
contents). I thought it was related to some aspect of the symlink stack
handling.

In that case, should they both be set to 0 on set_nameidata()? This will
mean that fd re-opening (or magic-link opening) through a
set_nameidata() would always fail.

> >  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)) {
> [...]
> 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.

The error path for LOOKUP_JUMPED comes from the old O_BENEATH patchset,
but all of the "bool trailing" logic is definitely my gaff (I was
quietly hoping you'd have a much better solution than the whole
get_link() thing -- it definitely felt very kludgey to write).

I will work on the suggestion in your follow-up email. Thanks!

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

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