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

From: Aleksa Sarai <cyphar@cyphar.com>
To: Al Viro <viro@zeniv.linux.org.uk>
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, linux-api@vger.kernel.org,
	linux-arch@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
	linux-fsdevel@vger.kernel.org, linux-ia64@vger.kernel.org,
	linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org,
	linux-m68k@lists.linux-m68k.org, linux-mips@vger.kernel.org,
	linux-parisc@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
	linux-s390@vger.kernel.org, linux-sh@vger.kernel.org,
	linux-xtensa@linux-xtensa.org, sparclinux@vger.kernel.org
Subject: Re: [PATCH v9 01/10] namei: obey trailing magic-link DAC permissions
Date: Fri, 12 Jul 2019 22:20:17 +1000	[thread overview]
Message-ID: <20190712122017.xkowq2cjreylpotm@yavin> (raw)
In-Reply-To: <20190712041454.GG17978@ZenIV.linux.org.uk>

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On 2019-07-12, Al Viro <viro@zeniv.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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