linux-kernel - Re: [PATCH v3 3/3] namei: aggressively check for nd->root escape on ".." resolution

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Message-ID: <20181013092702.mf4gjnq2cfqhkvah@ryuk>
Date:   Sat, 13 Oct 2018 20:27:02 +1100
From:   Aleksa Sarai <cyphar@...har.com>
To:     Al Viro <viro@...IV.linux.org.uk>
Cc:     Aleksa Sarai <asarai@...e.de>, Jann Horn <jannh@...gle.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>, jlayton@...nel.org,
        Bruce Fields <bfields@...ldses.org>,
        Arnd Bergmann <arnd@...db.de>,
        Andy Lutomirski <luto@...nel.org>,
        David Howells <dhowells@...hat.com>, christian@...uner.io,
        Tycho Andersen <tycho@...ho.ws>,
        David Drysdale <drysdale@...gle.com>, dev@...ncontainers.org,
        containers@...ts.linux-foundation.org,
        linux-fsdevel@...r.kernel.org,
        kernel list <linux-kernel@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>
Subject: Re: [PATCH v3 3/3] namei: aggressively check for nd->root escape on
 ".." resolution

On 2018-10-13, Al Viro <viro@...IV.linux.org.uk> wrote:
> On Sat, Oct 13, 2018 at 07:53:26PM +1100, Aleksa Sarai wrote:
> 
> > I didn't know about path_is_under() -- I just checked and it appears to
> > not take &rename_lock? From my understanding, in order to protect
> > against the rename attack you need to take &rename_lock (or check
> > against &rename_lock at least and retry if it changed).
> > 
> > I could definitely use path_is_under() if you prefer, though I think
> > that in this case we'd need to take &rename_lock (right?). Also is there
> > a speed issue with taking the write-side of a seqlock when we are just
> > reading -- is this more efficient than doing a retry like in __d_path?
> 
> ???
> 
> 1) it uses is_subdir(), which does deal with rename_lock

Oh -- complete brain-fart on my part. Sorry about that.

> 2) what it does is taking mount_lock.lock.  I.e. the same
> thing as the second retry in __d_path().  _If_ it shows
> up in profiles, we can switch it to read_seqbegin_or_lock(),
> but I'd like to see the profiling data first.

Sure, I'll switch it to use path_is_under().

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

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