linux-kernel - Re: file metadata via fs API (was: [GIT PULL] Filesystem Information)

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Message-ID: <CALCETrXRrMdbzcQPHQQgH3cKKhf87Piy7=gs3wVUX4z20bLyUw@mail.gmail.com>
Date:   Tue, 11 Aug 2020 14:17:46 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Miklos Szeredi <miklos@...redi.hu>
Cc:     Jann Horn <jannh@...gle.com>,
        Casey Schaufler <casey@...aufler-ca.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        David Howells <dhowells@...hat.com>,
        Al Viro <viro@...iv.linux.org.uk>, Karel Zak <kzak@...hat.com>,
        Jeff Layton <jlayton@...hat.com>,
        Miklos Szeredi <mszeredi@...hat.com>,
        Nicolas Dichtel <nicolas.dichtel@...nd.com>,
        Christian Brauner <christian@...uner.io>,
        Lennart Poettering <lennart@...ttering.net>,
        Linux API <linux-api@...r.kernel.org>,
        Ian Kent <raven@...maw.net>,
        LSM <linux-security-module@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: file metadata via fs API (was: [GIT PULL] Filesystem Information)

On Tue, Aug 11, 2020 at 1:56 PM Miklos Szeredi <miklos@...redi.hu> wrote:
>
> On Tue, Aug 11, 2020 at 10:37 PM Jann Horn <jannh@...gle.com> wrote:
> > If you change the semantics of path strings, you'd have to be
> > confident that the new semantics fit nicely with all the path
> > validation routines that exist scattered across userspace, and don't
> > expose new interfaces through file server software and setuid binaries
> > and so on.
>
> So that's where O_ALT comes in.   If the application is consenting,
> then that should prevent exploits.   Or?

We're going to be at risk from libraries that want to use the new
O_ALT mechanism but are invoked by old code that passes traditional
Linux paths.  Each library will have to sanitize paths, and some will
screw it up.

I much prefer Linus' variant where the final part of the extended path
is passed as a separate parameter.