linux-kernel - Re: [PATCH v6 5/6] binfmt_*: scope path resolution of interpreters

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Date:   Sun, 12 May 2019 04:00:43 +1000
From:   Aleksa Sarai <cyphar@...har.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Andy Lutomirski <luto@...capital.net>,
        Jann Horn <jannh@...gle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Jeff Layton <jlayton@...nel.org>,
        "J. Bruce Fields" <bfields@...ldses.org>,
        Arnd Bergmann <arnd@...db.de>,
        David Howells <dhowells@...hat.com>,
        Eric Biederman <ebiederm@...ssion.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Christian Brauner <christian@...uner.io>,
        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>,
        Linux Containers <containers@...ts.linux-foundation.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        kernel list <linux-kernel@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>
Subject: Re: [PATCH v6 5/6] binfmt_*: scope path resolution of interpreters

On 2019-05-11, Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> On Sat, May 11, 2019 at 1:31 PM Aleksa Sarai <cyphar@...har.com> wrote:
> > Yup, I've dropped the patch for the next version. (To be honest, I'm not
> > sure why I included any of the other flags -- the only one that would've
> > been necessary to deal with CVE-2019-5736 was AT_NO_MAGICLINKS.)
> 
> I do wonder if we could try to just set AT_NO_MAGICLINKS
> unconditionally for execve() (and certainly for the suid case).
> 
> I'd rather try to do these things across the board, than have "suid
> binaries are treated specially" if at all possible.
> 
> The main use case for having /proc/<pid>/exe thing is for finding open
> file descriptors, and for 'ps' kind of use, or to find the startup
> directory when people don't populate the execve() environment fully
> (ie "readlink(/proc/self/exe)" is afaik pretty common.
> 
> Sadly, googling for
> 
>     execve /proc/self/exe
> 
> does actually find hits, including one that implies that chrome does
> exactly that.  So it might not be possible.
> 
> Somewhat odd, but it does just confirm the whole "users will at some
> point do everything in their power to use every odd special case,
> intended or not".

*sheepishly* Actually we use this in runc very liberally.

It's done because we need to run namespace-related code but runc is
written in Go so (long story short) we re-exec ourselves in order to
run some __attribute__((constructor)) code which sets up the namespaces
and then lets the Go runtime boot.

I suspect just writing everything in C would've been orders of magnitude
simpler, but I wasn't around when that decision was made. :P

Also as Christian mentioned, fexecve(3) in glibc is implemented using
/proc/self/fd on old kernels (then again, if we change the behaviour on
new kernels it won't matter because glibc uses execveat(AT_EMPTY_PATH)
if it's available).

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

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