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

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Date:   Sat, 11 May 2019 19:48:47 +0200
From:   Christian Brauner <christian@...uner.io>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Aleksa Sarai <cyphar@...har.com>
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>,
        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 May 11, 2019 7:43:44 PM GMT+02:00, 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".
>
>                  Linus

Sadly I have to admit that we are using this.
Also, execveat on glibc is implemented via
/proc/self/fd/<nr> on kernels that do not
have a proper execveat.
See fexecve...

Christian