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From: Aleksa Sarai <cyphar@cyphar.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andy Lutomirski <luto@amacapital.net>,
	Jann Horn <jannh@google.com>, Andy Lutomirski <luto@kernel.org>,
	Al Viro <viro@zeniv.linux.org.uk>,
	Jeff Layton <jlayton@kernel.org>,
	"J. Bruce Fields" <bfields@fieldses.org>,
	Arnd Bergmann <arnd@arndb.de>,
	David Howells <dhowells@redhat.com>,
	Eric Biederman <ebiederm@xmission.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Alexei Starovoitov <ast@kernel.org>,
	Kees Cook <keescook@chromium.org>,
	Christian Brauner <christian@brauner.io>,
	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>,
	Linux Containers <containers@lists.linux-foundation.org>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	Linux API <linux-api@vger.kernel.org>,
	kernel list <linux-kernel@vger.kernel.org>,
	linux-arch <linux-arch@vger.kernel.org>
Subject: Re: [PATCH v6 5/6] binfmt_*: scope path resolution of interpreters
Date: Sun, 12 May 2019 04:00:43 +1000
Message-ID: <20190511180043.mfwwcz5j2fnxe6lp@yavin> (raw)
In-Reply-To: <CAHk-=wgo-X9pDbVf8khfDsgEKn3wSvLJkB890OxHL+42Hosypw@mail.gmail.com>


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On 2019-05-11, Linus Torvalds <torvalds@linux-foundation.org> wrote:
> On Sat, May 11, 2019 at 1:31 PM Aleksa Sarai <cyphar@cyphar.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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  parent reply index

Thread overview: 32+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-05-06 16:54 [PATCH v6 0/6] namei: resolveat(2) path resolution restriction API Aleksa Sarai
2019-05-06 16:54 ` [PATCH v6 1/6] namei: split out nd->dfd handling to dirfd_path_init Aleksa Sarai
2019-05-06 16:54 ` [PATCH v6 2/6] namei: O_BENEATH-style path resolution flags Aleksa Sarai
2019-05-06 16:54 ` [PATCH v6 3/6] namei: LOOKUP_IN_ROOT: chroot-like path resolution Aleksa Sarai
2019-05-06 16:54 ` [PATCH v6 4/6] namei: aggressively check for nd->root escape on ".." resolution Aleksa Sarai
2019-05-06 16:54 ` [PATCH v6 5/6] binfmt_*: scope path resolution of interpreters Aleksa Sarai
2019-05-06 18:37   ` Jann Horn
2019-05-06 19:17     ` Aleksa Sarai
2019-05-06 23:41       ` Andy Lutomirski
2019-05-08  0:54       ` Aleksa Sarai
2019-05-10 20:41       ` Jann Horn
2019-05-10 21:20         ` Andy Lutomirski
2019-05-10 22:55           ` Jann Horn
2019-05-10 23:36             ` Christian Brauner
2019-05-11 15:49               ` Aleksa Sarai
2019-05-11 17:00             ` Andy Lutomirski
2019-05-11 17:21               ` Linus Torvalds
2019-05-11 17:26                 ` Linus Torvalds
2019-05-11 17:31                   ` Aleksa Sarai
2019-05-11 17:43                     ` Linus Torvalds
2019-05-11 17:48                       ` Christian Brauner
2019-05-11 18:00                       ` Aleksa Sarai [this message]
2019-05-11 22:39                 ` Andy Lutomirski
     [not found]                   ` <CAHk-=wg3+3GfHsHdB4o78jNiPh_5ShrzxBuTN-Y8EZfiFMhCvw@mail.gmail.com>
2019-05-12 10:19                     ` Christian Brauner
     [not found]                     ` <9CD2B97D-A6BD-43BE-9040-B410D996A195@amacapital.net>
2019-05-12 10:44                       ` Linus Torvalds
2019-05-12 13:35                         ` Aleksa Sarai
2019-05-12 13:38                           ` Aleksa Sarai
2019-05-12 14:34                           ` Andy Lutomirski
2019-05-11 17:26               ` Aleksa Sarai
2019-05-08  0:38     ` Eric W. Biederman
2019-05-10 20:10       ` Jann Horn
2019-05-06 16:54 ` [PATCH v6 6/6] namei: resolveat(2) syscall Aleksa Sarai

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