From: Aleksa Sarai <cyphar@cyphar.com>
To: Al Viro <viro@zeniv.linux.org.uk>
Cc: linux-ia64@vger.kernel.org, linux-sh@vger.kernel.org,
Alexei Starovoitov <ast@kernel.org>,
linux-kernel@vger.kernel.org, David Howells <dhowells@redhat.com>,
linux-kselftest@vger.kernel.org, sparclinux@vger.kernel.org,
Shuah Khan <shuah@kernel.org>,
linux-arch@vger.kernel.org, linux-s390@vger.kernel.org,
Tycho Andersen <tycho@tycho.ws>, Aleksa Sarai <asarai@suse.de>,
linux-arm-kernel@lists.infradead.org, linux-mips@vger.kernel.org,
linux-xtensa@linux-xtensa.org, Kees Cook <keescook@chromium.org>,
Arnd Bergmann <arnd@arndb.de>, Jann Horn <jannh@google.com>,
linuxppc-dev@lists.ozlabs.org, linux-m68k@lists.linux-m68k.org,
Andy Lutomirski <luto@kernel.org>,
Shuah Khan <skhan@linuxfoundation.org>,
David Drysdale <drysdale@google.com>,
Christian Brauner <christian@brauner.io>,
"J. Bruce Fields" <bfields@fieldses.org>,
linux-parisc@vger.kernel.org, linux-api@vger.kernel.org,
Chanho Min <chanho.min@lge.com>, Jeff Layton <jlayton@kernel.org>,
Oleg Nesterov <oleg@redhat.com>,
Eric Biederman <ebiederm@xmission.com>,
linux-alpha@vger.kernel.org, linux-fsdevel@vger.kernel.org,
Andrew Morton <akpm@linux-foundation.org>,
Linus Torvalds <torvalds@linux-foundation.org>,
containers@lists.linux-foundation.org
Subject: Re: [PATCH v9 05/10] namei: O_BENEATH-style path resolution flags
Date: Thu, 18 Jul 2019 13:17:29 +1000 [thread overview]
Message-ID: <20190718031729.scehpjydhuxgxqjy@yavin> (raw)
In-Reply-To: <20190714143623.GR17978@ZenIV.linux.org.uk>
[-- Attachment #1: Type: text/plain, Size: 2207 bytes --]
On 2019-07-14, Al Viro <viro@zeniv.linux.org.uk> wrote:
> On Sun, Jul 14, 2019 at 05:00:29PM +1000, Aleksa Sarai wrote:
> > The basic property being guaranteed by LOOKUP_IN_ROOT is that it will
> > not result in resolution of a path component which was not inside the
> > root of the dirfd tree at some point during resolution (and that all
> > absolute symlink and ".." resolution will be done relative to the
> > dirfd). This may smell slightly of chroot(2), because unfortunately it
> > is a similar concept -- the reason for this is to allow for a more
> > efficient way to safely resolve paths inside a rootfs than spawning a
> > separate process to then pass back the fd to the caller.
>
> IDGI... If attacker can modify your subtree, you have already lost -
> after all, they can make anything appear inside that tree just before
> your syscall is made and bring it back out immediately afterwards.
> And if they can't, what is the race you are trying to protect against?
> Confused...
I'll be honest, this code mostly exists because Jann Horn said that it
was necessary in order for this interface to be safe against those kinds
of attacks. Though, it's also entirely possible I just am
mis-remembering the attack scenario he described when I posted v1 of
this series last year.
The use-case I need this functionality for (as do other container
runtimes) is one where you are trying to safely interact with a
directory tree that is a (malicious) container's root filesystem -- so
the container won't be able to move the directory tree root, nor can
they move things outside the rootfs into it (or the reverse). Users
dealing with FTP, web, or file servers probably have similar
requirements.
There is an obvious race condition if you allow the attacker to move the
root (I give an example and test-case of it in the last patch in the
series), and given that it is fairly trivial to defend against I don't
see the downside in including it? But it's obviously your call -- and
maybe Jann Horn can explain the reasoning behind this much better than I
can.
--
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>
[-- Attachment #2: signature.asc --]
[-- Type: application/pgp-signature, Size: 228 bytes --]
next prev parent reply other threads:[~2019-07-18 3:22 UTC|newest]
Thread overview: 45+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-07-06 14:57 [PATCH v9 00/10] namei: openat2(2) path resolution restrictions Aleksa Sarai
2019-07-06 14:57 ` [PATCH v9 01/10] namei: obey trailing magic-link DAC permissions Aleksa Sarai
2019-07-12 4:14 ` Al Viro
2019-07-12 6:36 ` Al Viro
2019-07-12 12:20 ` Aleksa Sarai
2019-07-12 13:10 ` Al Viro
2019-07-14 7:11 ` Aleksa Sarai
2019-07-06 14:57 ` [PATCH v9 02/10] procfs: switch magic-link modes to be more sane Aleksa Sarai
2019-07-06 14:57 ` [PATCH v9 03/10] open: O_EMPTYPATH: procfs-less file descriptor re-opening Aleksa Sarai
2019-07-06 14:57 ` [PATCH v9 04/10] namei: split out nd->dfd handling to dirfd_path_init Aleksa Sarai
2019-07-12 4:20 ` Al Viro
2019-07-12 12:07 ` Aleksa Sarai
2019-07-12 12:12 ` Aleksa Sarai
2019-07-06 14:57 ` [PATCH v9 05/10] namei: O_BENEATH-style path resolution flags Aleksa Sarai
2019-07-12 4:33 ` Al Viro
2019-07-12 10:57 ` Aleksa Sarai
2019-07-12 12:39 ` Al Viro
2019-07-12 12:55 ` Al Viro
2019-07-12 13:25 ` Al Viro
2019-07-12 15:00 ` Al Viro
2019-07-13 2:41 ` Al Viro
2019-07-14 3:58 ` Al Viro
2019-07-16 8:03 ` Aleksa Sarai
2019-07-14 7:00 ` Aleksa Sarai
2019-07-14 14:36 ` Al Viro
2019-07-18 3:17 ` Aleksa Sarai [this message]
2019-07-14 10:31 ` Aleksa Sarai
2019-07-06 14:57 ` [PATCH v9 06/10] namei: LOOKUP_IN_ROOT: chroot-like path resolution Aleksa Sarai
2019-07-06 14:57 ` [PATCH v9 07/10] namei: aggressively check for nd->root escape on ".." resolution Aleksa Sarai
2019-07-06 14:57 ` [PATCH v9 08/10] open: openat2(2) syscall Aleksa Sarai
2019-07-18 14:48 ` Rasmus Villemoes
2019-07-18 15:21 ` Aleksa Sarai
2019-07-18 15:10 ` Arnd Bergmann
2019-07-18 16:12 ` Aleksa Sarai
2019-07-18 21:29 ` Arnd Bergmann
2019-07-19 2:12 ` Dmitry V. Levin
2019-07-19 10:29 ` Christian Brauner
2019-07-19 1:59 ` Dmitry V. Levin
2019-07-19 2:19 ` Aleksa Sarai
2019-07-06 14:57 ` [PATCH v9 09/10] kselftest: save-and-restore errno to allow for %m formatting Aleksa Sarai
2019-07-06 14:57 ` [PATCH v9 10/10] selftests: add openat2(2) selftests Aleksa Sarai
2019-07-08 1:15 ` Michael Ellerman
2019-07-08 5:47 ` Aleksa Sarai
2019-07-12 15:11 ` [PATCH v9 00/10] namei: openat2(2) path resolution restrictions Al Viro
2019-07-12 15:32 ` Aleksa Sarai
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20190718031729.scehpjydhuxgxqjy@yavin \
--to=cyphar@cyphar.com \
--cc=akpm@linux-foundation.org \
--cc=arnd@arndb.de \
--cc=asarai@suse.de \
--cc=ast@kernel.org \
--cc=bfields@fieldses.org \
--cc=chanho.min@lge.com \
--cc=christian@brauner.io \
--cc=containers@lists.linux-foundation.org \
--cc=dhowells@redhat.com \
--cc=drysdale@google.com \
--cc=ebiederm@xmission.com \
--cc=jannh@google.com \
--cc=jlayton@kernel.org \
--cc=keescook@chromium.org \
--cc=linux-alpha@vger.kernel.org \
--cc=linux-api@vger.kernel.org \
--cc=linux-arch@vger.kernel.org \
--cc=linux-arm-kernel@lists.infradead.org \
--cc=linux-fsdevel@vger.kernel.org \
--cc=linux-ia64@vger.kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-kselftest@vger.kernel.org \
--cc=linux-m68k@lists.linux-m68k.org \
--cc=linux-mips@vger.kernel.org \
--cc=linux-parisc@vger.kernel.org \
--cc=linux-s390@vger.kernel.org \
--cc=linux-sh@vger.kernel.org \
--cc=linux-xtensa@linux-xtensa.org \
--cc=linuxppc-dev@lists.ozlabs.org \
--cc=luto@kernel.org \
--cc=oleg@redhat.com \
--cc=shuah@kernel.org \
--cc=skhan@linuxfoundation.org \
--cc=sparclinux@vger.kernel.org \
--cc=torvalds@linux-foundation.org \
--cc=tycho@tycho.ws \
--cc=viro@zeniv.linux.org.uk \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).