From: "Mickaël Salaün" <mic-WFhQfpSGs3bR7s880joybQ@public.gmane.org>
To: "Eric W. Biederman"
<ebiederm-aS9lmoZGLiVWk0Htik3J/w@public.gmane.org>,
Al Viro <viro-3bDd1+5oDREiFSDQTTA3OLVCufUGDwFn@public.gmane.org>,
David Drysdale <drysdale-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>
Cc: Linus Torvalds
<torvalds-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org>,
Jann Horn <jannh-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>,
Linux API <linux-api-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>,
Linux Kernel Mailing List
<linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>,
linux-fsdevel
<linux-fsdevel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>,
linux-security-module
<linux-security-module-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>,
Kees Cook <keescook-F7+t8E8rja9g9hUCZPvPmw@public.gmane.org>
Subject: Re: new ...at() flag: AT_NO_JUMPS
Date: Mon, 8 May 2017 21:34:09 +0200 [thread overview]
Message-ID: <2e28bd2d-44df-6850-c60d-1a12c584e632@digikod.net> (raw)
In-Reply-To: <8737cj6oao.fsf-aS9lmoZGLiVWk0Htik3J/w@public.gmane.org>
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On 05/05/2017 22:28, Eric W. Biederman wrote:
> Al Viro <viro-3bDd1+5oDREiFSDQTTA3OLVCufUGDwFn@public.gmane.org> writes:
>
>> On Thu, May 04, 2017 at 08:46:49PM -0700, Linus Torvalds wrote:
>>> On Thu, May 4, 2017 at 7:47 PM, Jann Horn <jannh-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org> wrote:
>>>>
>>>> Thread 1 starts an AT_BENEATH path walk using an O_PATH fd
>>>> pointing to /srv/www/example.org/foo; the path given to the syscall is
>>>> "bar/../../../../etc/passwd". The path walk enters the "bar" directory.
>>>> Thread 2 moves /srv/www/example.org/foo/bar to
>>>> /srv/www/example.org/bar.
>>>> Thread 1 processes the rest of the path ("../../../../etc/passwd"), never
>>>> hitting /srv/www/example.org/foo in the process.
>>>>
>>>> I'm not really familiar with the VFS internals, but from a coarse look
>>>> at the patch, it seems like it wouldn't block this?
>>>
>>> I think you're right.
>>>
>>> I guess it would be safe for the RCU case due to the sequence number
>>> check, but not the non-RCU case.
>>
>> Yes and no... FWIW, to exclude that it would suffice to have
>> mount --rbind /src/www/example.org/foo /srv/www/example.org/foo done first.
>> Then this kind of race will end up with -ENOENT due to path_connected()
>> logics in follow_dotdot_rcu()/follow_dotdot(). I'm not sure about the
>> intended applications, though - is that thing supposed to be used along with
>> some horror like seccomp, or...?
>
> As I recall the general idea is that if you have an application like a
> tftp server or a web server that gets a path from a possibly dubious
> source. Instead of implementing an error prone validation logic in
> userspace you can use AT_BENEATH and be certain the path resolution
> stays in bounds.
>
> As you can do stronger things as root this seems mostly targeted at
> non-root applications.
>
> I seem to recall part of the idea was to sometimes pair this to seccomp
> to be certain your application can't escape a sandbox. That plays to
> seccomp limitations that it can inspect flags as they reside in
> registers but seccomp can't follow pointers.
Here is the code and tests from David Drysdale:
https://github.com/google/capsicum-linux/commits/openat-v2
...and the latest patch: https://lkml.org/lkml/2015/3/9/407
The O_BENEATH flag have also been discussed for FreeBSD to support Capsicum.
>
> Which all suggests that we would want something similar to is_subdir
> when AT_BENEATH is specified that we check every time we follow ..
> that would verify that on the same filesystem we stay below and
> that we also stay on a mount that is below. mount --move has
> all of the same challenges for enforcing you stay within bounds
> as rename does.
FYI, I'm working on a new LSM [1] to work around the limitations of
seccomp-bpf, especially the pointer checks. The idea is to enable some
filtering as seccomp-bpf can do but instead of checking at the syscall
level, Landlock take advantage of LSM hooks. I had a first PoC of an
eBPF function and map type to check if a file was beneath another [2]. I
plan to create a new one that record a "snapshot" of the current mount
tree into an eBPF map to be able to check if a file is beneath or a
parent of another one.
[1] https://lkml.kernel.org/r/20170328234650.19695-1-mic-WFhQfpSGs3bR7s880joybQ@public.gmane.org
[2] https://lkml.kernel.org/r/20161026065654.19166-9-mic-WFhQfpSGs3bR7s880joybQ@public.gmane.org
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next prev parent reply other threads:[~2017-05-08 19:34 UTC|newest]
Thread overview: 25+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-04-29 22:04 new ...at() flag: AT_NO_JUMPS Al Viro
[not found] ` <20170429220414.GT29622-3bDd1+5oDREiFSDQTTA3OLVCufUGDwFn@public.gmane.org>
2017-04-29 23:17 ` Andy Lutomirski
[not found] ` <CALCETrXhOhG0tRDDOROwT9ghvQvKziM2PBN=CX5Soa2m7=0cFw-JsoAwUIsXosN+BqQ9rBEUg@public.gmane.org>
2017-04-29 23:25 ` Al Viro
2017-04-30 1:13 ` Andy Lutomirski
[not found] ` <20170429232504.GU29622-3bDd1+5oDREiFSDQTTA3OLVCufUGDwFn@public.gmane.org>
2017-04-30 4:38 ` Matthew Wilcox
[not found] ` <20170430043822.GE27790-PfSpb0PWhxZc2C7mugBRk2EX/6BAtgUQ@public.gmane.org>
2017-04-30 16:10 ` Al Viro
2017-05-01 4:52 ` Andy Lutomirski
[not found] ` <CALCETrX0dx3d6OQQ+1GJ_xgSz3iNVeRn+8o6b-+3f7awVOWdQg-JsoAwUIsXosN+BqQ9rBEUg@public.gmane.org>
2017-05-01 5:15 ` Al Viro
2017-05-01 17:36 ` Jann Horn
2017-05-01 19:37 ` Andy Lutomirski
[not found] ` <CAG48ez0wccvQ5i+XN_Q_yA9_ZwSaGb-W+zky0KQb_GU=9G+MSw-JsoAwUIsXosN+BqQ9rBEUg@public.gmane.org>
2017-05-05 0:30 ` Al Viro
2017-05-05 0:44 ` Andy Lutomirski
2017-05-05 1:06 ` Al Viro
[not found] ` <20170505003030.GM29622-3bDd1+5oDREiFSDQTTA3OLVCufUGDwFn@public.gmane.org>
2017-05-05 1:27 ` Linus Torvalds
[not found] ` <CA+55aFyOKM7DW7+0sdDFKdZFXgptb5r1id9=Wvhd8AgSP7qjwQ-JsoAwUIsXosN+BqQ9rBEUg@public.gmane.org>
2017-05-05 3:00 ` Al Viro
2017-05-05 4:01 ` Linus Torvalds
2017-05-05 4:31 ` Andy Lutomirski
2017-05-05 2:47 ` Jann Horn
2017-05-05 3:46 ` Linus Torvalds
[not found] ` <CA+55aFy1SokNNUgxBnFLdA1PRyeG13BqyYNg5xVrW-tNGqh2Bg-JsoAwUIsXosN+BqQ9rBEUg@public.gmane.org>
2017-05-05 4:39 ` Al Viro
2017-05-05 4:44 ` Andy Lutomirski
[not found] ` <CALCETrVQ2fwDZOsGSoLyRb6Qjp4nszfDjOPSYi0kzqt23Aw1NA-JsoAwUIsXosN+BqQ9rBEUg@public.gmane.org>
2017-05-05 20:04 ` Eric W. Biederman
2017-05-05 20:28 ` Eric W. Biederman
[not found] ` <8737cj6oao.fsf-aS9lmoZGLiVWk0Htik3J/w@public.gmane.org>
2017-05-08 19:34 ` Mickaël Salaün [this message]
2017-05-18 8:50 ` David Drysdale
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