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From: Aleksa Sarai <cyphar@cyphar.com>
To: Andy Lutomirski <luto@kernel.org>
Cc: 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>,
	Jann Horn <jannh@google.com>,
	Christian Brauner <christian@brauner.io>,
	David Drysdale <drysdale@google.com>,
	Tycho Andersen <tycho@tycho.ws>,
	Kees Cook <keescook@chromium.org>,
	Linux Containers <containers@lists.linux-foundation.org>,
	Linux FS Devel <linux-fsdevel@vger.kernel.org>,
	Linux API <linux-api@vger.kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Alexei Starovoitov <ast@kernel.org>,
	Chanho Min <chanho.min@lge.com>, Oleg Nesterov <oleg@redhat.com>,
	Aleksa Sarai <asarai@suse.de>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	LKML <linux-kernel@vger.kernel.org>,
	linux-arch <linux-arch@vger.kernel.org>
Subject: Re: [PATCH RESEND v5 0/5] namei: vfs flags to restrict path resolution
Date: Tue, 26 Mar 2019 00:04:29 +1100	[thread overview]
Message-ID: <20190325130429.dbrgjxnvq3w5cpb3@yavin> (raw)
In-Reply-To: <CALCETrV-Q4qZXGeavBPQbF6-VVz7hqvjRkbhFeYmbGiQgu5Omg@mail.gmail.com>

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On 2019-03-21, Andy Lutomirski <luto@kernel.org> wrote:
> On Wed, Mar 20, 2019 at 7:38 AM Aleksa Sarai <cyphar@cyphar.com> wrote:
> > Now that the holiday break is over, it's time to re-send this patch
> > series (with a few additions, due to new information we got from
> > CVE-2019-5736 -- which this patchset mostly protected against but had
> > some holes with regards to #!-style scripts).
> 
> I generally like this, but, as Linus pointed out, it will be
> unfortunate if application authors see this as just another
> non-portable weird Linux API and don't use it.  Would it be worthwhile
> to put some thought into making it an API that other OSes might be
> willing to implement?  As it stands, the openat(2) flags are getting
> rather crazy in this patch set.
> 
> Aleksa had a resolveat(2) proposal that really didn't seem too bad.

I agree having a bunch of O_* flags for resolution feels quite ugly (and
crazy) -- but the last time I pitched resolveat(2) the reaction was
lukewarm to say the least. It's basically an O_PATHv2 and I don't know
how popular that suggestion might be. I wouldn't mind pitching it again
though (now that I have a better idea of how to handle some of the UX
worries I had).

But, if we have a resolveat(2) we'll need to add O_EMPTYPATH support for
openat(2) so that you can open scoped paths without using the
/proc/self/fd/$n trick. However, you run into reopening permission
issues (as we saw in CVE-2019-5736 and countless other CVEs). There
might be a solution for this which Andy and I have talked about
off-list.

There is also the problem of the execveat(2) attack -- which is dealt
with in patch 5 of this series. In order to scope #!-script resolution
we need to have AT_* flags for the same resolution restriction
(defeating the point of a resolveat(2) slightly).

There is an argument that we shouldn't need AT_THIS_ROOT or AT_BENEATH
support (because ideally you should be doing execve(2) in a pivot_root
anyway) but AT_NO_MAGICLINKS is pretty important -- since it allows you
to block the circumvention mm->dumpable in certain situations (as we saw
in CVE-2019-5736).

The solution Andy and I have discussed is a way to make fd-reopening (a
seemingly little-known trick outside of container runtimes) much safer
such that we don't need mitigations like AT_NO_MAGICLINKS on
execveat(2). If we assume that idea works out (I'm still trying to get a
working patch for that idea) then resolveat(2) would be sufficient and
we don't need AT_* flags on execveat(2).

tl;dr: I think resolveat(2) is much nicer, and assuming it's not an
	   unpopular idea I think we should go for it. But there are several
	   other threads of discussion that we might want to have first
	   (such as how to improve the fd-reopening design so it's safer
	   before we expose it to everyone).

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

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  reply	other threads:[~2019-03-25 13:05 UTC|newest]

Thread overview: 14+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-03-20 14:37 Aleksa Sarai
2019-03-20 14:37 ` [PATCH RESEND v5 1/5] namei: split out nd->dfd handling to dirfd_path_init Aleksa Sarai
2019-03-20 14:37 ` [PATCH RESEND v5 2/5] namei: O_BENEATH-style path resolution flags Aleksa Sarai
2019-03-20 14:37 ` [PATCH RESEND v5 3/5] namei: O_THISROOT: chroot-like path resolution Aleksa Sarai
2019-03-20 14:37 ` [PATCH RESEND v5 4/5] namei: aggressively check for nd->root escape on ".." resolution Aleksa Sarai
2019-03-20 14:37 ` [PATCH RESEND v5 5/5] binfmt_*: scope path resolution of interpreters Aleksa Sarai
2019-03-21 17:06 ` [PATCH RESEND v5 0/5] namei: vfs flags to restrict path resolution Andy Lutomirski
2019-03-25 13:04   ` Aleksa Sarai [this message]
2019-04-23 20:13     ` Kees Cook
2019-04-23 20:24       ` Christian Brauner
2019-04-24 15:38       ` Aleksa Sarai
2019-04-25 13:22         ` Adam Borowski
2019-04-25 19:45         ` Aleksa Sarai
  -- strict thread matches above, loose matches on Subject: below --
2019-03-06 19:12 Aleksa Sarai

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