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

From: Aleksa Sarai <cyphar@cyphar.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andy Lutomirski <luto@amacapital.net>,
	Andrew Lutomirski <luto@kernel.org>, Jann Horn <jannh@google.com>,
	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 23:38:26 +1000	[thread overview]
Message-ID: <20190512133826.fcmfiqze7dnetews@yavin> (raw)
In-Reply-To: <20190512133549.ymx5yg5rdqvavzyq@yavin>

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On 2019-05-12, Aleksa Sarai <cyphar@cyphar.com> wrote:
> On 2019-05-12, Linus Torvalds <torvalds@linux-foundation.org> wrote:
> > On Sat, May 11, 2019 at 7:37 PM Andy Lutomirski <luto@amacapital.net> wrote:
> > > I bet this will break something that already exists. An execveat()
> > > flag to turn off /proc/self/exe would do the trick, though.
> > 
> > Thinking more about it, I suspect it is (once again) wrong to let the
> > thing that does the execve() control that bit.
> > 
> > Generally, the less we allow people to affect the lifetime and
> > environment of a suid executable, the better off we are.
> > 
> > But maybe we could limit /proc/*/exe to at least not honor suid'ness
> > of the target? Or does chrome/runc depend on that too?
> 
> Speaking on the runc side, we don't depend on this. It's possible
> someone depends on this for fexecve(3) -- but as mentioned before in
> newer kernels glibc uses execve(AT_EMPTY_PATH).
> 
> I would like to point out though that I'm a little bit cautious about
> /proc/self/exe-specific restrictions -- because a trivial way to get
> around them would be to just open it with O_PATH (and you end up with a
> /proc/self/fd/ which is equivalent). Unfortunately blocking setuid exec
> on all O_PATH descriptors would break even execve(AT_EMPTY_PATH) of
> setuid descriptors.
> 
> The patches I mentioned (which Andy and I discussed off-list) would
> effectively make the magiclink modes in /proc/ affect how you can
> operate on the path (no write bit in the mode, cannot re-open it write).
> One aspect of this is how to handle O_PATH and in particular how do we
> handle an O_PATH re-open of an already-restricted magiclink.
> 
> Maybe we could make it so that setuid is disallowed if you are dealing
> with an O_PATH fd which was a magiclink. Effectively, on O_PATH open you
> get an fmode_t saying FMODE_SETUID_EXEC_ALLOWED *but* if the path is a
> magiclink this fmode gets dropped and when the fd is given to
> execveat(AT_EMPTY_PATH) the fmode is checked and setuid-exec is not
> allowed.

... and obviously /proc/self/exe would have an fmode
~FMODE_SETUID_EXEC_ALLOWED from the outset. The reason for this slightly
odd semantic would be to continue to allow O_PATH setuid-exec as long as
the O_PATH was opened from an actual path rather than a magiclink.

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

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