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

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Date:   Sun, 12 May 2019 23:35:49 +1000
From:   Aleksa Sarai <cyphar@...har.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Andy Lutomirski <luto@...capital.net>,
        Andrew Lutomirski <luto@...nel.org>,
        Jann Horn <jannh@...gle.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Jeff Layton <jlayton@...nel.org>,
        "J. Bruce Fields" <bfields@...ldses.org>,
        Arnd Bergmann <arnd@...db.de>,
        David Howells <dhowells@...hat.com>,
        Eric Biederman <ebiederm@...ssion.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Christian Brauner <christian@...uner.io>,
        Tycho Andersen <tycho@...ho.ws>,
        David Drysdale <drysdale@...gle.com>,
        Chanho Min <chanho.min@....com>,
        Oleg Nesterov <oleg@...hat.com>, Aleksa Sarai <asarai@...e.de>,
        Linux Containers <containers@...ts.linux-foundation.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        kernel list <linux-kernel@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>
Subject: Re: [PATCH v6 5/6] binfmt_*: scope path resolution of interpreters

On 2019-05-12, Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> On Sat, May 11, 2019 at 7:37 PM Andy Lutomirski <luto@...capital.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.

[I assume in this discussion "setuid" means "setuid + setcap", right?]

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

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