linux-kernel - Re: [PATCH v2 0/2] implement OA2_INHERIT

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Date: Wed, 24 Apr 2024 17:43:02 -0700
From: Andy Lutomirski <luto@...capital.net>
To: stsp <stsp2@...dex.ru>
Cc: linux-kernel@...r.kernel.org, Stefan Metzmacher <metze@...ba.org>, 
	Eric Biederman <ebiederm@...ssion.com>, Alexander Viro <viro@...iv.linux.org.uk>, 
	Andy Lutomirski <luto@...nel.org>, Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>, 
	Jeff Layton <jlayton@...nel.org>, Chuck Lever <chuck.lever@...cle.com>, 
	Alexander Aring <alex.aring@...il.com>, linux-fsdevel@...r.kernel.org, 
	linux-api@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>, 
	Christian Göttsche <cgzones@...glemail.com>, 
	Aleksa Sarai <cyphar@...har.com>
Subject: Re: [PATCH v2 0/2] implement OA2_INHERIT_CRED flag for openat2()

On Wed, Apr 24, 2024 at 3:57 AM stsp <stsp2@...dex.ru> wrote:
>
> 23.04.2024 19:44, Andy Lutomirski пишет:
> >> On Apr 23, 2024, at 4:02 AM, Stas Sergeev <stsp2@...dex.ru> wrote:
> >>
> >> This patch-set implements the OA2_INHERIT_CRED flag for openat2() syscall.
> >> It is needed to perform an open operation with the creds that were in
> >> effect when the dir_fd was opened. This allows the process to pre-open
> >> some dirs and switch eUID (and other UIDs/GIDs) to the less-privileged
> >> user, while still retaining the possibility to open/create files within
> >> the pre-opened directory set.
> > I like the concept, as it’s a sort of move toward a capability system. But I think that making a dirfd into this sort of capability would need to be much more explicit. Right now, any program could do this entirely by accident, and applying OA2_INHERIT_CRED to an fd fished out of /proc seems hazardous.
>
> While I still don't quite understand
> the threat of /proc symlinks, I posted
> v4 which disallows them.
>

I like that, but you're blocking it the wrong way.  My concern is that
someone does dfd = open("/proc/PID/fd/3") and then openat(dfd, ...,
OA2_INHERIT_CRED);  IIRC open("/proc/PID/fd/3") is extremely magical
and returns the _same open file description_ (struct file) as PID's fd
3.

> > So perhaps if an open file description for a directory could have something like FMODE_CRED, and if OA2_INHERIT_CRED also blocked .., magic links, symlinks to anywhere above the dirfd (or maybe all symlinks) and absolute path lookups, then this would be okay.
>
> So I think this all is now done.

But you missed the FMODE_CRED part!

So here's the problem: right now, in current Linux, a dirfd pointing
to a directory that you can open anyway doesn't convey any new powers.
So, if I'm a regular program, and I do open("/etc", O_PATH), I get an
fd.  And if I get an fd pointing at /etc from somewhere else, I get
the same thing (possibly with different f_cred, but f_cred is largely
a hack to restrict things that would otherwise be insecure because
they were designed a bit wrong from the beginning).

But, with your patch, these fds suddenly convey a very strong
privilege: that of their f_cred *over the entire subtree to which they
refer*.  And you can attack it using exactly your intended use case:
if any program opens a dirfd and then drops privileges, well, oops, it
didn't actually fully drop privilege.

So I think that, if this whole concept has any chance of working well,
it needs to be opt-in *at the time of the original open*.  So a
privilege-carrying open would be an entirely new option like
O_CAPTURE_CREDS or FMODE_CREDS.  And OA2_INHERIT_CREDS is rejected if
the dirfd doesn't have that special mode.