linux-kernel - Re: [PATCH 2/2] openat2: add OA2_INHERIT

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Message-ID: <d1686d9d-b403-4985-bcaa-41d4f45a8ac0@yandex.ru>
Date: Thu, 25 Apr 2024 10:24:37 +0300
From: stsp <stsp2@...dex.ru>
To: Al Viro <viro@...iv.linux.org.uk>
Cc: linux-kernel@...r.kernel.org, Stefan Metzmacher <metze@...ba.org>,
 Eric Biederman <ebiederm@...ssion.com>, 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>,
 David Laight <David.Laight@...lab.com>, linux-fsdevel@...r.kernel.org,
 linux-api@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
 Christian Göttsche <cgzones@...glemail.com>
Subject: Re: [PATCH 2/2] openat2: add OA2_INHERIT_CRED flag

25.04.2024 05:31, Al Viro пишет:
> Consider the following, currently absolutely harmless situation:
> 	* process is owned by luser:students.
> 	* descriptor 69 refers to root-opened root directory (O_RDONLY)
> What's the expected result of
> 	fcntl(69, F_SEFTD, O_CLOEXEC);
> 	opening "etc/shadow" with dirfd equal to 69 and your flag given
> 	subsequent read() from the resulting descriptor?
>
> At which point will the kernel say "go fuck yourself, I'm not letting you
> read that file", provided that attacker passes that new flag of yours?
>
> As a bonus question, how about opening it for _write_, seeing that this
> is an obvious instant roothole?
>
> Again, currently the setup that has a root-opened directory in descriptor
> table of a non-root process is safe.
>
> Incidentally, suppose you have the same process run with stdin opened
> (r/o) by root.  F_SETFD it to O_CLOEXEC, then use your open with
> dirfd being 0, pathname - "" and flags - O_RDWR.

Ok, F_SETFD, how simple. :(

> AFAICS, without an explicit opt-in by the original opener it's
> a non-starter, and TBH I doubt that even with such opt-in (FMODE_CRED,
> whatever) it would be a good idea - it gives too much.
Yes, which is why I am quite sceptical
to this FMODE_CRED idea.

Please note that my O_CLOEXEC check
actually meant to check that exactly this
process have opened the dir. It just didn't
happen that way, as you pointed.
Can I replace the O_CLOEXEC check with
some explicit check that makes sure the
fd was opened by exactly that process?