linux-kernel - Re: [PATCH 1/2] fs/exec: Explicitly unshare fs

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Message-ID: <6C9B4DB6-0E73-471D-A278-39F61A825DB6@kernel.org>
Date: Tue, 13 May 2025 16:15:52 -0700
From: Kees Cook <kees@...nel.org>
To: Jann Horn <jannh@...gle.com>
CC: Mateusz Guzik <mjguzik@...il.com>, Kees Cook <keescook@...omium.org>,
 Christian Brauner <brauner@...nel.org>,
 Eric Biederman <ebiederm@...ssion.com>,
 Jorge Merlino <jorge.merlino@...onical.com>,
 Alexander Viro <viro@...iv.linux.org.uk>,
 Thomas Gleixner <tglx@...utronix.de>, Andy Lutomirski <luto@...nel.org>,
 Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
 Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
 linux-fsdevel@...r.kernel.org, John Johansen <john.johansen@...onical.com>,
 Paul Moore <paul@...l-moore.com>, James Morris <jmorris@...ei.org>,
 "Serge E. Hallyn" <serge@...lyn.com>,
 Stephen Smalley <stephen.smalley.work@...il.com>,
 Eric Paris <eparis@...isplace.org>,
 Richard Haines <richard_c_haines@...nternet.com>,
 Casey Schaufler <casey@...aufler-ca.com>, Xin Long <lucien.xin@...il.com>,
 "David S. Miller" <davem@...emloft.net>, Todd Kjos <tkjos@...gle.com>,
 Ondrej Mosnacek <omosnace@...hat.com>,
 Prashanth Prahlad <pprahlad@...hat.com>, Micah Morton <mortonm@...omium.org>,
 Fenghua Yu <fenghua.yu@...el.com>, Andrei Vagin <avagin@...il.com>,
 linux-kernel@...r.kernel.org, apparmor@...ts.ubuntu.com,
 linux-security-module@...r.kernel.org, selinux@...r.kernel.org,
 linux-hardening@...r.kernel.org, oleg@...hat.com
Subject: Re: [PATCH 1/2] fs/exec: Explicitly unshare fs_struct on exec

On May 13, 2025 2:09:48 PM PDT, Jann Horn <jannh@...gle.com> wrote:
>On Tue, May 13, 2025 at 10:57 PM Kees Cook <kees@...nel.org> wrote:
>> On May 13, 2025 6:05:45 AM PDT, Mateusz Guzik <mjguzik@...il.com> wrote:
>> >Here is my proposal: *deny* exec of suid/sgid binaries if fs_struct is
>> >shared. This will have to be checked for after the execing proc becomes
>> >single-threaded ofc.
>>
>> Unfortunately the above Chrome helper is setuid and uses CLONE_FS.
>
>Chrome first launches a setuid helper, and then the setuid helper does
>CLONE_FS. Mateusz's proposal would not impact this usecase.
>
>Mateusz is proposing to block the case where a process first does
>CLONE_FS, and *then* one of the processes sharing the fs_struct does a
>setuid execve(). Linux already downgrades such an execve() to be
>non-setuid, which probably means anyone trying to do this will get
>hard-to-understand problems. Mateusz' proposal would just turn this
>hard-to-debug edgecase, which already doesn't really work, into a
>clean error; I think that is a nice improvement even just from the
>UAPI standpoint.
>
>If this change makes it possible to clean up the kernel code a bit, even better.

Ah! Okay, I appreciate the clarification. :) I'm game to try making it an error instead of silent downgrading.

-Kees

-- 
Kees Cook
-- 
Kees Cook