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Date:   Thu, 6 Oct 2022 16:13:37 +0200
From:   Jann Horn <>
To:     Christian Brauner <>
Cc:     Kees Cook <>,
        Eric Biederman <>,
        Jorge Merlino <>,
        Alexander Viro <>,
        Thomas Gleixner <>,
        Andy Lutomirski <>,
        Sebastian Andrzej Siewior <>,
        Andrew Morton <>,,,
        John Johansen <>,
        Paul Moore <>,
        James Morris <>,
        "Serge E. Hallyn" <>,
        Stephen Smalley <>,
        Eric Paris <>,
        Richard Haines <>,
        Casey Schaufler <>,
        Xin Long <>,
        "David S. Miller" <>,
        Todd Kjos <>,
        Ondrej Mosnacek <>,
        Prashanth Prahlad <>,
        Micah Morton <>,
        Fenghua Yu <>,
        Andrei Vagin <>,,,,,
Subject: Re: [PATCH 1/2] fs/exec: Explicitly unshare fs_struct on exec

On Thu, Oct 6, 2022 at 11:05 AM Christian Brauner <> wrote:
> On Thu, Oct 06, 2022 at 01:27:34AM -0700, Kees Cook wrote:
> > The check_unsafe_exec() counting of n_fs would not add up under a heavily
> > threaded process trying to perform a suid exec, causing the suid portion
> > to fail. This counting error appears to be unneeded, but to catch any
> > possible conditions, explicitly unshare fs_struct on exec, if it ends up
> Isn't this a potential uapi break? Afaict, before this change a call to
> clone{3}(CLONE_FS) followed by an exec in the child would have the
> parent and child share fs information. So if the child e.g., changes the
> working directory post exec it would also affect the parent. But after
> this change here this would no longer be true. So a child changing a
> workding directoro would not affect the parent anymore. IOW, an exec is
> accompanied by an unshare(CLONE_FS). Might still be worth trying ofc but
> it seems like a non-trivial uapi change but there might be few users
> that do clone{3}(CLONE_FS) followed by an exec.

I believe the following code in Chromium explicitly relies on this
behavior, but I'm not sure whether this code is in active use anymore:;l=101?q=CLONE_FS&sq=&ss=chromium

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