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Date:   Fri, 29 May 2020 14:06:33 -0700
From:   Kees Cook <keescook@chromium.org>
To:     "Eric W. Biederman" <ebiederm@xmission.com>
Cc:     linux-kernel@vger.kernel.org,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Oleg Nesterov <oleg@redhat.com>, Jann Horn <jannh@google.com>,
        Greg Ungerer <gerg@linux-m68k.org>,
        Rob Landley <rob@landley.net>,
        Bernd Edlinger <bernd.edlinger@hotmail.de>,
        linux-fsdevel@vger.kernel.org, Al Viro <viro@ZenIV.linux.org.uk>,
        Alexey Dobriyan <adobriyan@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Casey Schaufler <casey@schaufler-ca.com>,
        linux-security-module@vger.kernel.org,
        James Morris <jmorris@namei.org>,
        "Serge E. Hallyn" <serge@hallyn.com>,
        Andy Lutomirski <luto@amacapital.net>
Subject: Re: [PATCH 1/2] exec: Add a per bprm->file version of per_clear
Message-ID: <202005291403.BCDBFA7D1@keescook>
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On Fri, May 29, 2020 at 11:46:40AM -0500, Eric W. Biederman wrote:
> 
> There is a small bug in the code that recomputes parts of bprm->cred
> for every bprm->file.  The code never recomputes the part of
> clear_dangerous_personality_flags it is responsible for.
> 
> Which means that in practice if someone creates a sgid script
> the interpreter will not be able to use any of:
> 	READ_IMPLIES_EXEC
> 	ADDR_NO_RANDOMIZE
> 	ADDR_COMPAT_LAYOUT
> 	MMAP_PAGE_ZERO.
> 
> This accentially clearing of personality flags probably does
> not matter in practice because no one has complained
> but it does make the code more difficult to understand.
> 
> Further remaining bug compatible prevents the recomputation from being
> removed and replaced by simply computing bprm->cred once from the
> final bprm->file.
> 
> Making this change removes the last behavior difference between
> computing bprm->creds from the final file and recomputing
> bprm->cred several times.  Which allows this behavior change
> to be justified for it's own reasons, and for any but hunts
> looking into why the behavior changed to wind up here instead
> of in the code that will follow that computes bprm->cred
> from the final bprm->file.
> 
> This small logic bug appears to have existed since the code
> started clearing dangerous personality bits.
> 
> History Tree: git://git.kernel.org/pub/scm/linux/kernel/git/tglx/history.git
> Fixes: 1bb0fa189c6a ("[PATCH] NX: clean up legacy binary support")
> Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

Yup, this looks good. Pointless nit because it's removed in the next
patch, but pf_per_clear is following the same behavioral pattern as
active_secureexec, it could be named active_per_clear, but since this
already been bikeshed in v1, it's fine! :)

Reviewed-by: Kees Cook <keescook@chromium.org>

I wish we had more robust execve tests. :(

-- 
Kees Cook