Re: [PATCH 2/2] exec: Compute file based creds only once

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 2/2] exec: Compute file based creds only once
Date: Fri, 29 May 2020 22:18:15 -0700	[thread overview]
Message-ID: <202005292216.6D96F7D@keescook> (raw)
In-Reply-To: <875zcenlsm.fsf@x220.int.ebiederm.org>

On Fri, May 29, 2020 at 10:28:41PM -0500, Eric W. Biederman wrote:
> The range-diff winds up being:
> 1:  c9258ef4879b ! 1:  a7868323c263 exec: Add a per bprm->file version of per_clear
>     @@ Commit message
>      
>          History Tree: git://git.kernel.org/pub/scm/linux/kernel/git/tglx/history.git
>          Fixes: 1bb0fa189c6a ("[PATCH] NX: clean up legacy binary support")
>     +    Reviewed-by: Kees Cook <keescook@chromium.org>
>          Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
>      
>       ## fs/exec.c ##
>     @@ include/linux/lsm_hooks.h
>        *	transitions between security domains).
>        *	The hook must set @bprm->active_secureexec to 1 if AT_SECURE should be set to
>        *	request libc enable secure mode.
>     -+ *	The hook must set @bprm->pf_per_clear to the personality flags that
>     -+ *	should be cleared from current->personality.
>     ++ *	The hook must add to @bprm->pf_per_clear any personality flags that
>     ++ * 	should be cleared from current->personality.
>        *	@bprm contains the linux_binprm structure.
>        *	Return 0 if the hook is successful and permission is granted.
>        * @bprm_check_security:
> 2:  e6f20c69b96e ! 2:  56305aa9b6fa exec: Compute file based creds only once
>     @@ Commit message
>          secureity attribute and derive capabilities from the fact the
>          user had uid 0 has been added.
>      
>     +    Reviewed-by: Kees Cook <keescook@chromium.org>
>          Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
>      
>       ## fs/binfmt_misc.c ##
>     @@ include/linux/lsm_hooks.h
>      + *	between security domains).
>      + *	The hook must set @bprm->secureexec to 1 if AT_SECURE should be set to
>        *	request libc enable secure mode.
>     -- *	The hook must set @bprm->pf_per_clear to the personality flags that
>     -+ *	The hook must set @bprm->per_clear to the personality flags that
>     -  *	should be cleared from current->personality.
>     +- *	The hook must add to @bprm->pf_per_clear any personality flags that
>     ++ *	The hook must add to @bprm->per_clear any personality flags that
>     +  * 	should be cleared from current->personality.
>        *	@bprm contains the linux_binprm structure.
>        *	Return 0 if the hook is successful and permission is granted.

Awesome; thanks!

> > The cap_ambient_invariant_ok() test is needlessly repeated: it doesn't
> > examine securebits, and nonroot_raised_pE appears to have no
> > side-effects.
> >
> > One of those can be dropped, yes?
> 
> That is what it looks like to me.

Okay, cool. I was worried I was missing something in the mess of tiny
helper calls. :)

> I hope that when the dust clears the function can become a
> straightforward implementation of the capability equations.
> We will see.

Yeah, this looks better and better every day! I'm glad you're able to
dig through all of this.

-- 
Kees Cook