Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Mon, 6 Apr 2020 13:27:32 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Jessica Yu <jeyu@kernel.org>
Cc:     Miroslav Benes <mbenes@suse.cz>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        linux-kernel@vger.kernel.org, Thomas Gleixner <tglx@linutronix.de>,
        keescook@chromium.org
Subject: Re: [PATCH] module: Harden STRICT_MODULE_RWX
Message-ID: <20200406112732.GK20730@hirez.programming.kicks-ass.net>
References: <20200403163716.GV20730@hirez.programming.kicks-ass.net>
 <20200403165631.hrxxm3pnzqa4vxln@treble>
 <alpine.LSU.2.21.2004061146590.26870@pobox.suse.cz>
 <20200406104615.GA9629@linux-8ccs>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20200406104615.GA9629@linux-8ccs>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Mon, Apr 06, 2020 at 12:46:17PM +0200, Jessica Yu wrote:
> +++ Miroslav Benes [06/04/20 11:55 +0200]:
> > On Fri, 3 Apr 2020, Josh Poimboeuf wrote:
> > 
> > > On Fri, Apr 03, 2020 at 06:37:16PM +0200, Peter Zijlstra wrote:
> > > > +{
> > > > +	int i;
> > > > +
> > > > +	for (i = 0; i < hdr->e_shnum; i++) {
> > > > +		if (sechdrs[i].sh_flags & (SHF_EXECINSTR|SHF_WRITE))
> > > > +			return -ENOEXEC;
> > > 
> > > I think you only want the error when both are set?
> > > 
> > > 		if (sechdrs[i].sh_flags & (SHF_EXECINSTR|SHF_WRITE) == (SHF_EXECINSTR|SHF_WRITE))
> > 
> > A section with SHF_EXECINSTR and SHF_WRITE but without SHF_ALLOC would be
> > strange though, no? It wouldn't be copied to the final module later
> > anyway.
> 
> That's right - move_module() ignores !SHF_ALLOC sections and does not
> copy them over to their final location. So I think we want to look for
> SHF_EXECINSTR|SHF_WRITE|SHF_ALLOC here..

So I did notice that !SHF_ALLOC sections get ignored, but since this
check is about W^X we don't strictly care about SHF_ALLOC. What we care
about it never allowing a writable and executable map.

Adding ALLOC to the test only allows for future mistakes and doesn't
make the check any better.