linux-kernel - Re: [PATCH] module: Harden STRICT_MODULE

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Date:   Mon, 6 Apr 2020 13:27:32 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Jessica Yu <jeyu@...nel.org>
Cc:     Miroslav Benes <mbenes@...e.cz>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        keescook@...omium.org
Subject: Re: [PATCH] module: Harden STRICT_MODULE_RWX

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.