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Date:   Tue, 7 Apr 2020 09:43:17 +0200 (CEST)
From:   Miroslav Benes <mbenes@suse.cz>
To:     Peter Zijlstra <peterz@infradead.org>
cc:     Jessica Yu <jeyu@kernel.org>, 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
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On Mon, 6 Apr 2020, Peter Zijlstra wrote:

> 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.

Ok, fair enough.

I am still wondering if there are modules out there with sections flags 
combination which would cause the same problem with layout_sections() and 
move_module() logic I described earlier. But that it is a separate issue.

Thanks
Miroslav