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

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Date:   Fri, 3 Apr 2020 19:08:50 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     jeyu@...nel.org, linux-kernel@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>, keescook@...omium.org,
        Miroslav Benes <mbenes@...e.cz>
Subject: Re: [PATCH] module: Harden STRICT_MODULE_RWX

On Fri, Apr 03, 2020 at 11:56:31AM -0500, Josh Poimboeuf wrote:
> On Fri, Apr 03, 2020 at 06:37:16PM +0200, Peter Zijlstra wrote:
> > 
> > We're very close to enforcing W^X memory, refuse to load modules that
> > violate this principle per construction.
> > 
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> > Cc: Jessica Yu <jeyu@...nel.org>
> > Cc: Kees Cook <keescook@...omium.org>
> > ---
> >  kernel/module.c |   24 ++++++++++++++++++++++++
> >  1 file changed, 24 insertions(+)
> > 
> > --- a/kernel/module.c
> > +++ b/kernel/module.c
> > @@ -2044,9 +2044,28 @@ static void module_enable_x(const struct
> >  	frob_text(&mod->core_layout, set_memory_x);
> >  	frob_text(&mod->init_layout, set_memory_x);
> >  }
> > +
> > +static int module_rwx_sections(Elf_Ehdr *hdr, Elf_Shdr *sechdrs,
> > +			       char *secstrings, struct module *mod)
> 
> A verb would be nice: "module_enforce_rwx_sections"?
> 
> Shouldn't this be under STRICT_MODULE_RWX instead of
> ARCH_HAS_STRICT_MODULE_RWX?
> 
> > +{
> > +	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))
> 

Duh. yes. Let me respin.