linux-kernel - Re: [PATCH v2] module: Harden STRICT_MODULE

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Date:   Wed, 12 Aug 2020 13:56:46 +0100
From:   Will Deacon <will@...nel.org>
To:     peterz@...radead.org, szabolcs.nagy@....com
Cc:     Ard Biesheuvel <ardb@...nel.org>, Jessica Yu <jeyu@...nel.org>,
        Mauro Carvalho Chehab <mchehab+huawei@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Kees Cook <keescook@...omium.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Miroslav Benes <mbenes@...e.cz>,
        Mark Rutland <mark.rutland@....com>
Subject: Re: [PATCH v2] module: Harden STRICT_MODULE_RWX

On Wed, Aug 12, 2020 at 12:40:05PM +0200, peterz@...radead.org wrote:
> On Wed, Aug 12, 2020 at 10:56:56AM +0200, Ard Biesheuvel wrote:
> > The module .lds has BYTE(0) in the section contents to prevent the
> > linker from pruning them entirely. The (NOLOAD) is there to ensure
> > that this byte does not end up in the .ko, which is more a matter of
> > principle than anything else, so we can happily drop that if it helps.
> > 
> > However, this should only affect the PROGBITS vs NOBITS designation,
> > and so I am not sure whether it makes a difference.
> > 
> > Depending on where the w^x check occurs, we might simply override the
> > permissions of these sections, and strip the writable permission if it
> > is set in the PLT handling init code, which manipulates the metadata
> > of all these 3 sections before the module space is vmalloc'ed.
> 
> What's curious is that this seems the result of some recent binutils
> change. Every build with binutils-2.34 (or older) does not seem to
> generate these as WAX, but has the much more sensible WA.
> 
> I suppose we can change the kernel check and 'allow' W^X for 0 sized
> sections, but I think we should still figure out why binutils-2.35 is
> now generating WAX sections all of a sudden, it might come bite us
> elsewhere.

Agreed, I think it's important to figure out what's going on here before we
try to bodge around it.

Adding Szabolcs, in case he has any ideas.

To save him reading the whole thread, here's a summary:

AArch64 kernel modules built with binutils 2.35 end up with a couple of
ELF sections marked as SHF_WRITE | SHF_ALLOC | SHF_EXECINSTR:

[ 5] .plt PROGBITS 0000000000000388 01d000 000008 00 WAX  0   0  1
[ 6] .init.plt NOBITS 0000000000000390 01d008 000008 00  WA  0   0  1
[ 7] .text.ftrace_trampoline PROGBITS 0000000000000398 01d008 000008 00 WAX  0   0  1

This results in the module being rejected by our loader, because we don't
permit writable, executable mappings.

Our linker script for these entries uses NOLOAD, so it's odd to see PROGBITS
appearing in the readelf output above (and older binutils emits NOBITS
sections). Anyway, here's the linker script:

SECTIONS {
	.plt (NOLOAD) : { BYTE(0) }
	.init.plt (NOLOAD) : { BYTE(0) }
	.text.ftrace_trampoline (NOLOAD) : { BYTE(0) }
}

It appears that the name of the section influences the behaviour, as
Jessica observed [1] that sections named .text.* end up with PROGBITS,
whereas random naming such as ".test" ends up with NOBITS, as before.

We've looked at the changelog between binutils 2.34 and 2.35, but nothing
stands out. Any clues? Is this intentional binutils behaviour?

Thanks,

Will

[1] https://lore.kernel.org/r/20200812114127.GA10824@linux-8ccs.fritz.box