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

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Message-ID: <20200812141557.GQ14398@arm.com>
Date:   Wed, 12 Aug 2020 15:15:59 +0100
From:   Szabolcs Nagy <szabolcs.nagy@....com>
To:     Will Deacon <will@...nel.org>
Cc:     peterz@...radead.org, 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>, nd@....com
Subject: Re: [PATCH v2] module: Harden STRICT_MODULE_RWX

The 08/12/2020 13:56, Will Deacon wrote:
> 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?

for me it bisects to

https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=8c803a2dd7d3d742a3d0071914f557ef465afe71

i will have to investigate further what's going on.

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