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Date:   Wed, 12 Aug 2020 17:42:05 +0100
From:   Szabolcs Nagy <szabolcs.nagy@arm.com>
To:     Ard Biesheuvel <ardb@kernel.org>
Cc:     Jessica Yu <jeyu@kernel.org>, Will Deacon <will@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Mauro Carvalho Chehab <mchehab+huawei@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Kees Cook <keescook@chromium.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Miroslav Benes <mbenes@suse.cz>,
        Mark Rutland <mark.rutland@arm.com>, nd <nd@arm.com>
Subject: Re: [PATCH v2] module: Harden STRICT_MODULE_RWX
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The 08/12/2020 18:37, Ard Biesheuvel wrote:
> module_frob_arch_sections
> 
> On Wed, 12 Aug 2020 at 18:00, Jessica Yu <jeyu@kernel.org> wrote:
> >
> > +++ Szabolcs Nagy [12/08/20 15:15 +0100]:
> > >The 08/12/2020 13:56, Will Deacon wrote:
> > >> On Wed, Aug 12, 2020 at 12:40:05PM +0200, peterz@infradead.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 for the hint. I'm almost certain it's due to this excerpt from
> > the changelog: "we now init sh_type and sh_flags for all known ABI sections
> > in _bfd_elf_new_section_hook."
> >
> > Indeed, .plt and .text.* are listed as special sections in bfd/elf.c.
> > The former requires an exact match and the latter only has to match
> > the prefix ".text." Since the code considers ".plt" and
> > ".text.ftrace_trampoline" special sections, their sh_type and sh_flags
> > are now set by default. Now I guess the question is whether this can
> > be overriden by a linker script..
> >
> 
> If this is even possible to begin with, doing this in a way that is
> portable across the various linkers that we claim to support is going
> to be tricky.
> 
> I suppose this is the downside of using partially linked objects as
> our module format - using ordinary shared libraries (along with the
> appropriate dynamic relocations which are mostly portable across
> architectures) would get rid of most of the PLT and trampoline issues,
> and of a lot of complex static relocation processing code.
> 
> I know there is little we can do at this point, apart from ignoring
> the permissions - perhaps we should just defer the w^x check until
> after calling module_frob_arch_sections()?

i opened

https://sourceware.org/bugzilla/show_bug.cgi?id=26378

and waiting for binutils maintainers if this is intentional.