Re: [PATCH v2] module: Harden STRICT_MODULE_RWX

[Ard Biesheuvel]

module_frob_arch_sections

On Wed, 12 Aug 2020 at 18:00, Jessica Yu <jeyu@xxxxxxxxxx> 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@xxxxxxxxxxxxx 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()?