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

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Message-ID: <20200812160017.GA30302@linux-8ccs>
Date:   Wed, 12 Aug 2020 18:00:17 +0200
From:   Jessica Yu <jeyu@...nel.org>
To:     Szabolcs Nagy <szabolcs.nagy@....com>
Cc:     Will Deacon <will@...nel.org>, peterz@...radead.org,
        Ard Biesheuvel <ardb@...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

+++ 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@...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 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..