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From: Jessica Yu <jeyu@kernel.org>
To: Szabolcs Nagy <szabolcs.nagy@arm.com>
Cc: Will Deacon <will@kernel.org>,
	peterz@infradead.org, Ard Biesheuvel <ardb@kernel.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@arm.com
Subject: Re: [PATCH v2] module: Harden STRICT_MODULE_RWX
Date: Wed, 12 Aug 2020 18:00:17 +0200	[thread overview]
Message-ID: <20200812160017.GA30302@linux-8ccs> (raw)
In-Reply-To: <20200812141557.GQ14398@arm.com>

+++ 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..


  reply	other threads:[~2020-08-12 16:00 UTC|newest]

Thread overview: 43+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-04-03 17:13 Peter Zijlstra
2020-04-03 20:31 ` Kees Cook
2020-04-08 15:32 ` Jessica Yu
2020-04-08 15:43   ` [PATCH] module: break nested ARCH_HAS_STRICT_MODULE_RWX and STRICT_MODULE_RWX #ifdefs Jessica Yu
2020-04-08 15:57   ` [PATCH v2] module: Harden STRICT_MODULE_RWX Peter Zijlstra
2020-04-08 16:20     ` Jessica Yu
2020-08-08  8:12 ` Mauro Carvalho Chehab
2020-08-10  9:25   ` Jessica Yu
2020-08-10 15:06     ` Jessica Yu
2020-08-11 14:34       ` Mauro Carvalho Chehab
2020-08-11 14:55         ` peterz
2020-08-11 15:27           ` Mauro Carvalho Chehab
2020-08-11 16:01             ` Jessica Yu
2020-08-11 16:57               ` Will Deacon
2020-08-11 17:59               ` peterz
2020-08-11 21:29                 ` Peter Zijlstra
2020-08-12  8:56               ` Ard Biesheuvel
2020-08-12 10:40                 ` peterz
2020-08-12 11:41                   ` Jessica Yu
2020-08-12 13:14                     ` H.J. Lu
2020-08-12 12:56                   ` Will Deacon
2020-08-12 14:15                     ` Szabolcs Nagy
2020-08-12 16:00                       ` Jessica Yu [this message]
2020-08-12 16:37                         ` Ard Biesheuvel
2020-08-12 16:42                           ` Szabolcs Nagy
2020-08-13  9:00                             ` Will Deacon
2020-08-12 20:00                           ` Peter Zijlstra
2020-08-13  8:36                             ` Ard Biesheuvel
2020-08-13 13:04                               ` Jessica Yu
2020-08-13 13:07                                 ` Ard Biesheuvel
2020-08-21 12:20                                   ` Will Deacon
2020-08-21 12:27                                     ` Ard Biesheuvel
2020-08-21 12:30                                       ` Will Deacon
2020-08-22 13:47                                         ` Ard Biesheuvel
2020-08-24 15:24                                           ` Jessica Yu
2020-08-25  1:54                                             ` Masahiro Yamada
2020-08-31  9:46                                         ` Jessica Yu
2020-08-31 10:42                                           ` Masahiro Yamada
2020-08-31 13:25                                             ` Ard Biesheuvel
2020-08-31 15:31                                               ` Jessica Yu
2020-08-31 15:46                                               ` Masahiro Yamada
2020-09-03 12:37                                             ` Jessica Yu
2020-09-01 12:51                                           ` Will Deacon

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