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SubjectRe: [PATCH v2] module: Harden STRICT_MODULE_RWX
+++ Mauro Carvalho Chehab [11/08/20 17:27 +0200]:
>Em Tue, 11 Aug 2020 16:55:24 +0200
>peterz@infradead.org escreveu:
>
>> On Tue, Aug 11, 2020 at 04:34:27PM +0200, Mauro Carvalho Chehab wrote:
>> > [33] .plt PROGBITS 0000000000000340 00035c80
>> > 0000000000000001 0000000000000000 WAX 0 0 1
>> > [34] .init.plt NOBITS 0000000000000341 00035c81
>> > 0000000000000001 0000000000000000 WA 0 0 1
>> > [35] .text.ftrace[...] PROGBITS 0000000000000342 00035c81
>> > 0000000000000001 0000000000000000 WAX 0 0 1
>>
>> .plt and .text.ftrace_tramplines are buggered.
>>
>> arch/arm64/kernel/module.lds even marks then as NOLOAD.
>
>Hmm... Shouldn't the code at module_enforce_rwx_sections() or at
>load_module() ignore such sections instead of just rejecting probing
>all modules?
>
>I mean, if the existing toolchain is not capable of excluding
>those sections, either the STRICT_MODULE_RWX hardening should be
>disabled, if a broken toolchain is detected or some runtime code
>should handle such sections on a different way.

Hi Mauro, thanks for providing the readelf output. The sections marked 'WAX'
are indeed the reason why the module loader is rejecting them.

Interesting, my cross-compiled modules do not have the executable flag -

[38] .plt NOBITS 0000000000000340 00038fc0
0000000000000001 0000000000000000 WA 0 0 1
[39] .init.plt NOBITS 0000000000000341 00038fc0
0000000000000001 0000000000000000 WA 0 0 1
[40] .text.ftrace_tram NOBITS 0000000000000342 00038fc0
0000000000000001 0000000000000000 WA 0 0 1

ld version:

GNU ld (GNU Binutils) 2.34
Copyright (C) 2020 Free Software Foundation, Inc.
This program is free software; you may redistribute it under the terms of
the GNU General Public License version 3 or (at your option) a later version.

And gcc:

aarch64-linux-gcc (GCC) 9.3.0
Copyright (C) 2019 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

I'm a bit confused about what NOLOAD actually implies in this context. From the
ld documentation - "The `(NOLOAD)' directive will mark a section to not be
loaded at run time." But these sections are marked SHF_ALLOC and are referenced
to in the module plt code. Or does it just tell the linker to not initialize it?

Adding Ard to CC, I'm sure he'd know more about the section flag specifics.

Jessica

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