linux-kernel - Re: [PATCH v5 1/2] powerpc/32: add stack protector support

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Date:   Mon, 14 Jan 2019 11:46:40 +1100
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Samuel Holland <samuel@...lland.org>,
        Christophe Leroy <christophe.leroy@....fr>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>
Cc:     linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        "Jason A. Donenfeld" <Jason@...c4.com>,
        Alexey Kardashevskiy <aik@...abs.ru>
Subject: Re: [PATCH v5 1/2] powerpc/32: add stack protector support

Samuel Holland <samuel@...lland.org> writes:
> Hello all,
>
> On 09/27/18 02:05, Christophe Leroy wrote:
> [..snip..]
>> diff --git a/arch/powerpc/Makefile b/arch/powerpc/Makefile
>> index 07d9dce7eda6..45b8eb4d8fe7 100644
>> --- a/arch/powerpc/Makefile
>> +++ b/arch/powerpc/Makefile
>> @@ -112,6 +112,9 @@ KBUILD_LDFLAGS	+= -m elf$(BITS)$(LDEMULATION)
>>  KBUILD_ARFLAGS	+= --target=elf$(BITS)-$(GNUTARGET)
>>  endif
>>  
>> +cflags-$(CONFIG_STACKPROTECTOR)	+= -mstack-protector-guard=tls
>> +cflags-$(CONFIG_STACKPROTECTOR)	+= -mstack-protector-guard-reg=r2
>> +
>>  LDFLAGS_vmlinux-y := -Bstatic
>>  LDFLAGS_vmlinux-$(CONFIG_RELOCATABLE) := -pie
>>  LDFLAGS_vmlinux	:= $(LDFLAGS_vmlinux-y)
>> @@ -404,6 +407,13 @@ archclean:
>>  
>>  archprepare: checkbin
>>  
>> +ifdef CONFIG_STACKPROTECTOR
>> +prepare: stack_protector_prepare
>> +
>> +stack_protector_prepare: prepare0
>> +	$(eval KBUILD_CFLAGS += -mstack-protector-guard-offset=$(shell awk '{if ($$2 == "TASK_CANARY") print $$3;}' include/generated/asm-offsets.h))
>> +endif
>> +
>
> This breaks when building out-of-tree kernel modules. GCC is not getting passed
> the -mstack-protector-guard-offset argument, so the default offset is used. The
> kernel then panics the first time a function with stack protector is called.
>
> I'm seeing this on powerpc64. It looks like it was reported for powerpc on
> kernel bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=201891

Thanks for the bug report.

Alexey also hit this and sent an RFC fix.

  https://patchwork.ozlabs.org/patch/1022751/

But Masahiro thought we could do something less hacky, and hopefully he
can help us come up with something better.

cheers