linux-kernel - Re: [PATCH v5 3/3] powerpc/32: Add KASAN support

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Message-ID: <8654376c-2c55-89b0-cd79-0bcf02338519@c-s.fr>
Date:   Tue, 19 Feb 2019 19:03:38 +0100
From:   Christophe Leroy <christophe.leroy@....fr>
To:     Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Nicholas Piggin <npiggin@...il.com>,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Alexander Potapenko <glider@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Daniel Axtens <dja@...ens.net>
Cc:     linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        kasan-dev@...glegroups.com, linux-mm@...ck.org
Subject: Re: [PATCH v5 3/3] powerpc/32: Add KASAN support



Le 18/02/2019 à 10:27, Michael Ellerman a écrit :
> Christophe Leroy <christophe.leroy@....fr> writes:
> 
>> diff --git a/arch/powerpc/include/asm/ppc_asm.h b/arch/powerpc/include/asm/ppc_asm.h
>> index e0637730a8e7..dba2c1038363 100644
>> --- a/arch/powerpc/include/asm/ppc_asm.h
>> +++ b/arch/powerpc/include/asm/ppc_asm.h
>> @@ -251,6 +251,10 @@ GLUE(.,name):
>>   
>>   #define _GLOBAL_TOC(name) _GLOBAL(name)
>>   
>> +#define KASAN_OVERRIDE(x, y) \
>> +	.weak x;	     \
>> +	.set x, y
>> +
> 
> Can you add a comment describing what that does and why?

It's gone. Hope the new approach is more clear. It's now in a dedicated 
patch.

> 
>> diff --git a/arch/powerpc/kernel/Makefile b/arch/powerpc/kernel/Makefile
>> index 879b36602748..fc4c42262694 100644
>> --- a/arch/powerpc/kernel/Makefile
>> +++ b/arch/powerpc/kernel/Makefile
>> @@ -16,8 +16,9 @@ CFLAGS_prom_init.o      += -fPIC
>>   CFLAGS_btext.o		+= -fPIC
>>   endif
>>   
>> -CFLAGS_cputable.o += $(DISABLE_LATENT_ENTROPY_PLUGIN)
>> -CFLAGS_prom_init.o += $(DISABLE_LATENT_ENTROPY_PLUGIN)
>> +CFLAGS_early_32.o += -DDISABLE_BRANCH_PROFILING
>> +CFLAGS_cputable.o += $(DISABLE_LATENT_ENTROPY_PLUGIN) -DDISABLE_BRANCH_PROFILING
>> +CFLAGS_prom_init.o += $(DISABLE_LATENT_ENTROPY_PLUGIN) -DDISABLE_BRANCH_PROFILING
> 
> Why do we need to disable branch profiling now?

Recommended by Andrey, see https://patchwork.ozlabs.org/patch/1023887/

Maybe it should be only when KASAN is active ? For prom_init it should 
probably be all the time, for the others I don't know. Can't remember 
why I did it that way.

> 
> I'd probably be happier if all the CFLAGS changes were done in a leadup
> patch to make them more obvious.

Oops, I forgot to read your mail entirely before sending out v6. Indeed 
I only read first part. Anyway, that's probably not the last run.

> 
>> diff --git a/arch/powerpc/kernel/prom_init_check.sh b/arch/powerpc/kernel/prom_init_check.sh
>> index 667df97d2595..da6bb16e0876 100644
>> --- a/arch/powerpc/kernel/prom_init_check.sh
>> +++ b/arch/powerpc/kernel/prom_init_check.sh
>> @@ -16,8 +16,16 @@
>>   # If you really need to reference something from prom_init.o add
>>   # it to the list below:
>>   
>> +grep CONFIG_KASAN=y .config >/dev/null
> 
> Just to be safe "^CONFIG_KASAN=y$" ?

ok

> 
>> +if [ $? -eq 0 ]
>> +then
>> +	MEMFCT="__memcpy __memset"
>> +else
>> +	MEMFCT="memcpy memset"
>> +fi
> 
> MEM_FUNCS ?

Yes, I change it now before I forget.

> 
>> diff --git a/arch/powerpc/lib/Makefile b/arch/powerpc/lib/Makefile
>> index 3bf9fc6fd36c..ce8d4a9f810a 100644
>> --- a/arch/powerpc/lib/Makefile
>> +++ b/arch/powerpc/lib/Makefile
>> @@ -8,6 +8,14 @@ ccflags-$(CONFIG_PPC64)	:= $(NO_MINIMAL_TOC)
>>   CFLAGS_REMOVE_code-patching.o = $(CC_FLAGS_FTRACE)
>>   CFLAGS_REMOVE_feature-fixups.o = $(CC_FLAGS_FTRACE)
>>   
>> +KASAN_SANITIZE_code-patching.o := n
>> +KASAN_SANITIZE_feature-fixups.o := n
>> +
>> +ifdef CONFIG_KASAN
>> +CFLAGS_code-patching.o += -DDISABLE_BRANCH_PROFILING
>> +CFLAGS_feature-fixups.o += -DDISABLE_BRANCH_PROFILING
>> +endif
> 
> There's that branch profiling again, though here it's only if KASAN is enabled.
> 
>> diff --git a/arch/powerpc/mm/kasan_init.c b/arch/powerpc/mm/kasan_init.c
>> new file mode 100644
>> index 000000000000..bd8e0a263e12
>> --- /dev/null
>> +++ b/arch/powerpc/mm/kasan_init.c
>> @@ -0,0 +1,114 @@
>> +// SPDX-License-Identifier: GPL-2.0
>> +
>> +#define DISABLE_BRANCH_PROFILING
>> +
>> +#include <linux/kasan.h>
>> +#include <linux/printk.h>
>> +#include <linux/memblock.h>
>> +#include <linux/sched/task.h>
>> +#include <asm/pgalloc.h>
>> +
>> +void __init kasan_early_init(void)
>> +{
>> +	unsigned long addr = KASAN_SHADOW_START;
>> +	unsigned long end = KASAN_SHADOW_END;
>> +	unsigned long next;
>> +	pmd_t *pmd = pmd_offset(pud_offset(pgd_offset_k(addr), addr), addr);
> 
> Can none of those fail?

map_kernel_page() in pgtable_32.c does exactly the same.

pud_offset() and pmd_offset() are no-ops and only serve as type 
modifiers, so pmd will get the value returned by pgd_offset_k() which 
should always be valid unless init_mm->pgd is bad.

Christophe

> 
> 
> cheers
>