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Subject: Re: [PATCH] kasan: Always respect CONFIG_KASAN_STACK
From:   Alexandre ghiti <alex@ghiti.fr>
To:     Nathan Chancellor <nathan@kernel.org>,
        Palmer Dabbelt <palmer@dabbelt.com>
Cc:     elver@google.com, akpm@linux-foundation.org,
        ryabinin.a.a@gmail.com, glider@google.com, andreyknvl@gmail.com,
        dvyukov@google.com, ndesaulniers@google.com,
        Arnd Bergmann <arnd@arndb.de>, kasan-dev@googlegroups.com,
        linux-kernel@vger.kernel.org, llvm@lists.linux.dev,
        linux-riscv@lists.infradead.org,
        Paul Walmsley <paul.walmsley@sifive.com>,
        aou@eecs.berkeley.edu, linux-mm@kvack.org
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Date:   Fri, 15 Oct 2021 15:04:51 +0200
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On 10/14/21 8:31 PM, Alex Ghiti wrote:
> Hi Nathan,
>
> Le 14/10/2021 à 18:55, Nathan Chancellor a écrit :
>> On Fri, Oct 08, 2021 at 11:46:55AM -0700, Palmer Dabbelt wrote:
>>> On Thu, 23 Sep 2021 07:59:46 PDT (-0700), nathan@kernel.org wrote:
>>>> On Thu, Sep 23, 2021 at 12:07:17PM +0200, Marco Elver wrote:
>>>>> On Wed, 22 Sept 2021 at 22:55, Nathan Chancellor
>>>>> <nathan@kernel.org> wrote:
>>>>>> Currently, the asan-stack parameter is only passed along if
>>>>>> CFLAGS_KASAN_SHADOW is not empty, which requires
>>>>>> KASAN_SHADOW_OFFSET to
>>>>>> be defined in Kconfig so that the value can be checked. In RISC-V's
>>>>>> case, KASAN_SHADOW_OFFSET is not defined in Kconfig, which means
>>>>>> that
>>>>>> asan-stack does not get disabled with clang even when
>>>>>> CONFIG_KASAN_STACK
>>>>>> is disabled, resulting in large stack warnings with allmodconfig:
>>>>>>
>>>>>> drivers/video/fbdev/omap2/omapfb/displays/panel-lgphilips-lb035q02.c:117:12:
>>>>>>
>>>>>> error: stack frame size (14400) exceeds limit (2048) in function
>>>>>> 'lb035q02_connect' [-Werror,-Wframe-larger-than]
>>>>>> static int lb035q02_connect(struct omap_dss_device *dssdev)
>>>>>>             ^
>>>>>> 1 error generated.
>>>>>>
>>>>>> Ensure that the value of CONFIG_KASAN_STACK is always passed
>>>>>> along to
>>>>>> the compiler so that these warnings do not happen when
>>>>>> CONFIG_KASAN_STACK is disabled.
>>>>>>
>>>>>> Link: https://github.com/ClangBuiltLinux/linux/issues/1453
>>>>>> References: 6baec880d7a5 ("kasan: turn off asan-stack for clang-8
>>>>>> and earlier")
>>>>>> Signed-off-by: Nathan Chancellor <nathan@kernel.org>
>>>>>
>>>>> Reviewed-by: Marco Elver <elver@google.com>
>>>>
>>>> Thanks!
>>>>
>>>>> [ Which tree are you planning to take it through? ]
>>>>
>>>> Gah, I was intending for it to go through -mm, then I cc'd neither
>>>> Andrew nor linux-mm... :/ Andrew, do you want me to resend or can you
>>>> grab it from LKML?
>>>
>>> Acked-by: Palmer Dabbelt <palmerdabbelt@google.com>
>>>
>>> (assuming you still want it through somewhere else)
>>
>> Thanks, it is now in mainline as commit 19532869feb9 ("kasan: always
>> respect CONFIG_KASAN_STACK").
>>
>>>>> Note, arch/riscv/include/asm/kasan.h mentions KASAN_SHADOW_OFFSET in
>>>>> comment (copied from arm64). Did RISC-V just forget to copy over the
>>>>> Kconfig option?
>>>>
>>>> I do see it defined in that file as well but you are right that
>>>> they did
>>>> not copy the Kconfig logic, even though it was present in the tree
>>>> when
>>>> RISC-V KASAN was implemented. Perhaps they should so that they get
>>>> access to the other flags in the "else" branch?
>>>
>>> Ya, looks like we just screwed this up.  I'm seeing some warnings like
>>>
>>>     cc1: warning: ‘-fsanitize=kernel-address’ with stack protection
>>> is not supported without ‘-fasan-shadow-offset=’ for this target
>>
>> Hmmm, I thought I did a GCC build with this change but I must not have
>> :/
>>
>>> which is how I ended up here, I'm assuming that's what you're
>>> talking about
>>> here?  LMK if you were planning on sending along a fix or if you
>>> want me to
>>> go figure it out.
>>
>> I took a look at moving the logic into Kconfig like arm64 before sending
>> this change and I did not really understand it well enough to do so. I
>> think it would be best if you were able to do that so that nothing gets
>> messed up.
>>
>
> I'll do it tomorrow, I'm the last one who touched kasan on riscv :)
>

Adding KASAN_SHADOW_OFFSET config makes kasan kernel fails to boot.
It receives a *write* fault at the beginning of a memblock_alloc
function while populating the kernel shadow memory: the trap address is
in the kasan shadow virtual address range and this corresponds to a
kernel address in init_stack. The question is: how do I populate the
stack shadow mapping without using memblock API? It's weird, I don't
find anything on other architectures.

And just a short note: I have realized this will break with the sv48
patchset as we decide at runtime the address space width and the kasan
shadow start address is different between sv39 and sv48. I will have to
do like x86 and move the kasan shadow start at the end of the address
space so that it is the same for both sv39 and sv48.

Thanks,

Alex


> Thanks,
>
> Alex
>
>> Cheers,
>> Nathan
>>
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