[PATCH] kasan: Always respect CONFIG_KASAN_STACK

[Alexandre Ghiti]

Hi,

On Fri, Oct 15, 2021 at 3:08 PM Alexandre ghiti <alex at ghiti.fr> wrote:
>
> 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 at 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 at 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 at kernel.org>
> >>>>>
> >>>>> Reviewed-by: Marco Elver <elver at 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 at 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.

@kasan: Any idea what we are doing wrong in riscv to encounter the
above situation?

Thanks,

Alex

>
> 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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