Re: [PATCH v2 1/4] arm64: kasan: don't populate vmalloc area for CONFIG_KASAN_VMALLOC

Lecopzer Chen
2021-02-04T14:49:11Z

> On Sat, Jan 09, 2021 at 06:32:49PM +0800, Lecopzer Chen wrote:
> > Linux support KAsan for VMALLOC since commit 3c5c3cfb9ef4da9
> > ("kasan: support backing vmalloc space with real shadow memory")
> >
> > Like how the MODULES_VADDR does now, just not to early populate
> > the VMALLOC_START between VMALLOC_END.
> > similarly, the kernel code mapping is now in the VMALLOC area and
> > should keep these area populated.
> >
> > Signed-off-by: Lecopzer Chen <lecopzer.chen@mediatek.com>
> > ---
> > arch/arm64/mm/kasan_init.c | 23 ++++++++++++++++++-----
> > 1 file changed, 18 insertions(+), 5 deletions(-)
> >
> > diff --git a/arch/arm64/mm/kasan_init.c b/arch/arm64/mm/kasan_init.c
> > index d8e66c78440e..39b218a64279 100644
> > --- a/arch/arm64/mm/kasan_init.c
> > +++ b/arch/arm64/mm/kasan_init.c
> > @@ -214,6 +214,7 @@ static void __init kasan_init_shadow(void)
> > {
> > u64 kimg_shadow_start, kimg_shadow_end;
> > u64 mod_shadow_start, mod_shadow_end;
> > + u64 vmalloc_shadow_start, vmalloc_shadow_end;
> > phys_addr_t pa_start, pa_end;
> > u64 i;
> >
> > @@ -223,6 +224,9 @@ static void __init kasan_init_shadow(void)
> > mod_shadow_start = (u64)kasan_mem_to_shadow((void *)MODULES_VADDR);
> > mod_shadow_end = (u64)kasan_mem_to_shadow((void *)MODULES_END);
> >
> > + vmalloc_shadow_start = (u64)kasan_mem_to_shadow((void *)VMALLOC_START);
> > + vmalloc_shadow_end = (u64)kasan_mem_to_shadow((void *)VMALLOC_END);
> > +
> > /*
> > * We are going to perform proper setup of shadow memory.
> > * At first we should unmap early shadow (clear_pgds() call below).
> > @@ -241,12 +245,21 @@ static void __init kasan_init_shadow(void)
> >
> > kasan_populate_early_shadow(kasan_mem_to_shadow((void *)PAGE_END),
> > (void *)mod_shadow_start);
> > - kasan_populate_early_shadow((void *)kimg_shadow_end,
> > - (void *)KASAN_SHADOW_END);
> > + if (IS_ENABLED(CONFIG_KASAN_VMALLOC)) {
>
> Do we really need yet another CONFIG option for KASAN? What's the use-case
> for *not* enabling this if you're already enabling one of the KASAN
> backends?

As I know, KASAN_VMALLOC now only supports KASAN_GENERIC and also
KASAN_VMALLOC uses more memory to map real shadow memory (1/8 of vmalloc va).

There should be someone can enable KASAN_GENERIC but can't use VMALLOC
due to memory issue.

> > + kasan_populate_early_shadow((void *)vmalloc_shadow_end,
> > + (void *)KASAN_SHADOW_END);
> > + if (vmalloc_shadow_start > mod_shadow_end)
>
> To echo Ard's concern: when is the above 'if' condition true?

After reviewing this code,
since VMALLOC_STAR is a compiler defined macro of MODULES_END,
this if-condition will never be true.

I also test it with removing this and works fine.

I'll remove this in the next version patch,
thanks a lot for pointing out this.

BRs,
Lecopzer