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

From: Lecopzer Chen <lecopzer@gmail.com>
To: will@kernel.org
Cc: akpm@linux-foundation.org, andreyknvl@google.com,
	ardb@kernel.org, aryabinin@virtuozzo.com, broonie@kernel.org,
	catalin.marinas@arm.com, dan.j.williams@intel.com,
	dvyukov@google.com, glider@google.com, gustavoars@kernel.org,
	kasan-dev@googlegroups.com, lecopzer.chen@mediatek.com,
	lecopzer@gmail.com, linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org, linux-mediatek@lists.infradead.org,
	linux-mm@kvack.org, linux@roeck-us.net, robin.murphy@arm.com,
	rppt@kernel.org, tyhicks@linux.microsoft.com,
	vincenzo.frascino@arm.com, yj.chiang@mediatek.com
Subject: Re: [PATCH v2 1/4] arm64: kasan: don't populate vmalloc area for CONFIG_KASAN_VMALLOC
Date: Thu,  4 Feb 2021 22:46:12 +0800	[thread overview]
Message-ID: <20210204144612.75582-1-lecopzer@gmail.com> (raw)
In-Reply-To: <20210204124543.GA20468@willie-the-truck>

> 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