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From:   Lecopzer Chen <lecopzer@gmail.com>
To:     will@kernel.org
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Subject: Re: [PATCH v2 1/4] arm64: kasan: don't populate vmalloc area for CONFIG_KASAN_VMALLOC
Date:   Fri,  5 Feb 2021 00:37:21 +0800
Message-Id: <20210204163721.91295-1-lecopzer@gmail.com>
In-Reply-To: <20210204150100.GE20815@willie-the-truck>
References: <20210204150100.GE20815@willie-the-truck>
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> On Thu, Feb 04, 2021 at 10:46:12PM +0800, Lecopzer Chen wrote:
> > > 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).
>
> The shadow is allocated dynamically though, isn't it?

Yes, but It's still a cost.

> > There should be someone can enable KASAN_GENERIC but can't use VMALLOC
> > due to memory issue.
>
> That doesn't sound particularly realistic to me. The reason I'm pushing here
> is because I would _really_ like to move to VMAP stack unconditionally, and
> that would effectively force KASAN_VMALLOC to be set if KASAN is in use.
>
> So unless there's a really good reason not to do that, please can we make
> this unconditional for arm64? Pretty please?

I think it's fine since we have a good reason.
Also if someone have memory issue in KASAN_VMALLOC,
they can use SW_TAG, right?

However the SW_TAG/HW_TAG is not supported VMALLOC yet.
So the code would be like

	if (IS_ENABLED(CONFIG_KASAN_GENERIC))
		/* explain the relationship between 
		 * KASAN_GENERIC and KASAN_VMALLOC in arm64
		 * XXX: because we want VMAP stack....
		 */
		kasan_populate_early_shadow((void *)vmalloc_shadow_end,
					    (void *)KASAN_SHADOW_END);
	else {
		kasan_populate_early_shadow((void *)kimg_shadow_end,
					    (void *)KASAN_SHADOW_END);
		if (kimg_shadow_start > mod_shadow_end)
			kasan_populate_early_shadow((void *)mod_shadow_end,
						    (void *)kimg_shadow_start);
	}

and the arch/arm64/Kconfig will add
	select KASAN_VMALLOC if KASAN_GENERIC

Is this code same as your thought?

BRs,
Lecopzer