Re: [PATCH 3/3] x86/kasan: support KASAN_VMALLOC

[Andy Lutomirski <luto@amacapital.net>]

> On Jul 25, 2019, at 12:49 AM, Dmitry Vyukov <dvyukov@google.com> wrote:
> 
>> On Thu, Jul 25, 2019 at 7:55 AM Daniel Axtens <dja@axtens.net> wrote:
>> 
>> In the case where KASAN directly allocates memory to back vmalloc
>> space, don't map the early shadow page over it.
>> 
>> Not mapping the early shadow page over the whole shadow space means
>> that there are some pgds that are not populated on boot. Allow the
>> vmalloc fault handler to also fault in vmalloc shadow as needed.
>> 
>> Signed-off-by: Daniel Axtens <dja@axtens.net>
> 
> 
> Would it make things simpler if we pre-populate the top level page
> tables for the whole vmalloc region? That would be
> (16<<40)/4096/512/512*8 = 131072 bytes?
> The check in vmalloc_fault in not really a big burden, so I am not
> sure. Just brining as an option.

I prefer pre-populating them. In particular, I have already spent far too much time debugging the awful explosions when the stack doesn’t have KASAN backing, and the vmap stack code is very careful to pre-populate the stack pgds — vmalloc_fault fundamentally can’t recover when the stack itself isn’t mapped.

So the vmalloc_fault code, if it stays, needs some careful analysis to make sure it will actually survive all the various context switch cases.  Or you can pre-populate it.

> 
> Acked-by: Dmitry Vyukov <dvyukov@google.com>
> 
>> ---
>> arch/x86/Kconfig            |  1 +
>> arch/x86/mm/fault.c         | 13 +++++++++++++
>> arch/x86/mm/kasan_init_64.c | 10 ++++++++++
>> 3 files changed, 24 insertions(+)
>> 
>> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
>> index 222855cc0158..40562cc3771f 100644
>> --- a/arch/x86/Kconfig
>> +++ b/arch/x86/Kconfig
>> @@ -134,6 +134,7 @@ config X86
>>        select HAVE_ARCH_JUMP_LABEL
>>        select HAVE_ARCH_JUMP_LABEL_RELATIVE
>>        select HAVE_ARCH_KASAN                  if X86_64
>> +       select HAVE_ARCH_KASAN_VMALLOC          if X86_64
>>        select HAVE_ARCH_KGDB
>>        select HAVE_ARCH_MMAP_RND_BITS          if MMU
>>        select HAVE_ARCH_MMAP_RND_COMPAT_BITS   if MMU && COMPAT
>> diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
>> index 6c46095cd0d9..d722230121c3 100644
>> --- a/arch/x86/mm/fault.c
>> +++ b/arch/x86/mm/fault.c
>> @@ -340,8 +340,21 @@ static noinline int vmalloc_fault(unsigned long address)
>>        pte_t *pte;
>> 
>>        /* Make sure we are in vmalloc area: */
>> +#ifndef CONFIG_KASAN_VMALLOC
>>        if (!(address >= VMALLOC_START && address < VMALLOC_END))
>>                return -1;
>> +#else
>> +       /*
>> +        * Some of the shadow mapping for the vmalloc area lives outside the
>> +        * pgds populated by kasan init. They are created dynamically and so
>> +        * we may need to fault them in.
>> +        *
>> +        * You can observe this with test_vmalloc's align_shift_alloc_test
>> +        */
>> +       if (!((address >= VMALLOC_START && address < VMALLOC_END) ||
>> +             (address >= KASAN_SHADOW_START && address < KASAN_SHADOW_END)))
>> +               return -1;
>> +#endif
>> 
>>        /*
>>         * Copy kernel mappings over when needed. This can also
>> diff --git a/arch/x86/mm/kasan_init_64.c b/arch/x86/mm/kasan_init_64.c
>> index 296da58f3013..e2fe1c1b805c 100644
>> --- a/arch/x86/mm/kasan_init_64.c
>> +++ b/arch/x86/mm/kasan_init_64.c
>> @@ -352,9 +352,19 @@ void __init kasan_init(void)
>>        shadow_cpu_entry_end = (void *)round_up(
>>                        (unsigned long)shadow_cpu_entry_end, PAGE_SIZE);
>> 
>> +       /*
>> +        * If we're in full vmalloc mode, don't back vmalloc space with early
>> +        * shadow pages.
>> +        */
>> +#ifdef CONFIG_KASAN_VMALLOC
>> +       kasan_populate_early_shadow(
>> +               kasan_mem_to_shadow((void *)VMALLOC_END+1),
>> +               shadow_cpu_entry_begin);
>> +#else
>>        kasan_populate_early_shadow(
>>                kasan_mem_to_shadow((void *)PAGE_OFFSET + MAXMEM),
>>                shadow_cpu_entry_begin);
>> +#endif
>> 
>>        kasan_populate_shadow((unsigned long)shadow_cpu_entry_begin,
>>                              (unsigned long)shadow_cpu_entry_end, 0);
>> --
>> 2.20.1
>> 
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