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

[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.
>

No worries - I'll have another crack at prepopulating them for v2. 

I tried prepopulating them at first, but because I'm really a powerpc
developer rather than an x86 developer (and because I find mm code
confusing at the best of times) I didn't have a lot of luck. I think on
reflection I stuffed up the pgd/p4d stuff and I think I know how to fix
it. So I'll give it another go and ask for help here if I get stuck :)

Regards,
Daniel


>> 
>> 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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