[PATCH v2] arm64: add alignment fault hanling

[Robin Murphy]

On 16/02/16 10:31, Will Deacon wrote:
> On Tue, Feb 16, 2016 at 04:44:35AM +0000, EunTaik Lee wrote:
>> Userspace memory is mapped as below:
>> F2A7F000--F2A7FFFF Normal Memory
>> F2A80000--F2A80FFF Device nGnRnE
>>
>> And that userspace application makes a system call
>> as below:
>>
>> -009 |do_strncpy_from_user(inline)
>> -009 |strncpy_from_user()
>> -010 |getname_flags()
>> -011 |user_path_at_empty()
>> -012 |user_path_at()
>> -013 |SYSC_faccessat(inline)
>> -013 |sys_faccessat()
>> -014 |__sys_trace(asm)
>>   --> |exception
>>
>> The string spans from 0xF2A7FFC1 to 0xF2A7FFFB.
>>
>> When do_strncpy_from_user() reads the last (unsigned long)
>> value, the alignement fault is triggered. The 8 byte
>> from 0xF2A7FFC1 spans to the next page that is mapped as
>> Device nGnRnE, which does not allow an unaligned access,
>> causes the abort.
>>
>> The instruction which caused the alignment fault is registered
>> in the fixup table but the exception handler does not reach there.
>>
>> This patch registers a alignment fault handler and fixes up the
>> pc if appropriate.
>
> As discussed with Catalin previously, we should solve this by adding a
> guard page rather than handling the fault.

...especially since we may not even get a fault. See "Crossing a page 
boundary with different memory types or Shareability attributes" in the 
UNPREDICTABLE spec (K1.2.10 in the latest ARMv8 ARM).

Robin.

>
> Will
>
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