linux-kernel - Re: [PATCH V7 04/10] arm64: exception: handle Synchronous External Abort

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Date:   Mon, 23 Jan 2017 10:01:14 +0000
From:   James Morse <james.morse@....com>
To:     "Baicar, Tyler" <tbaicar@...eaurora.org>
CC:     christoffer.dall@...aro.org, marc.zyngier@....com,
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Subject: Re: [PATCH V7 04/10] arm64: exception: handle Synchronous External
 Abort

Hi Tyler,

On 20/01/17 20:35, Baicar, Tyler wrote:
> On 1/19/2017 10:55 AM, James Morse wrote:
>> On 18/01/17 23:26, Baicar, Tyler wrote:
>>> On 1/17/2017 3:31 AM, James Morse wrote:
>>>> On 12/01/17 18:15, Tyler Baicar wrote:
>>>>> +    info.si_addr  = (void __user *)addr;
>>>> addr here was read from FAR_EL1, but for some of the classes of exception you
>>>> have listed below this register isn't updated with the faulting address.
>>>>
>>>> The ARM-ARM version 'k' in D1.10.5 "Summary of registers on faults taken to an
>>>> Exception level that is using Aarch64" has:
>>>>> The architecture permits that the FAR_ELx is UNKNOWN for Synchronous External
>>>>> Aborts other than Synchronous External Aborts on Translation Table Walks. In
>>>>> this case, the ISS.FnV bit returned in ESR_ELx  indicates whether FAR_ELx is
>>>>> valid.
>>>> This is a problem if we get 'synchronous external abort' or 'synchronous parity
>>>> error' while a user space process was running.
>>> It looks like this would just cause an incorrect address to be printed in the
>>> above pr_err.
>>> Unless I'm missing something, I don't see arm64_notify_die or anything that gets
>>> called from
>>> there using the info.si_addr variable.
>> I may be misreading something here...
>>
>> This patch has:
>>>     info.si_addr  = (void __user *)addr;
>>>     arm64_notify_die("", regs, &info, esr);
>>  From arch/arm64/kernel/traps.c:arm64_notify_die():
>>>     if (user_mode(regs)) {
>>>         current->thread.fault_address = 0;
>>>         current->thread.fault_code = err;
>>>         force_sig_info(info->si_signo, info, current);
>>>     }
>> So if the SEA interrupted userspace, we put maybe-unknown addr into
>> force_sig_info() to deliver a signal to user space. User-space then gets a copy
>> of the info struct containing the maybe-unknown addr.
>>
>> I think this is an existing bug, but if we are separating the synchronous
>> external aborts from the generic do_bad handler, we should probably check the
>> FnV bit. (I think we should still print it out)
>>
>>
>>> What do you suggest I do here? The firmware should be reporting the physical and
>>> virtual
>>> address information if it is available in the HEST entry that the kernel will
>>> parse.
>> Its not just firmware that may trigger this, other SoCs may use it for parity or
>> ECC errors, and they may not always have a valid address in FAR_EL1.
>>
>> I think we should check the FnV bit in the esr variable and set info.si_addr to
>> 0 if the addr we have isn't valid:
>> 'For some implementations, the value of si_addr may be inaccurate.' [0]

> Okay, that makes sense, we don't want userspace to be notified with an incorrect
> address.
> I will add the check to verify it's valid. Which bit in the ESR is the FnV bit?
> I'm not finding
> the bit breakdown of the ISS that shows it.

The bits in ISS vary depending on the EC, so a little digging is required.
"D7.2.27 ESR_ELx, Exception Syndrome Register (ELx)" lists the EC values, from
there 'Instruction Abort' and 'Data Abort' both list FnV as bit 10. Version 'k'
of the ARM-ARM has this on pages D7-1953 to D7-1956.


Thanks,

James