Re: [PATCH] arm64: kpti: Update arm64_kernel_use_ng_mappings() when forced on

[Hanjun Guo <guohanjun@huawei.com>]

On 2019/3/1 21:02, Will Deacon wrote:
> On Fri, Mar 01, 2019 at 08:59:19PM +0800, Hanjun Guo wrote:
>> On 2019/3/1 19:46, Ard Biesheuvel wrote:
>>> On Fri, 1 Mar 2019 at 12:45, Will Deacon <will.deacon@arm.com> wrote:
>>>>
>>>> On Fri, Mar 01, 2019 at 12:43:01PM +0100, Ard Biesheuvel wrote:
>>>>> On Fri, 1 Mar 2019 at 12:35, Will Deacon <will.deacon@arm.com> wrote:
>>>>>>
>>>>>> On Fri, Mar 01, 2019 at 11:18:21AM +0000, John Garry wrote:
>>>>>>> On 16/01/2019 09:38, John Garry wrote:
>>>>>>>> On 15/01/2019 18:49, James Morse wrote:
>>>>>>>>> Since commit b89d82ef01b3 ("arm64: kpti: Avoid rewriting early page
>>>>>>>>
>>>>>>>> About b89d82ef01b3, I got an *unconfirmed* (emphasis on this, I don't
>>>>>>>> want to cry wolf) report yesterday that the symptom I saw (boot delay)
>>>>>>>> has been seen on 5.0-rc2 on our D06 board. I could not see it.
>>>>>>>>
>>>>>>>> Please note that this would be same board which we saw this on:
>>>>>>>> https://lkml.org/lkml/2018/6/20/589
>>>>>>>>
>>>>>>>> I only witnessed the issue on the predecessor A72-based D05 board.
>>>>>>>>
>>>>>>>> The reporter is very busy, but we'll keep tabs on it.
>>>>>>>>
>>>>>>>
>>>>>>> FYI, this just came in this morning:
>>>>>>
>>>>>> This isn't completely surprising, because the kernel doesn't know about
>>>>>> the CPU in D06:
>>>>>>
>>>>>>> [    0.000000] Booting Linux on physical CPU 0x0000010000 [0x480fd010]
>>>>>>
>>>>>> [...]
>>>>>>
>>>>>>> [    0.000000] CPU features: detected: Kernel page table isolation (KPTI)
>>>>>>
>>>>>> If the CPU is not affected by meltdown, you can add the MIDR above to the
>>>>>> kpti_safe_list[] table. If it *is* affected by meltdown, then I'm afraid
>>>>>> that you're stuck with the delay at boot for !KASLR kernels (which is an
>>>>>> awful lot better than getting pwned by userspace :)
>>>>>>
>>>>>> I think D06 is 8.2, so I would've hoped that you'd implemented
>>>>>> AA64PFR0_EL1.CSV3, which would make this MIDR-based whitelisting
>>>>>> unnecessary.
>>
>> D06 has the function of AA64PFR0_EL1.CSV3 but didn't set the bit, and
>> it's not affected by meltdown...
>>
>>>>>>
>>>>>
>>>>> Note that this is also still tied to the new rodata handling. The new
>>>>> default is rodata=full, and reverting back to rodata=on (which used to
>>>>> be the default in prior releases) or rodata=off (not recommended)
>>>>> should get around this as well.
>>>>
>>>> Or, somewhat perversely, enabling CONFIG_RANDOMIZE_BASE and passing a
>>>> valid kaslr seed. Performance /and/ security!
>>>>
>>>
>>> Yeah but unfortunately, it seems the UEFI on that box does not
>>> implement EFI_RNG_PROTOCOL.
>>
>> Eagle eye :) So if we implement EFI_RNG_PROTOCOL (will take some time) in
>> firmware and enable CONFIG_RANDOMIZE_BASE, the issue will gone, right?
> 
> Yup, but you can also update the MIDR whitelist since you're not affected by
> meltdown.

Thanks for the guidance, Performance /and/ security is what I prefer :)

I take a look at the code, the logic in unmap_kernel_at_el0() shows that
even I add D06's CPU in kpti_safe_list[] table, I can still enable
CONFIG_RANDOMIZE_BASE + UEFI EFI_RNG_PROTOCOL to get performance and
security.

So it would be useful to prepare a patch to add D06 CPU's MIDR to
kpti_safe_list[] table, to prevent some mis-use case such as above.

Thanks
Hanjun


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