linux-kernel - Re: [PATCH 11/11] arm64: Implement branch predictor hardening for affected Cortex-A CPUs

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Date:   Thu, 4 Jan 2018 17:14:07 +0000
From:   Marc Zyngier <marc.zyngier@....com>
To:     Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Will Deacon <will.deacon@....com>
Cc:     linux-arm-kernel@...ts.infradead.org,
        Catalin Marinas <catalin.marinas@....com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Christoffer Dall <christoffer.dall@...aro.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 11/11] arm64: Implement branch predictor hardening for
 affected Cortex-A CPUs

On 04/01/18 16:31, Ard Biesheuvel wrote:
> On 4 January 2018 at 15:08, Will Deacon <will.deacon@....com> wrote:
>> Cortex-A57, A72, A73 and A75 are susceptible to branch predictor aliasing
>> and can theoretically be attacked by malicious code.
>>
>> This patch implements a PSCI-based mitigation for these CPUs when available.
>> The call into firmware will invalidate the branch predictor state, preventing
>> any malicious entries from affecting other victim contexts.
>>
>> Signed-off-by: Marc Zyngier <marc.zyngier@....com>
>> Signed-off-by: Will Deacon <will.deacon@....com>
>> ---
>>  arch/arm64/kernel/bpi.S        | 24 ++++++++++++++++++++++++
>>  arch/arm64/kernel/cpu_errata.c | 42 ++++++++++++++++++++++++++++++++++++++++++
>>  2 files changed, 66 insertions(+)
>>
>> diff --git a/arch/arm64/kernel/bpi.S b/arch/arm64/kernel/bpi.S
>> index 06a931eb2673..2b10d52a0321 100644
>> --- a/arch/arm64/kernel/bpi.S
>> +++ b/arch/arm64/kernel/bpi.S
>> @@ -53,3 +53,27 @@ ENTRY(__bp_harden_hyp_vecs_start)
>>         vectors __kvm_hyp_vector
>>         .endr
>>  ENTRY(__bp_harden_hyp_vecs_end)
>> +ENTRY(__psci_hyp_bp_inval_start)
>> +       stp     x0, x1, [sp, #-16]!
>> +       stp     x2, x3, [sp, #-16]!
>> +       stp     x4, x5, [sp, #-16]!
>> +       stp     x6, x7, [sp, #-16]!
>> +       stp     x8, x9, [sp, #-16]!
>> +       stp     x10, x11, [sp, #-16]!
>> +       stp     x12, x13, [sp, #-16]!
>> +       stp     x14, x15, [sp, #-16]!
>> +       stp     x16, x17, [sp, #-16]!
>> +       stp     x18, x19, [sp, #-16]!
> 
> Would it be better to update sp only once here?

Maybe. I suppose that's quite uarch dependent, but worth trying.

> Also, do x18 and x19 need to be preserved/restored here?

My bad. I misread the SMCCC and though I needed to save it too. For the
reference, the text says:

"Registers  X18-X30 and stack pointers SP_EL0 and SP_ELx are saved by
the function that is called, and must be preserved over the SMC or HVC
call."

I'll amend the patch.

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny...