Subject: Re: [PATCH v7 11/11] arm64: scs: add shadow stacks for SDEI

From: James Morse <[email-masked]>
Date: Fri, 14 Feb 2020 18:13:15 +0000

Hi Sami,

On 12/02/2020 20:59, Sami Tolvanen wrote:
> On Tue, Feb 11, 2020 at 5:57 AM James Morse <[email-masked]> wrote:
>> On 28/01/2020 18:49, Sami Tolvanen wrote:
>>> This change adds per-CPU shadow call stacks for the SDEI handler.
>>> Similarly to how the kernel stacks are handled, we add separate shadow
>>> stacks for normal and critical events.
>>
>> Reviewed-by: James Morse <[email-masked]>
>> Tested-by: James Morse <[email-masked]>

>>> diff --git a/arch/arm64/kernel/scs.c b/arch/arm64/kernel/scs.c
>>> index eaadf5430baa..dddb7c56518b 100644
>>> --- a/arch/arm64/kernel/scs.c
>>> +++ b/arch/arm64/kernel/scs.c
>>
>>> +static int scs_alloc_percpu(unsigned long * __percpu *ptr, int cpu)
>>> +{
>>> +     unsigned long *p;
>>> +
>>> +     p = __vmalloc_node_range(PAGE_SIZE, SCS_SIZE,
>>> +                              VMALLOC_START, VMALLOC_END,
>>> +                              GFP_SCS, PAGE_KERNEL,
>>> +                              0, cpu_to_node(cpu),
>>> +                              __builtin_return_address(0));
>>
>> (What makes this arch specific? arm64 has its own calls like this for the regular vmap
>> stacks because it plays tricks with the alignment. Here the alignment requirement comes
>> from the core SCS code... Would another architecture implement these
>> scs_alloc_percpu()/scs_free_percpu() differently?)
> 
> You are correct, these aren't necessarily specific to arm64. However,
> right now, we are not allocating per-CPU shadow stacks anywhere else,
> so this was a natural place for the helper functions.

Fair enough,

> Would you prefer me to move these to kernel/scs.c instead?

I have no preference, as long as they don't get duplicated later!

Thanks,

James

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