linux-kernel - Re: [PATCH v5 1/5] arm64: Add utilities to save restore pointer authentication keys

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Message-ID: <adbeb25c-fa88-41fd-7819-5f412a762b64@arm.com>
Date:   Wed, 13 Feb 2019 17:32:37 +0000
From:   Kristina Martsenko <kristina.martsenko@....com>
To:     James Morse <james.morse@....com>,
        Amit Daniel Kachhap <amit.kachhap@....com>
Cc:     linux-arm-kernel@...ts.infradead.org,
        Marc Zyngier <marc.zyngier@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        kvmarm@...ts.cs.columbia.edu,
        Ramana Radhakrishnan <ramana.radhakrishnan@....com>,
        Dave Martin <Dave.Martin@....com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 1/5] arm64: Add utilities to save restore pointer
 authentication keys

On 31/01/2019 16:20, James Morse wrote:
> Hi Amit,
> 
> On 28/01/2019 06:58, Amit Daniel Kachhap wrote:
>> The keys can be switched either inside an assembly or such
>> functions which do not have pointer authentication checks, so a GCC
>> attribute is added to enable it.
>>
>> A function ptrauth_keys_store is added which is similar to existing
>> function ptrauth_keys_switch but saves the key values in memory.
>> This may be useful for save/restore scenarios when CPU changes
>> privilege levels, suspend/resume etc.
> 
> 
>> diff --git a/arch/arm64/include/asm/pointer_auth.h b/arch/arm64/include/asm/pointer_auth.h
>> index 15d4951..98441ce 100644
>> --- a/arch/arm64/include/asm/pointer_auth.h
>> +++ b/arch/arm64/include/asm/pointer_auth.h
>> @@ -11,6 +11,13 @@
>>  
>>  #ifdef CONFIG_ARM64_PTR_AUTH
>>  /*
>> + * Compile the function without pointer authentication instructions. This
>> + * allows pointer authentication to be enabled/disabled within the function
>> + * (but leaves the function unprotected by pointer authentication).
>> + */
>> +#define __no_ptrauth	__attribute__((target("sign-return-address=none")))
> 
> The documentation[0] for this says 'none' is the default. Will this only
> take-effect once the kernel supports pointer-auth for the host? (Is this just
> documentation until then?)

Yes, I don't think this should be in this series, since we're not
building the kernel with pointer auth yet.

> 
> ('noptrauth' would fit with 'notrace' slightly better)

(But worse with e.g. __noreturn, __notrace_funcgraph, __init,
__always_inline, __exception. Not sure what the pattern is. Would
__noptrauth be better?)

Thanks,
Kristina

> 
> [0]
> https://gcc.gnu.org/onlinedocs/gcc/AArch64-Function-Attributes.html#AArch64-Function-Attributes
>