[PATCH v6 2/3] arm64: Introduce prctl(PR_PAC_{SET,GET}_ENABLED_KEYS)

[Will Deacon]

On Tue, Dec 29, 2020 at 10:59:14PM -0800, Peter Collingbourne wrote:
> This change introduces a prctl that allows the user program to control
> which PAC keys are enabled in a particular task. The main reason
> why this is useful is to enable a userspace ABI that uses PAC to
> sign and authenticate function pointers and other pointers exposed
> outside of the function, while still allowing binaries conforming
> to the ABI to interoperate with legacy binaries that do not sign or
> authenticate pointers.
> 
> The idea is that a dynamic loader or early startup code would issue
> this prctl very early after establishing that a process may load legacy
> binaries, but before executing any PAC instructions.
> 
> This change adds a small amount of overhead to kernel entry and exit
> due to additional required instruction sequences.
> 
> On a DragonBoard 845c (Cortex-A75) with the powersave governor, the
> overhead of similar instruction sequences was measured as 4.9ns when
> simulating the common case where IA is left enabled, or 43.7ns when
> simulating the uncommon case where IA is disabled. These numbers can
> be seen as the worst case scenario, since in more realistic scenarios
> a better performing governor would be used and a newer chip would be
> used that would support PAC unlike Cortex-A75 and would be expected
> to be faster than Cortex-A75.
> 
> On an Apple M1 under a hypervisor, the overhead of the entry/exit
> instruction sequences introduced by this patch was measured as 0.3ns
> in the case where IA is left enabled, and 33.0ns in the case where
> IA is disabled.
> 
> Signed-off-by: Peter Collingbourne <pcc at google.com>
> Reviewed-by: Dave Martin <Dave.Martin at arm.com>
> Link: https://linux-review.googlesource.com/id/Ibc41a5e6a76b275efbaa126b31119dc197b927a5

[...]

> diff --git a/arch/arm64/kernel/pointer_auth.c b/arch/arm64/kernel/pointer_auth.c
> index adb955fd9bdd..f03e5bfe4490 100644
> --- a/arch/arm64/kernel/pointer_auth.c
> +++ b/arch/arm64/kernel/pointer_auth.c
> @@ -46,3 +46,65 @@ int ptrauth_prctl_reset_keys(struct task_struct *tsk, unsigned long arg)
>  
>  	return 0;
>  }
> +
> +static u64 arg_to_enxx_mask(unsigned long arg)
> +{
> +	u64 sctlr_enxx_mask = 0;
> +
> +	WARN_ON(arg & ~PR_PAC_ENABLED_KEYS_MASK);
> +	if (arg & PR_PAC_APIAKEY)
> +		sctlr_enxx_mask |= SCTLR_ELx_ENIA;
> +	if (arg & PR_PAC_APIBKEY)
> +		sctlr_enxx_mask |= SCTLR_ELx_ENIB;
> +	if (arg & PR_PAC_APDAKEY)
> +		sctlr_enxx_mask |= SCTLR_ELx_ENDA;
> +	if (arg & PR_PAC_APDBKEY)
> +		sctlr_enxx_mask |= SCTLR_ELx_ENDB;
> +	return sctlr_enxx_mask;
> +}
> +
> +int ptrauth_set_enabled_keys(struct task_struct *tsk, unsigned long keys,
> +			     unsigned long enabled)
> +{
> +	u64 sctlr = tsk->thread.sctlr_user;
> +
> +	if (!system_supports_address_auth())
> +		return -EINVAL;
> +
> +	if (is_compat_thread(task_thread_info(tsk)))
> +		return -EINVAL;
> +
> +	if ((keys & ~PR_PAC_ENABLED_KEYS_MASK) || (enabled & ~keys))
> +		return -EINVAL;
> +
> +	sctlr &= ~arg_to_enxx_mask(keys);
> +	sctlr |= arg_to_enxx_mask(enabled);
> +	if (tsk == current)
> +		set_task_sctlr_el1(sctlr);
> +	else
> +		tsk->thread.sctlr_user = sctlr;

Who synchronizes all these modifications to 'sctlr_user'? Seems like it gets
hit by two independent prctl()s as well as ptrace.

Will