[PATCH v5 19/30] arm64: add POE signal support

[Kevin Brodsky]

On 22/08/2024 17:11, Joey Gouly wrote:
> @@ -1178,6 +1237,9 @@ static void setup_return(struct pt_regs *regs, struct k_sigaction *ka,
>  		sme_smstop();
>  	}
>  
> +	if (system_supports_poe())
> +		write_sysreg_s(POR_EL0_INIT, SYS_POR_EL0);

At the point where setup_return() is called, the signal frame has
already been written to the user stack. In other words, we write to the
user stack first, and then reset POR_EL0. This may be problematic,
especially if we are using the alternate signal stack, which the
interrupted POR_EL0 may not grant access to. In that situation uaccess
will fail and we'll end up with a SIGSEGV.

This issue has already been discussed on the x86 side, and as it happens
patches to reset PKRU early [1] have just landed. I don't think this is
a blocker for getting this series landed, but we should try and align
with x86. If there's no objection, I'm planning to work on a counterpart
to the x86 series (resetting POR_EL0 early during signal delivery).

Kevin

[1]
https://lore.kernel.org/lkml/20240802061318.2140081-2-aruna.ramakrishna@oracle.com/

> +
>  	if (ka->sa.sa_flags & SA_RESTORER)
>  		sigtramp = ka->sa.sa_restorer;
>  	else