arm64: poe: fix stale POR_EL0 values for ptrace

If a process wrote to POR_EL0 and then crashed before a context switch
happened, the coredump would contain an incorrect value for POR_EL0.

The value read in poe_get() would be a stale value left in thread.por_el0.  Fix
this by reading the value from the system register, if the target thread is the
current thread.

This matches what gcs/fpsimd do.

Fixes: 1751981 ("arm64/ptrace: add support for FEAT_POE")
Reported-by: David Spickett <david.spickett@arm.com>
Cc: stable@vger.kernel.org
Signed-off-by: Joey Gouly <joey.gouly@arm.com>
Cc: Kevin Brodsky <kevin.brodsky@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: Kevin Brodsky <kevin.brodsky@arm.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>

Author: jgouly

arch/arm64/kernel/ptrace.c

@@ -1486,6 +1486,9 @@ static int poe_get(struct task_struct *target,
 	if (!system_supports_poe())
 		return -EINVAL;
 
+	if (target == current)
+		current->thread.por_el0 = read_sysreg_s(SYS_POR_EL0);
+
 	return membuf_write(&to, &target->thread.por_el0,
 			    sizeof(target->thread.por_el0));
 }