Re: [PATCH RFC] arm64: entry: PSTATE_I_SET is leaking on pseudo NMI mode

[Vladimir Murzin <vladimir.murzin@arm.com>]

Hi Will,

On 8/10/26 12:43, Will Deacon wrote:
> On Fri, Aug 07, 2026 at 09:29:12AM -0700, Breno Leitao wrote:
>> On Fri, Aug 07, 2026 at 07:58:21AM -0700, Breno Leitao wrote:
>>> Meanwhile, I will try to ftrace the writes to PMR and regs->pmr to get
>>> a better grasp of the states machine we are in (probably on Monday).
>> It seems LLM found a very easy to reproduce this:
>>
>> 	bash-5.1# dmesg
>>
>> 	bash-5.1#  cd /sys/kernel/tracing
>> 	echo 'r:pmr vfs_read bad=+0($retval):u64' >> kprobe_events
>> 	echo 1 > events/kprobes/pmr/enable
> Nice, that triggers straightforwardly in QEMU for me. The diff below
> (which implements my suggestion from [1]) seems to fix the issue, but
> it would be good to hear feedback from one of the Arm folks.
> 
> Will
> 
> [1] https://lore.kernel.org/all/anXgWRmcjwPKG7N5@willie-the-truck/
> 
> --->8
> 
> diff --git a/arch/arm64/include/asm/daifflags.h b/arch/arm64/include/asm/daifflags.h
> index 795b35128467..691ee5f86dbe 100644
> --- a/arch/arm64/include/asm/daifflags.h
> +++ b/arch/arm64/include/asm/daifflags.h
> @@ -132,7 +132,7 @@ static __always_inline void local_daif_inherit(struct pt_regs *regs)
>                 trace_hardirqs_on();
> 
>         if (system_uses_irq_prio_masking())
> -               gic_write_pmr(regs->pmr);
> +               gic_write_pmr(regs->pmr & ~GIC_PRIO_PSR_I_SET);
> 
>         /*
>          * We can't use local_daif_restore(regs->pstate) here as
> 

I have no strong opinion on the change, but I struggle to see how it
fits into the big picture.

Specifically, I have difficulty explaining this change in isolation.
Everywhere else, we try to keep DAIF.I and GIC_PRIO_PSR_I_SET in sync,
so it is not clear why we should allow them to go out of sync when
inheriting the exception state from the previous context.

At the same time, IIUC, the only reason we call local_irq_disable()
(which also causes the states to become unsynchronized) in
arm64_exit_to_kernel_mode() is for preemption path. So avoiding
local_irq_disable() (and preemption) when we interrupted a
non-preemptible context seems easier to follow.

Alternatively, using local_daif_restore(DAIF_PROCCTX_NOIRQ) directly,
if we believe it is OK to downgrade the inherited context, would
achieve the same effect as local_irq_disable() while keeping DAIF and
PMR in sync.

Cheers
Vladimir