linux-hardening - Re: [PATCH v4 22/30] context_tracking: Exit CT_STATE

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Message-ID: <Z5A6NPqVGoZ32YsN@pavilion.home>
Date: Wed, 22 Jan 2025 01:22:12 +0100
From: Frederic Weisbecker <frederic@...nel.org>
To: Valentin Schneider <vschneid@...hat.com>
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Subject: Re: [PATCH v4 22/30] context_tracking: Exit CT_STATE_IDLE upon
 irq/nmi entry

Le Tue, Jan 14, 2025 at 06:51:35PM +0100, Valentin Schneider a écrit :
> ct_nmi_{enter, exit}() only touches the RCU watching counter and doesn't
> modify the actual CT state part context_tracking.state. This means that
> upon receiving an IRQ when idle, the CT_STATE_IDLE->CT_STATE_KERNEL
> transition only happens in ct_idle_exit().
> 
> One can note that ct_nmi_enter() can only ever be entered with the CT state
> as either CT_STATE_KERNEL or CT_STATE_IDLE, as an IRQ/NMI happenning in the
> CT_STATE_USER or CT_STATE_GUEST states will be routed down to ct_user_exit().

Are you sure? An NMI can fire between guest_state_enter_irqoff() and
__svm_vcpu_run(). And NMIs interrupting userspace don't call
enter_from_user_mode(). In fact they don't call irqentry_enter_from_user_mode()
like regular IRQs but irqentry_nmi_enter() instead. Well that's for archs
implementing common entry code, I can't speak for the others.

Unifying the behaviour between user and idle such that the IRQs/NMIs exit the
CT_STATE can be interesting but I fear this may not come for free. You would
need to save the old state on IRQ/NMI entry and restore it on exit.

Do we really need it?

Thanks.