Re: [PATCH v3 28/46] kmsan: entry: handle register passing from uninstrumented code

[Thomas Gleixner <tglx@linutronix.de>]

Alexander,

On Thu, May 05 2022 at 20:04, Alexander Potapenko wrote:
> On Tue, May 3, 2022 at 12:00 AM Thomas Gleixner <tglx@linutronix.de> wrote:
>> > Regarding the regs, you are right. It should be enough to unpoison the
>> > regs at idtentry prologue instead.
>> > I tried that initially, but IIRC it required patching each of the
>> > DEFINE_IDTENTRY_XXX macros, which already use instrumentation_begin().
>>
>> Exactly 4 instances :)
>>
>
> Not really, I had to add a call to `kmsan_unpoison_memory(regs,
> sizeof(*regs));` to the following places in
> arch/x86/include/asm/idtentry.h:
> - DEFINE_IDTENTRY()
> - DEFINE_IDTENTRY_ERRORCODE()
> - DEFINE_IDTENTRY_RAW()
> - DEFINE_IDTENTRY_RAW_ERRORCODE()
> - DEFINE_IDTENTRY_IRQ()
> - DEFINE_IDTENTRY_SYSVEC()
> - DEFINE_IDTENTRY_SYSVEC_SIMPLE()
> - DEFINE_IDTENTRY_DF()
>
> , but even that wasn't enough. For some reason I also had to unpoison
> pt_regs directly in
> DEFINE_IDTENTRY_SYSVEC(sysvec_apic_timer_interrupt) and
> DEFINE_IDTENTRY_IRQ(common_interrupt).
> In the latter case, this could have been caused by
> asm_common_interrupt being entered from irq_entries_start(), but I am
> not sure what is so special about sysvec_apic_timer_interrupt().
>
> Ideally, it would be great to find that single point where pt_regs are
> set up before being passed to all IDT entries.
> I used to do that by inserting calls to kmsan_unpoison_memory right
> into arch/x86/entry/entry_64.S
> (https://github.com/google/kmsan/commit/3b0583f45f74f3a09f4c7e0e0588169cef918026),
> but that required storing/restoring all GP registers. Maybe there's a
> better way?

Yes. Something like this should cover all exceptions and syscalls before
anything instrumentable can touch @regs. Anything up to those points is
either off-limit for instrumentation or does not deal with @regs.

--- a/kernel/entry/common.c
+++ b/kernel/entry/common.c
@@ -24,6 +24,7 @@ static __always_inline void __enter_from
 	user_exit_irqoff();
 
 	instrumentation_begin();
+	unpoison(regs);
 	trace_hardirqs_off_finish();
 	instrumentation_end();
 }
@@ -352,6 +353,7 @@ noinstr irqentry_state_t irqentry_enter(
 		lockdep_hardirqs_off(CALLER_ADDR0);
 		rcu_irq_enter();
 		instrumentation_begin();
+		unpoison(regs);
 		trace_hardirqs_off_finish();
 		instrumentation_end();
 
@@ -367,6 +369,7 @@ noinstr irqentry_state_t irqentry_enter(
 	 */
 	lockdep_hardirqs_off(CALLER_ADDR0);
 	instrumentation_begin();
+	unpoison(regs);
 	rcu_irq_enter_check_tick();
 	trace_hardirqs_off_finish();
 	instrumentation_end();
@@ -452,6 +455,7 @@ irqentry_state_t noinstr irqentry_nmi_en
 	rcu_nmi_enter();
 
 	instrumentation_begin();
+	unpoison(regs);
 	trace_hardirqs_off_finish();
 	ftrace_nmi_enter();
 	instrumentation_end();

As I said: 4 places :)

> Fortunately, I don't think we need to insert extra instrumentation
> into instrumentation_begin()/instrumentation_end() regions.
>
> What I have in mind is adding a bool flag to kmsan_context_state, that
> the instrumentation sets to true before the function call.
> When entering an instrumented function, KMSAN would check that flag
> and set it to false, so that the context state can be only used once.
> If a function is called from another instrumented function, the
> context state is properly set up, and there is nothing to worry about.
> If it is called from non-instrumented code (either noinstr or the
> skipped files that have KMSAN_SANITIZE:=n), KMSAN would detect that
> and wipe the context state before use.

Sounds good.

Thanks,

	tglx