On Fri, May 06 2022 at 19:41, Alexander Potapenko wrote:
> On Fri, May 6, 2022 at 6:14 PM Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
>> sysvec_apic_timer_interrupt() invokes irqentry_enter() _before_
>> set_irq_regs() and irqentry_enter() unpoisons @reg.
>>
>> Confused...
>
> As far as I can tell in this case sysvect_apic_timer_interrupt() is
> called by the following code in arch/x86/kernel/idt.c:
>
> INTG(LOCAL_TIMER_VECTOR, asm_sysvec_apic_timer_interrupt),
>
> , which does not use IDTENTRY_SYSVEC framework and thus does not call
> irqentry_enter().
asm_sysvec_apic_timer_interrupt != sysvec_apic_timer_interrupt
arch/x86/kernel/apic/apic.c:
DEFINE_IDTENTRY_SYSVEC(sysvec_apic_timer_interrupt)
{
....
#define DEFINE_IDTENTRY_SYSVEC(func) \
static void __##func(struct pt_regs *regs); \
\
__visible noinstr void func(struct pt_regs *regs) \
{ \
irqentry_state_t state = irqentry_enter(regs); \
....
__##func (regs); \
....
} \
\
static noinline void __##func(struct pt_regs *regs)
So it goes through that code path _before_ the actual implementation
which does set_irq_regs() is reached.
The callchain is:
asm_sysvec_apic_timer_interrupt <- ASM entry in gate
sysvec_apic_timer_interrupt(regs) <- noinstr C entry point
irqentry_enter(regs) <- unpoisons @reg
__sysvec_apic_timer_interrupt(regs) <- the actual handler
set_irq_regs(regs) <- stores regs
local_apic_timer_interrupt()
...
tick_handler() <- One of the 4 variants
regs = get_irq_regs(); <- retrieves regs
update_process_times(user_tick = user_mode(regs))
account_process_tick(user_tick)
irqtime_account_process_tick(user_tick)
line 382: } else if { user_tick } <- KMSAN complains
I'm even more confused now.
> I guess handling those will require wrapping every interrupt gate into
> a function that performs register unpoisoning?
No, guessing does not help here.
The gates point to the ASM entry point, which then invokes the C entry
point. All C entry points use a DEFINE_IDTENTRY variant.
Some of the DEFINE_IDTENTRY_* C entry points are not doing anything in
the macro, but the C function either invokes irqentry_enter() or
irqentry_nmi_enter() open coded _before_ invoking any instrumentable
function. So the unpoisoning of @regs in these functions should tell
KMSAN that @regs or something derived from @regs are not some random
uninitialized values.
There should be no difference between unpoisoning @regs in
irqentry_enter() or in set_irq_regs(), right?
If so, then the problem is definitely _not_ the idt entry code.
> By the way, if it helps, I think we don't necessarily have to call
> kmsan_unpoison_memory() from within the
> instrumentation_begin()/instrumentation_end() region?
> We could move the call to the beginning of irqentry_enter(), removing
> unnecessary duplication.
We could, but then you need to mark unpoison_memory() noinstr too and you
have to add the unpoison into the syscall code. No win and irrelevant to
the problem at hand.
Thanks,
tglx
