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SubjectRe: [PATCH v3 28/46] kmsan: entry: handle register passing from uninstrumented code
Date
On Thu, May 12 2022 at 18:17, Thomas Gleixner wrote:
> On Thu, May 12 2022 at 14:24, Alexander Potapenko wrote:
>> We could try to figure out the places in idtentry code where normal
>> kmsan_unpoison_memory() can be called in IRQ context, but as far as I
>> can see it will depend on the type of the entry point.
>
> NMI is covered as it increments before it invokes the unpoison().
>
> Let me figure out why we increment the preempt count late for
> interrupts. IIRC it's for symmetry reasons related to softirq processing
> on return, but let me double check.

It's even documented:

https://www.kernel.org/doc/html/latest/core-api/entry.html#interrupts-and-regular-exceptions

But who reads documentation? :)

So, I think the simplest and least intrusive solution is to have special
purpose unpoison functions. See the patch below for illustration.

The reasons why I used specific ones:

1) User entry

Whether that's a syscall or interrupt/exception does not
matter. It's always on the task stack and your machinery cannot be
running at that point because it came from user space.

2) Interrupt/exception/NMI entry kernel

Those can nest into an already active context, so you really want
to unpoison @regs.

Also while regular interrupts cannot nest because of interrupts
staying disabled, exceptions triggered in the interrupt handler and
NMIs can nest.

-> device interrupt()
irqentry_enter(regs)

-> NMI()
irqentry_nmi_enter(regs)

-> fault()
irqentry_enter(regs)

--> debug_exception()
irqentry_nmi_enter(regs)

Soft interrupt processing on return from interrupt makes it more
interesting:

interrupt()
handler()
do_softirq()
local_irq_enable()
interrupt()
NMI
....

And everytime you get a new @regs pointer to deal with.

Wonderful, isn't it?

Thanks,

tglx

---
--- 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_user(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_irq(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_irq(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_irq(regs);
trace_hardirqs_off_finish();
ftrace_nmi_enter();
instrumentation_end();
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