Re: [PATCH] x86/entry/64: Fix CR3 restore order in paranoid_exit()

[Ingo Molnar]

* Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:

> I haven't actually seen any real-world bugs caused by this, so I'm not
> sure how theoretical it is.  I just stumbled upon it in code review when
> looking for another bug.

I believe it's a real bug, but the fix is wrong with irq tracing or lockdep 
enabled as Dave points out.

I think the reason we haven't seen this bug yet is that "paranoid" entry points 
are limited to:

idtentry double_fault			do_double_fault			has_error_code=1 paranoid=2
idtentry debug			do_debug		has_error_code=0	paranoid=1 shift_ist=DEBUG_STACK
idtentry int3			do_int3			has_error_code=0	paranoid=1 shift_ist=DEBUG_STACK
idtentry machine_check		do_mce			has_error_code=0	paranoid=1

Only machine_check is one that will interrupt an IRQS-off critical section 
asynchronously - and machine check events are rare.

The other main asynchronous entries are NMI entries, which can be very high-freq 
with perf profiling, but they are special: they don't use the 'idtentry' macro but 
are open coded and restore user CR3 unconditionally so don't seem to have this 
bug.

Thanks,

	Ingo