Re: PANIC: double fault, error_code: 0x0 in 4.0.0-rc3-2, kvm related?

[Denys Vlasenko]

On 03/18/2015 08:26 PM, Andy Lutomirski wrote:
> Hi Linus-
> 
> You seem to enjoy debugging these things.  Want to give this a shot?
> My guess is a vmalloc fault accessing either old_rsp or kernel_stack
> right after swapgs in syscall entry.

The code is:

ENTRY(system_call)
        SWAPGS_UNSAFE_STACK
GLOBAL(system_call_after_swapgs)
        movq    %rsp,PER_CPU_VAR(rsp_scratch)
	movq    PER_CPU_VAR(kernel_stack),%rsp

If PER_CPU_VAR(var) memory access can page fault
(I was thinking this is ensured to never fault),
then on these two instructions such page fault
will be fatal: we will still have userspace %rsp.

I thought we can only get a NMI or debug interrupt here,
and they are both set up to use IST stacks
to prevent this scenario (among other reasons).

-- 
vda
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