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SubjectRe: [PATCH v2 6/8] x86/entry: add unwind hint annotations
On Thu, Jun 29, 2017 at 10:53 AM, Josh Poimboeuf <jpoimboe@redhat.com> wrote:
> There's a bug here that will need a small change to the entry code.
>
> Mike Galbraith reported:
>
> WARNING: can't dereference registers at ffffc900089d7e08 for ip ffffffff81740bbb
>
> After some looking I found that it's caused by the following code
> snippet in the 'interrupt' macro in entry_64.S:
>
> /*
> * Save previous stack pointer, optionally switch to interrupt stack.
> * irq_count is used to check if a CPU is already on an interrupt stack
> * or not. While this is essentially redundant with preempt_count it is
> * a little cheaper to use a separate counter in the PDA (short of
> * moving irq_enter into assembly, which would be too much work)
> */
> movq %rsp, %rdi
> incl PER_CPU_VAR(irq_count)
> cmovzq PER_CPU_VAR(irq_stack_ptr), %rsp
> UNWIND_HINT_REGS base=rdi
> pushq %rdi
> UNWIND_HINT_REGS indirect=1
>
> The problem is that it's changing the stack pointer *before* writing the
> previous stack pointer (push %rdi). So when unwinding from an NMI which
> hit between the rsp write and the rdi push, the unwinder tries to access
> the regs on the previous stack (by reading rdi), but the previous stack
> pointer isn't there yet, so the access is considered out of bounds.

Ugh, that code. Does this problem go away with this patch applied:

https://git.kernel.org/pub/scm/linux/kernel/git/luto/linux.git/commit/?h=x86/entry_ist&id=2231ec7e0bcc1a2bc94a17081511ab54cc6badd1

If so, want to update the patch for new kernels (shouldn't conflict
with anything except your unwind hints)?

--Andy

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