Re: [PATCH] x86/traps: Re-enable IRQs after reading cr2 in the #PF handler

[Frediano Ziglio <frediano.ziglio@xxxxxxxxx>]

On Wed, Sep 11, 2024 at 3:58 PM Alejandro Vallejo
<alejandro.vallejo@xxxxxxxxx> wrote:
>
> Moves sti directly after the cr2 read and immediately after the #PF
> handler.
>
> While in the area, remove redundant q suffix to a movq in entry.S
>
> Signed-off-by: Alejandro Vallejo <alejandro.vallejo@xxxxxxxxx>
> ---
> I don't think this is a bug as much as an accident about to happen. Even if
> there's no cases at the moment in which the IRQ handler may page fault, that
> might change in the future.
>
> Note: I haven't tested it extensively beyond running it on GitLab.
>
> pipeline:
>     https://gitlab.com/xen-project/people/agvallejo/xen/-/pipelines/1449182525
>
> ---
>  xen/arch/x86/traps.c        |  2 ++
>  xen/arch/x86/x86_64/entry.S | 11 +++++++++--
>  2 files changed, 11 insertions(+), 2 deletions(-)
>
> diff --git a/xen/arch/x86/traps.c b/xen/arch/x86/traps.c
> index 708136f625..1c04c03d9f 100644
> --- a/xen/arch/x86/traps.c
> +++ b/xen/arch/x86/traps.c
> @@ -1600,6 +1600,8 @@ void asmlinkage do_page_fault(struct cpu_user_regs 
> *regs)
>
>      addr = read_cr2();
>
> +    local_irq_enable();
> +
>      /* fixup_page_fault() might change regs->error_code, so cache it here. */
>      error_code = regs->error_code;
>
> diff --git a/xen/arch/x86/x86_64/entry.S b/xen/arch/x86/x86_64/entry.S
> index b8482de8ee..ef803f6288 100644
> --- a/xen/arch/x86/x86_64/entry.S
> +++ b/xen/arch/x86/x86_64/entry.S
> @@ -844,8 +844,7 @@ handle_exception_saved:
>  #elif !defined(CONFIG_PV)
>          ASSERT_CONTEXT_IS_XEN
>  #endif /* CONFIG_PV */
> -        sti
> -1:      movq  %rsp,%rdi
> +1:      mov   %rsp,%rdi
>          movzbl UREGS_entry_vector(%rsp),%eax
>  #ifdef CONFIG_PERF_COUNTERS
>          lea   per_cpu__perfcounters(%rip), %rcx
> @@ -866,7 +865,15 @@ handle_exception_saved:
>          jmp   .L_exn_dispatch_done;    \
>  .L_ ## vec ## _done:
>
> +        /*
> +         * IRQs kept off to derisk being hit by a nested interrupt before
> +         * reading %cr2. Otherwise a page fault in the nested interrupt 
> hadnler


Minor, typo: hadnler -> handler

>
> +         * would corrupt %cr2.
> +         */
>          DISPATCH(X86_EXC_PF, do_page_fault)
> +
> +        sti
> +
>          DISPATCH(X86_EXC_GP, do_general_protection)
>          DISPATCH(X86_EXC_UD, do_invalid_op)
>          DISPATCH(X86_EXC_NM, do_device_not_available)
>

Frediano