Re: [PATCH] x86/entry/64/paravirt: Use paravirt-safe macro to access eflags

[Juergen Gross]

On 27/11/17 19:05, Boris Ostrovsky wrote:
> Commit 1d3e53e8624a ("x86/entry/64: Refactor IRQ stacks and make
> them NMI-safe") added DEBUG_ENTRY_ASSERT_IRQS_OFF macro that acceses
> eflags using 'pushfq' instruction when testing for IF bit. On PV Xen
> guests looking at IF flag directly will always see it set, resulting
> in 'ud2'.
> 
> Introduce SAVE_FLAGS() macro that will use appropriate save_fl pv op
> when running paravirt.
> 
> Signed-off-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
> ---
>  arch/x86/entry/entry_64.S        |    5 ++---
>  arch/x86/include/asm/irqflags.h  |    3 +++
>  arch/x86/include/asm/paravirt.h  |    9 +++++++++
>  arch/x86/kernel/asm-offsets_64.c |    3 +++
>  4 files changed, 17 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
> index f81d50d..4bb7719 100644
> --- a/arch/x86/entry/entry_64.S
> +++ b/arch/x86/entry/entry_64.S
> @@ -466,12 +466,11 @@ END(irq_entries_start)
>  
>  .macro DEBUG_ENTRY_ASSERT_IRQS_OFF
>  #ifdef CONFIG_DEBUG_ENTRY
> -	pushfq
> -	testl $X86_EFLAGS_IF, (%rsp)
> +	SAVE_FLAGS(CLBR_ANY)
> +	testl $X86_EFLAGS_IF, %eax

Are you sure %eax is allowed to be modified?

>  	jz .Lokay_\@
>  	ud2
>  .Lokay_\@:
> -	addq $8, %rsp
>  #endif
>  .endm
>  
> diff --git a/arch/x86/include/asm/irqflags.h b/arch/x86/include/asm/irqflags.h
> index c8ef23f..7f65f3f 100644
> --- a/arch/x86/include/asm/irqflags.h
> +++ b/arch/x86/include/asm/irqflags.h
> @@ -142,6 +142,9 @@ static inline notrace unsigned long arch_local_irq_save(void)
>  	swapgs;					\
>  	sysretl
>  
> +#ifdef CONFIG_DEBUG_ENTRY
> +#define SAVE_FLAGS(x)		pushfq

Isn't there a "pop %rax" missing (assuming %rax is allowed to be
modified) ?


Juergen