Re: [RFC PATCH] x86_64,entry,xen: Do not invoke espfix64 on Xen

[Boris Ostrovsky]

On 07/15/2014 12:23 PM, Andy Lutomirski wrote:
> This moves the espfix64 logic into native_iret.  To make this work,
> it gets rid of the native patch for INTERRUPT_RETURN:
> INTERRUPT_RETURN on native kernels is now 'jmp native_iret'.
>
> This changes the 16-bit SS behavior on Xen from OOPSing to leaking
> some bits of the Xen hypervisor's RSP (I think).
>
> Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
> ---
>
> I just pushed this to kernel.org, so the kbuild robot should be pounding
> on it soon.  It passes all my tests on bare metal, and it fails them
> without crashing on Xen.
>
> It's not exactly beautiful, but I think it's an improvement.
>
>   arch/x86/include/asm/irqflags.h     |  2 +-
>   arch/x86/kernel/entry_64.S          | 28 ++++++++++------------------
>   arch/x86/kernel/paravirt_patch_64.c |  2 --
>   3 files changed, 11 insertions(+), 21 deletions(-)
>
> diff --git a/arch/x86/include/asm/irqflags.h b/arch/x86/include/asm/irqflags.h
> index bba3cf8..0a8b519 100644
> --- a/arch/x86/include/asm/irqflags.h
> +++ b/arch/x86/include/asm/irqflags.h
> @@ -129,7 +129,7 @@ static inline notrace unsigned long arch_local_irq_save(void)
>   
>   #define PARAVIRT_ADJUST_EXCEPTION_FRAME	/*  */
>   
> -#define INTERRUPT_RETURN	iretq
> +#define INTERRUPT_RETURN	jmp native_iret
>   #define USERGS_SYSRET64				\
>   	swapgs;					\
>   	sysretq;
> diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
> index b25ca96..c844f08 100644
> --- a/arch/x86/kernel/entry_64.S
> +++ b/arch/x86/kernel/entry_64.S
> @@ -830,27 +830,24 @@ restore_args:
>   	RESTORE_ARGS 1,8,1
>   
>   irq_return:
> +	INTERRUPT_RETURN
> +
> +ENTRY(native_iret)
>   	/*
>   	 * Are we returning to a stack segment from the LDT?  Note: in
>   	 * 64-bit mode SS:RSP on the exception stack is always valid.
>   	 */
>   #ifdef CONFIG_X86_ESPFIX64
>   	testb $4,(SS-RIP)(%rsp)
> -	jnz irq_return_ldt
> +	jnz native_irq_return_ldt
>   #endif
>   
> -irq_return_iret:
> -	INTERRUPT_RETURN
> -	_ASM_EXTABLE(irq_return_iret, bad_iret)
> -
> -#ifdef CONFIG_PARAVIRT
> -ENTRY(native_iret)
> +native_irq_return_iret:
>   	iretq
> -	_ASM_EXTABLE(native_iret, bad_iret)
> -#endif
> +	_ASM_EXTABLE(native_irq_return_iret, bad_iret)
>   
>   #ifdef CONFIG_X86_ESPFIX64
> -irq_return_ldt:
> +native_irq_return_ldt:
>   	pushq_cfi %rax
>   	pushq_cfi %rdi
>   	SWAPGS
> @@ -872,7 +869,7 @@ irq_return_ldt:
>   	SWAPGS
>   	movq %rax,%rsp
>   	popq_cfi %rax
> -	jmp irq_return_iret
> +	jmp native_irq_return_iret
>   #endif
>   
>   	.section .fixup,"ax"
> @@ -956,13 +953,8 @@ __do_double_fault:
>   	cmpl $__KERNEL_CS,CS(%rdi)
>   	jne do_double_fault
>   	movq RIP(%rdi),%rax
> -	cmpq $irq_return_iret,%rax
> -#ifdef CONFIG_PARAVIRT
> -	je 1f
> -	cmpq $native_iret,%rax
> -#endif
> +	cmpq $native_irq_return_iret,%rax
>   	jne do_double_fault		/* This shouldn't happen... */
> -1:
>   	movq PER_CPU_VAR(kernel_stack),%rax
>   	subq $(6*8-KERNEL_STACK_OFFSET),%rax	/* Reset to original stack */
>   	movq %rax,RSP(%rdi)
> @@ -1428,7 +1420,7 @@ error_sti:
>    */
>   error_kernelspace:
>   	incl %ebx
> -	leaq irq_return_iret(%rip),%rcx
> +	leaq native_irq_return_iret(%rip),%rcx
>   	cmpq %rcx,RIP+8(%rsp)
>   	je error_swapgs
>   	movl %ecx,%eax	/* zero extend */
> diff --git a/arch/x86/kernel/paravirt_patch_64.c b/arch/x86/kernel/paravirt_patch_64.c
> index 3f08f34..a1da673 100644
> --- a/arch/x86/kernel/paravirt_patch_64.c
> +++ b/arch/x86/kernel/paravirt_patch_64.c
> @@ -6,7 +6,6 @@ DEF_NATIVE(pv_irq_ops, irq_disable, "cli");
>   DEF_NATIVE(pv_irq_ops, irq_enable, "sti");
>   DEF_NATIVE(pv_irq_ops, restore_fl, "pushq %rdi; popfq");
>   DEF_NATIVE(pv_irq_ops, save_fl, "pushfq; popq %rax");
> -DEF_NATIVE(pv_cpu_ops, iret, "iretq");
>   DEF_NATIVE(pv_mmu_ops, read_cr2, "movq %cr2, %rax");
>   DEF_NATIVE(pv_mmu_ops, read_cr3, "movq %cr3, %rax");
>   DEF_NATIVE(pv_mmu_ops, write_cr3, "movq %rdi, %cr3");
> @@ -50,7 +49,6 @@ unsigned native_patch(u8 type, u16 clobbers, void *ibuf,
>   		PATCH_SITE(pv_irq_ops, save_fl);
>   		PATCH_SITE(pv_irq_ops, irq_enable);
>   		PATCH_SITE(pv_irq_ops, irq_disable);
> -		PATCH_SITE(pv_cpu_ops, iret);


Does this mean that we are no longer patching IRET with a jump to a 
hypercall?

-boris


>   		PATCH_SITE(pv_cpu_ops, irq_enable_sysexit);
>   		PATCH_SITE(pv_cpu_ops, usergs_sysret32);
>   		PATCH_SITE(pv_cpu_ops, usergs_sysret64);

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