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

Boris Ostrovsky
2014-07-15T17:17:33Z

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@amacapital.net>
> ---
>
> 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);