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

Andy Lutomirski
2014-07-15T17:23:50Z

On Tue, Jul 15, 2014 at 10:19 AM, Boris Ostrovsky
<boris.ostrovsky@oracle.com> wrote:
> 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?
>

IIUC this means that, on native, we are no longer patching
INTERRUPT_RETURN with an "iretq" instruction, so INTERRUPT_RETURN will
remain a "jmp native_iret". I'm not sure why this patch was there in
the first place. On Xen, it should still get patched with the
hypercall (although someone should verify this).

--Andy

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

--
Andy Lutomirski
AMA Capital Management, LLC