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SubjectRe: [PATCH v2] x86/xen/64: Rearrange the SYSCALL entries
On Sun, Aug 13, 2017 at 7:44 PM, Brian Gerst <brgerst@gmail.com> wrote:
> On Mon, Aug 7, 2017 at 11:59 PM, Andy Lutomirski <luto@kernel.org> wrote:
>> /* Normal 64-bit system call target */
>> ENTRY(xen_syscall_target)
>> - undo_xen_syscall
>> - jmp entry_SYSCALL_64_after_swapgs
>> + popq %rcx
>> + popq %r11
>> + jmp entry_SYSCALL_64_after_hwframe
>> ENDPROC(xen_syscall_target)
>>
>> #ifdef CONFIG_IA32_EMULATION
>>
>> /* 32-bit compat syscall target */
>> ENTRY(xen_syscall32_target)
>> - undo_xen_syscall
>> - jmp entry_SYSCALL_compat
>> + popq %rcx
>> + popq %r11
>> + jmp entry_SYSCALL_compat_after_hwframe
>> ENDPROC(xen_syscall32_target)
>>
>> /* 32-bit compat sysenter target */
>> ENTRY(xen_sysenter_target)
>> - undo_xen_syscall
>> + mov 0*8(%rsp), %rcx
>> + mov 1*8(%rsp), %r11
>> + mov 5*8(%rsp), %rsp
>> jmp entry_SYSENTER_compat
>> ENDPROC(xen_sysenter_target)
>
> This patch causes the iopl_32 and ioperm_32 self-tests to fail on a
> 64-bit PV kernel. The 64-bit versions pass. It gets a seg fault after
> "parent: write to 0x80 (should fail)", and the fault isn't caught by
> the signal handler. It just dumps back to the shell. The tests pass
> after reverting this.

I can reproduce it if I emulate an AMD machine. I can "fix" it like this:

diff --git a/arch/x86/xen/xen-asm_64.S b/arch/x86/xen/xen-asm_64.S
index a8a4f4c460a6..6255e00f425e 100644
--- a/arch/x86/xen/xen-asm_64.S
+++ b/arch/x86/xen/xen-asm_64.S
@@ -97,6 +97,9 @@ ENDPROC(xen_syscall_target)
ENTRY(xen_syscall32_target)
popq %rcx
popq %r11
+ movq $__USER32_DS, 4*8(%rsp)
+ movq $__USER32_CS, 1*8(%rsp)
+ movq %r11, 2*8(%rsp)
jmp entry_SYSCALL_compat_after_hwframe
ENDPROC(xen_syscall32_target)

but I haven't tried to diagnose precisely what's going on.
Xen seems to be putting the 0xe0?? values in ss and cs, which oughtn't
to be a problem, but it kills opportunistic sysretl. Maybe that's
triggering a preexisting bug?

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