Re: the x86 sysret_rip test fails on the Intel FRED architecture

H. Peter Anvin
2023-01-23T19:46:32Z

On January 23, 2023 1:02:22 AM PST, Ammar Faizi <ammarfaizi2@gnuweeb.org> wrote:
>On 1/23/23 6:45 AM, H. Peter Anvin wrote:
>> static enum regs_ok check_regs_syscall(int syscall,
>>     unsigned long arg1, unsigned long arg2)
>> {
>>
>>     register unsigned long r11 asm("%r11");
>>     unsigned long rcx, rbx, tmp;
>
>tmp is unused.
>
>>     r11 = r11_sentinel;
>>     rcx = rcx_sentinel;
>>
>>     asm volatile("push %3; popf; "
>>              "lea 1f(%%rip),%2; "
>>              "syscall; "
>>              "1:"
>>              : "+r" (r11), "+c" (rcx), "=b" (rbx)
>>              : "g" (rflags_sentinel),
>>                "a" (syscall), "D" (arg1), "S" (arg2));
>
>BTW, I just realized this "push" is unsafe for userspace code if the
>compiler decides to inline this inside a leaf function that uses the
>redzone.
>
>Reason: Because this "push;" clobbers redzone.
>
>It doesn't always happen, but when that happens it can be confusing to
>debug.
>
>A simple workaround is: just compile it with "-mno-red-zone" flag.
>
>Alternative, without using that flag, maybe preserve the value like:
>
> movq -8(%rsp), %r12
> pushq %[rflags_sentinel]
> popf
> movq %r12, -8(%rsp)
> syscall
>
>with "r12" and "memory" added to the clobber list.
>
>What do you think?
>

Good spotting. %rax needs to be marked clobbered, too.