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

Ammar Faizi
2023-01-23T09:02:34Z

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?

--
Ammar Faizi