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

[Andrew Cooper <Andrew.Cooper3@citrix.com>]

On 23/01/2023 9:02 am, Ammar Faizi 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.

You can't compile userspace like that, unless you recompile and
statically link everything including libc.

The proper way to fix this is to add a "+r" constraint on the stack
pointer, at which point the compiler will arrange for nothing in the
redzone to be live around the asm block.

~Andrew