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

H. Peter Anvin
2023-01-20T20:51:52Z

On January 20, 2023 10:52:02 AM PST, Andrew Cooper <Andrew.Cooper3@citrix.com> wrote:
>On 20/01/2023 5:45 pm, Dave Hansen wrote:
>> On 1/19/23 23:49, Li, Xin3 wrote:
>>> The x86 sysret_rip test has the following assertion:
>>>
>>> /* R11 and EFLAGS should already match. */
>>> assert(ctx->uc_mcontext.gregs[REG_EFL] ==
>>> ctx->uc_mcontext.gregs[REG_R11]);
>>>
>>> This is being tested to avoid kernel state leak due to sysret vs iret,
>>> but that on FRED r11 is *always* preserved, and the test just fails.
>> Let's figure out the reason that FRED acts differently, first. Right
>> now, the SDM says:
>>
>> SYSCALL also saves RFLAGS into R11
>>
>> so that behavior of SYSCALL _looks_ architectural to me. Was this
>> change in SYSCALL behavior with FRED intentional?
>
>FRED 3.0 Section 7.4 says the only changes for the SYSCALL and SYSENTER
>instructions are the enablement conditions.  Nowhere else is there
>mention of a FRED OS needing to emulate legacy syscall behaviour by
>adjusting %r11/%rcx
>
>However, ERETU does handle flags different to SYSRET (in particular, I
>think you can establish TF on the instruction boundary after SYSCALL
>now).  What are the raw values of REG_EFL and REG_R11 ?
>
>~Andrew
>

Just to avoid any confusion:

Syscall and sysenter in a FRED system are treated equivalently to software interrupts, e.g. INT 0x80. They do not modify any registers.