linux-kernel - Re: the x86 sysret_rip test fails on the Intel FRED architecture

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Message-ID: <e1df7cdf-b265-4bb9-92d7-f20a656931fd@citrix.com>
Date:   Fri, 20 Jan 2023 21:29:30 +0000
From:   Andrew Cooper <Andrew.Cooper3@...rix.com>
To:     "H. Peter Anvin" <hpa@...or.com>,
        Dave Hansen <dave.hansen@...el.com>,
        "Li, Xin3" <xin3.li@...el.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "bp@...en8.de" <bp@...en8.de>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        Andrew Cooper <Andrew.Cooper3@...rix.com>
CC:     "x86@...nel.org" <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: the x86 sysret_rip test fails on the Intel FRED architecture

On 20/01/2023 9:17 pm, H. Peter Anvin wrote:
> On January 20, 2023 1:10:09 PM PST, Andrew Cooper <Andrew.Cooper3@...rix.com> wrote:
>> On 20/01/2023 8:50 pm, H. Peter Anvin wrote:
>>> On January 20, 2023 10:52:02 AM PST, Andrew Cooper <Andrew.Cooper3@...rix.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.
>> In which case can Intel please publish a v4 spec which actually says this?
>>
>> I can't see anything in the v3 spec which mentions a change in register
>> behaviour for SYSCALL.
>>
>> ~Andrew
>>
> I'll make sure it makes it into the next update.

Thankyou!

~Andrew