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

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Message-ID: <A87231FB-5B5D-46B7-9888-5770039B7EA1@zytor.com>
Date:   Fri, 20 Jan 2023 12:50:51 -0800
From:   "H. Peter Anvin" <hpa@...or.com>
To:     Andrew Cooper <Andrew.Cooper3@...rix.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>
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 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.