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

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Message-ID: <509443c8-e0fd-935f-63d8-7264f5dd3c05@zytor.com>
Date:   Mon, 23 Jan 2023 15:58:12 -0800
From:   "H. Peter Anvin" <hpa@...or.com>
To:     Ammar Faizi <ammarfaizi2@...weeb.org>
Cc:     Dave Hansen <dave.hansen@...el.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Xin Li <xin3.li@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Peter Zijlstra <peterz@...radead.org>,
        x86 Mailing List <x86@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: the x86 sysret_rip test fails on the Intel FRED architecture

On 1/23/23 15:43, Ammar Faizi wrote:
> 
> Align them to spot differences:
> 
>      0x200893 = 0b1000000000100010010011
>      0x200a93 = 0b1000000000101010010011
>                               ^
> 
> Or just xor them to find the differences:
> 
>      (gdb) p/x 0x200893 ^ 0x200a93
>      $3 = 0x200
> 
> ** Checks my Intel SDM cheat sheets. **
> 
> Then, I was like "Oh, that's (1 << 9) a.k.a. IF. Of course we can't
> change rflags[IF] from userspace!!!".
> 
> In short, we can't use 0x200893 as the rflags_sentinel value because it
> clears the interrupt flag.
> 

Right, my mistake.

	-hpa