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

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Message-ID: <fb1cab9f-a373-38e6-92e6-456332010653@gnuweeb.org>
Date:   Mon, 23 Jan 2023 16:02:22 +0700
From:   Ammar Faizi <ammarfaizi2@...weeb.org>
To:     "H. Peter Anvin" <hpa@...or.com>
Cc:     Dave Hansen <dave.hansen@...el.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "Li, Xin3" <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@...nel.org" <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 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.

Alternative, without using that flag, maybe preserve the value like:

     movq    -8(%rsp), %r12
     pushq   %[rflags_sentinel]
     popf
     movq    %r12, -8(%rsp)
     syscall

with "r12" and "memory" added to the clobber list.

What do you think?

-- 
Ammar Faizi