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Date:   Sun, 8 Jan 2023 20:55:06 +0100
From:   Borislav Petkov <bp@alien8.de>
To:     "Maciej W. Rozycki" <macro@orcam.me.uk>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        "H. Peter Anvin" <hpa@zytor.com>, x86@kernel.org,
        linux-kernel@vger.kernel.org, Kees Cook <keescook@chromium.org>
Subject: Re: [PATCH] x86: Disable kernel stack offset randomization for !TSC
Message-ID: <Y7sfdxJ5/2DSQK/l@zn.tnic>
References: <alpine.DEB.2.21.2301081919550.65308@angie.orcam.me.uk>
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On Sun, Jan 08, 2023 at 07:42:54PM +0000, Maciej W. Rozycki wrote:
> For x86 kernel stack offset randomization uses the RDTSC instruction, 
> which causes an invalid opcode exception with hardware that does not 
> implement this instruction:

Out of pure curiosity, what hw is that?

Also, I guess the fix should be something like this instead:

---
diff --git a/arch/x86/include/asm/entry-common.h b/arch/x86/include/asm/entry-common.h
index 117903881fe4..57b372ca6ce7 100644
--- a/arch/x86/include/asm/entry-common.h
+++ b/arch/x86/include/asm/entry-common.h
@@ -85,7 +85,8 @@ static inline void arch_exit_to_user_mode_prepare(struct pt_regs *regs,
 	 * Therefore, final stack offset entropy will be 5 (x86_64) or
 	 * 6 (ia32) bits.
 	 */
-	choose_random_kstack_offset(rdtsc() & 0xFF);
+	if (cpu_feature_enabled(X86_FEATURE_TSC))
+		choose_random_kstack_offset(rdtsc() & 0xFF);
 }
 #define arch_exit_to_user_mode_prepare arch_exit_to_user_mode_prepare
 

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette