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Message-ID: <20231025065610.GA31201@noisy.programming.kicks-ass.net>
Date: Wed, 25 Oct 2023 08:56:10 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
Cc: Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
"H. Peter Anvin" <hpa@...or.com>,
Josh Poimboeuf <jpoimboe@...nel.org>,
Andy Lutomirski <luto@...nel.org>,
Jonathan Corbet <corbet@....net>,
Sean Christopherson <seanjc@...gle.com>,
Paolo Bonzini <pbonzini@...hat.com>, tony.luck@...el.com,
ak@...ux.intel.com, tim.c.chen@...ux.intel.com,
linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
kvm@...r.kernel.org,
Alyssa Milburn <alyssa.milburn@...ux.intel.com>,
Daniel Sneddon <daniel.sneddon@...ux.intel.com>,
antonio.gomez.iglesias@...ux.intel.com,
Alyssa Milburn <alyssa.milburn@...el.com>
Subject: Re: [PATCH v2 1/6] x86/bugs: Add asm helpers for executing VERW
On Tue, Oct 24, 2023 at 09:00:29PM -0700, Pawan Gupta wrote:
> config1: mitigations=on, 32-bit mode, post-boot
>
> entry_SYSENTER_32:
> ...
> 0xc1a3748e <+222>: pop %eax
> 0xc1a3748f <+223>: verw 0xc1a38240
> 0xc1a37496 <+230>: sti
> 0xc1a37497 <+231>: sysexit
>
> ---------------------------------------------
>
> config2: mitigations=off, 32-bit mode, post-boot
>
> entry_SYSENTER_32:
> ...
> 0xc1a3748e <+222>: pop %eax
> 0xc1a3748f <+223>: lea 0x0(%esi,%eiz,1),%esi <---- Doesn't look right
> 0xc1a37496 <+230>: sti
> 0xc1a37497 <+231>: sysexit
>
> ---------------------------------------------
>
> config3: 32-bit mode, pre-boot objdump
>
> entry_SYSENTER_32:
> ...
> c8e: 58 pop %eax
> c8f: 90 nop
> c90: 90 nop
> c91: 90 nop
> c92: 90 nop
> c93: 90 nop
> c94: 90 nop
> c95: 90 nop
> c96: fb sti
> c97: 0f 35 sysexit
>
If you look at arch/x86/include/asm/nops.h, you'll find (for 32bit):
* 7: leal 0x0(%esi,%eiz,1),%esi
Which reads as:
load-effective-address of %esi[0] into %esi
which is, of course, just %esi.
You can also get this from GAS using:
.nops 7
which results in:
0: 8d b4 26 00 00 00 00 lea 0x0(%esi,%eiz,1),%esi
It is basically abusing bizarro x86 instruction encoding rules to create
a 7 byte nop without using NOPL. If you really want to know, volume 2
of the SDM has a ton of stuff on instruction encoding, also the interweb
:-)
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