linux-hardening - RE: [RFC PATCH v2 20/21] x86: Add support for CONFIG_CFI

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Date:   Tue, 17 May 2022 07:56:05 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Sami Tolvanen' <samitolvanen@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Kees Cook <keescook@...omium.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        "x86@...nel.org" <x86@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        "Nathan Chancellor" <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Joao Moreira <joao@...rdrivepizza.com>,
        Sedat Dilek <sedat.dilek@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        "linux-hardening@...r.kernel.org" <linux-hardening@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "llvm@...ts.linux.dev" <llvm@...ts.linux.dev>
Subject: RE: [RFC PATCH v2 20/21] x86: Add support for CONFIG_CFI_CLANG

From: Sami Tolvanen
> Sent: 16 May 2022 23:03
> 
> On Mon, May 16, 2022 at 2:44 PM Peter Zijlstra <peterz@...radead.org> wrote:
> >
> > On Mon, May 16, 2022 at 09:32:55PM +0000, David Laight wrote:
> >
> > > > The compiler always generates this specific instruction sequence.
> > >
> > > Yes, but there are several ways to encode 'cmpl imm,-6(reg)'.
> >
> > Yes, but we don't care. This *always* uses the 32bit immediate form.
> > Even if the immediate is small.
> 
> Yes, that part is not a problem, but it's a valid point that LLVM
> might not always use r8-r15 here, so I will have to check for the REX
> prefix before blindly attempting to decode the instruction.

Are you allowing for the REX prefix at all?
The encoding of:
> > > +      *   cmpl    <id>, -6(%reg)     ; 7 bytes
is
	<opcode><mod/TTT/rm><off8><imm32>
which is 7 bytes without the REX.
If reg is r11 there is an extra REX byte - for 8 in total.

Without the REX byte the decode will be using %bx.
So the testing should all have failed.
Which means that something else is wrong as well.

	David

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