linux-kernel - Re: [PATCH v7 14/26] x86/insn-eval: Indicate a 32-bit displacement if ModRM.mod is 0 and ModRM.rm is 5

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Date:   Thu, 15 Jun 2017 12:36:06 -0700
From:   Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
To:     Borislav Petkov <bp@...e.de>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Brian Gerst <brgerst@...il.com>,
        Chris Metcalf <cmetcalf@...lanox.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Huang Rui <ray.huang@....com>, Jiri Slaby <jslaby@...e.cz>,
        Jonathan Corbet <corbet@....net>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Paul Gortmaker <paul.gortmaker@...driver.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Chen Yucong <slaoub@...il.com>,
        Alexandre Julliard <julliard@...ehq.org>,
        Stas Sergeev <stsp@...t.ru>, Fenghua Yu <fenghua.yu@...el.com>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Shuah Khan <shuah@...nel.org>, linux-kernel@...r.kernel.org,
        x86@...nel.org, linux-msdos@...r.kernel.org, wine-devel@...ehq.org,
        Adam Buchbinder <adam.buchbinder@...il.com>,
        Colin Ian King <colin.king@...onical.com>,
        Lorenzo Stoakes <lstoakes@...il.com>,
        Qiaowei Ren <qiaowei.ren@...el.com>,
        Arnaldo Carvalho de Melo <acme@...hat.com>,
        Adrian Hunter <adrian.hunter@...el.com>,
        Kees Cook <keescook@...omium.org>,
        Thomas Garnier <thgarnie@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>
Subject: Re: [PATCH v7 14/26] x86/insn-eval: Indicate a 32-bit displacement
 if ModRM.mod is 0 and ModRM.rm is 5

On Wed, 2017-06-07 at 15:15 +0200, Borislav Petkov wrote:
> On Fri, May 05, 2017 at 11:17:12AM -0700, Ricardo Neri wrote:
> > Section 2.2.1.3 of the Intel 64 and IA-32 Architectures Software
> > Developer's Manual volume 2A states that when ModRM.mod is zero and
> > ModRM.rm is 101b, a 32-bit displacement follows the ModRM byte. This means
> > that none of the registers are used in the computation of the effective
> > address. A return value of -EDOM signals callers that they should not use
> > the value of registers when computing the effective address for the
> > instruction.
> > 
> > In IA-32e 64-bit mode (long mode), the effective address is given by the
> > 32-bit displacement plus the value of RIP of the next instruction.
> > In IA-32e compatibility mode (protected mode), only the displacement is
> > used.
> > 
> > The instruction decoder takes care of obtaining the displacement.
> 
> ...
> 
> > diff --git a/arch/x86/lib/insn-eval.c b/arch/x86/lib/insn-eval.c
> > index 693e5a8..4f600de 100644
> > --- a/arch/x86/lib/insn-eval.c
> > +++ b/arch/x86/lib/insn-eval.c
> > @@ -379,6 +379,12 @@ static int get_reg_offset(struct insn *insn, struct pt_regs *regs,
> >  	switch (type) {
> >  	case REG_TYPE_RM:
> >  		regno = X86_MODRM_RM(insn->modrm.value);
> 
> 
> <---- newline here.

Will add the new line.
> 
> > +		/*
> > +		 * ModRM.mod == 0 and ModRM.rm == 5 means a 32-bit displacement
> > +		 * follows the ModRM byte.
> > +		 */
> > +		if (!X86_MODRM_MOD(insn->modrm.value) && regno == 5)
> > +			return -EDOM;
> >  		if (X86_REX_B(insn->rex_prefix.value))
> >  			regno += 8;
> >  		break;
> > @@ -730,9 +736,21 @@ void __user *insn_get_addr_ref(struct insn *insn, struct pt_regs *regs)
> >  			eff_addr = base + indx * (1 << X86_SIB_SCALE(sib));
> >  		} else {
> >  			addr_offset = get_reg_offset(insn, regs, REG_TYPE_RM);
> > -			if (addr_offset < 0)
> 
> ditto.

Will add the new line.
> 
> > +			/*
> > +			 * -EDOM means that we must ignore the address_offset.
> > +			 * In such a case, in 64-bit mode the effective address
> > +			 * relative to the RIP of the following instruction.
> > +			 */
> > +			if (addr_offset == -EDOM) {
> > +				eff_addr = 0;
> > +				if (user_64bit_mode(regs))
> > +					eff_addr = (long)regs->ip +
> > +						   insn->length;
> 
> Let that line stick out and write it balanced:
> 
>                         if (addr_offset == -EDOM) {
>                                 if (user_64bit_mode(regs))
>                                         eff_addr = (long)regs->ip + insn->length;
>                                 else
>                                         eff_addr = 0;
> 
> should be easier parseable this way.

Will rewrite as you suggest.

Thanks and BR,
Ricardo