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Re: [v6 PATCH 03/21] x86/mpx: Do not use R/EBP as base in the SIB byte with Mod = 0

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  Re: [v6 PATCH 03/21] x86/mpx: Do not use R/EBP as base in the SIB  byte with Mod = 0 Ricardo Neri <ricardo.neri-calderon@linux.intel.com> - 2017-04-26 04:10 +0200
    Re: [v6 PATCH 03/21] x86/mpx: Do not use R/EBP as base in the SIB  byte with Mod = 0 Borislav Petkov <bp@suse.de> - 2017-04-26 10:20 +0200
      Re: [v6 PATCH 03/21] x86/mpx: Do not use R/EBP as base in the SIB  byte with Mod = 0 Ricardo Neri <ricardo.neri-calderon@linux.intel.com> - 2017-04-28 01:00 +0200

#1631103 — Re: [v6 PATCH 03/21] x86/mpx: Do not use R/EBP as base in the SIB byte with Mod = 0

On Wed, 2017-04-12 at 00:08 +0200, Borislav Petkov wrote:
> On Tue, Mar 07, 2017 at 04:32:36PM -0800, Ricardo Neri wrote:
> > Section 2.2.1.2 of the Intel 64 and IA-32 Architectures Software
> > Developer's Manual volume 2A states that when a SIB byte is used and the
> > base of the SIB byte points to R/EBP (i.e., base = 5) and the mod part
> > of the ModRM byte is zero, the value of such register will not be used
> > as part of the address computation. To signal this, a -EDOM error is
> > returned to indicate callers that they should ignore the value.
> > 
> > Also, for this particular case, a displacement of 32-bits should follow
> > the SIB byte if the mod part of ModRM is equal to zero. The instruction
> > decoder ensures that this is the case.
> > 
> > Cc: Dave Hansen <dave.hansen@linux.intel.com>
> > Cc: Adam Buchbinder <adam.buchbinder@gmail.com>
> > Cc: Colin Ian King <colin.king@canonical.com>
> > Cc: Lorenzo Stoakes <lstoakes@gmail.com>
> > Cc: Qiaowei Ren <qiaowei.ren@intel.com>
> > Cc: Peter Zijlstra <peterz@infradead.org>
> > Cc: Nathan Howard <liverlint@gmail.com>
> > Cc: Adan Hawthorn <adanhawthorn@gmail.com>
> > Cc: Joe Perches <joe@perches.com>
> > Cc: Ravi V. Shankar <ravi.v.shankar@intel.com>
> > Cc: x86@kernel.org
> > Signed-off-by: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
> > ---
> >  arch/x86/mm/mpx.c | 29 ++++++++++++++++++++++-------
> >  1 file changed, 22 insertions(+), 7 deletions(-)
> > 
> > diff --git a/arch/x86/mm/mpx.c b/arch/x86/mm/mpx.c
> > index d9e92d6..ef7eb67 100644
> > --- a/arch/x86/mm/mpx.c
> > +++ b/arch/x86/mm/mpx.c
> > @@ -121,6 +121,17 @@ static int get_reg_offset(struct insn *insn, struct pt_regs *regs,
> >  
> >  	case REG_TYPE_BASE:
> >  		regno = X86_SIB_BASE(insn->sib.value);
> > +		/*
> > +		 * If mod is 0 and register R/EBP (regno=5) is indicated in the
> > +		 * base part of the SIB byte,
> 
> you can simply say here: "if SIB.base == 5, the base of the
> register-indirect addressing is 0."

This is better wording. I will change it.
> 
> > the value of such register should
> > +		 * not be used in the address computation. Also, a 32-bit
> 
> Not "Also" but "In this case, a 32-bit displacement..."

Will change.
> 
> > +		 * displacement is expected in this case; the instruction
> > +		 * decoder takes care of it. This is true for both R13 and
> > +		 * R/EBP as REX.B will not be decoded.
> 
> You don't need that sentence as the only thing that matters is ModRM.mod
> being 0.

For the specific case of ModRM.mod being 0, I feel I need to clarify
that REX.B is not decoded and if SIB.base is %r13 the base is also 0.
This comment adds clarity because REX.X is decoded when determining
SIB.index.
> 
> > +		 */
> > +		if (regno == 5 && X86_MODRM_MOD(insn->modrm.value) == 0)
> 
> The 0 test we normally do with the ! (also flip parts of if-condition):
> 
> 		if (!X86_MODRM_MOD(insn->modrm.value) && regno == 5)

Will change it.
> 
> > +			return -EDOM;
> > +
> >  		if (X86_REX_B(insn->rex_prefix.value))
> >  			regno += 8;
> >  		break;
> > @@ -161,16 +172,21 @@ static void __user *mpx_get_addr_ref(struct insn *insn, struct pt_regs *regs)
> >  		eff_addr = regs_get_register(regs, addr_offset);
> >  	} else {
> >  		if (insn->sib.nbytes) {
> > +			/*
> > +			 * Negative values in the base and index offset means
> > +			 * an error when decoding the SIB byte. Except -EDOM,
> > +			 * which means that the registers should not be used
> > +			 * in the address computation.
> > +			 */
> >  			base_offset = get_reg_offset(insn, regs, REG_TYPE_BASE);
> > -			if (base_offset < 0)
> > +			if (unlikely(base_offset == -EDOM))
> > +				base = 0;
> > +			else if (unlikely(base_offset < 0))
> 
> Bah, unlikely's in something which is not really a hot path. They only
> encumber readability, no need for them.

I will remove them.

Thanks and BR,
Ricardo

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#1631224

On Tue, Apr 25, 2017 at 07:04:20PM -0700, Ricardo Neri wrote:
> For the specific case of ModRM.mod being 0, I feel I need to clarify
> that REX.B is not decoded and if SIB.base is %r13 the base is also 0.

Well, that all doesn't matter. The rule is this:

ModRM.mod == 00b and ModRM.r/m == 101b -> effective address: disp32

See Table 2-2. "32-Bit Addressing Forms with the ModR/M Byte" in the SDM.

So the base register is not used. How that base register is specified
then doesn't matter (undecoded REX bits or not).

> This comment adds clarity because REX.X is decoded when determining
> SIB.index.

Well, that's a different thing. The REX bits participating in the SIB
fields don't matter about this particular case. We only want to say that
we're returning a disp32 without a base register and the comment should
keep it simple without extraneous information.

I know, you want to mention what Table 2-5. "Special Cases of REX
Encodings" says but we should avoid unnecessary content in the comment.
People who want details can stare at the manuals - the comment should
only document what that particular case is.

Btw, you could write it even better:

	if (!X86_MODRM_MOD(insn->modrm.value) && X86_MODRM_RM(insn->modrm.value) == 5)

and then it is basically a 1:1 copy of the rule from Table 2-2.

-- 
Regards/Gruss,
    Boris.

SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nürnberg)
-- 

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#1632430

On Wed, 2017-04-26 at 10:05 +0200, Borislav Petkov wrote:
> On Tue, Apr 25, 2017 at 07:04:20PM -0700, Ricardo Neri wrote:
> > For the specific case of ModRM.mod being 0, I feel I need to clarify
> > that REX.B is not decoded and if SIB.base is %r13 the base is also 0.
> 
> Well, that all doesn't matter. The rule is this:
> 
> ModRM.mod == 00b and ModRM.r/m == 101b -> effective address: disp32
> 
> See Table 2-2. "32-Bit Addressing Forms with the ModR/M Byte" in the SDM.

You are right. This summarizes the rule. Then I will shorten the
comment.
> 
> So the base register is not used. How that base register is specified
> then doesn't matter (undecoded REX bits or not).
> 
> > This comment adds clarity because REX.X is decoded when determining
> > SIB.index.
> 
> Well, that's a different thing. The REX bits participating in the SIB
> fields don't matter about this particular case. We only want to say that
> we're returning a disp32 without a base register and the comment should
> keep it simple without extraneous information.
> 
> I know, you want to mention what Table 2-5. "Special Cases of REX
> Encodings" says but we should avoid unnecessary content in the comment.
> People who want details can stare at the manuals - the comment should
> only document what that particular case is.
> 
> Btw, you could write it even better:
> 
> 	if (!X86_MODRM_MOD(insn->modrm.value) && X86_MODRM_RM(insn->modrm.value) == 5)
> 
> and then it is basically a 1:1 copy of the rule from Table 2-2.

It is!

Thanks and BR,
Ricardo

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