Re: [PATCH v7 04/26] x86/mpx: Do not use SIB.index if its value is 100b and ModRM.mod is not 11b

[Ricardo Neri]

On Wed, 2017-05-24 at 15:37 +0200, Borislav Petkov wrote:
> On Fri, May 05, 2017 at 11:17:02AM -0700, 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 ModRM.mod !=11b and
> > ModRM.rm = 100b indexed register-indirect addressing is used. In other
> > words, a SIB byte follows the ModRM byte. In the specific case of
> > SIB.index = 100b, the scale*index portion of the computation of the
> > effective address is null. To signal callers of this particular situation,
> > get_reg_offset() can return -EDOM (-EINVAL continues to indicate that an
> > error when decoding the SIB byte).
> > 
> > An example of this situation can be the following instruction:
> > 
> >    8b 4c 23 80       mov -0x80(%rbx,%riz,1),%rcx
> >    ModRM:            0x4c [mod:1b][reg:1b][rm:100b]
> >    SIB:              0x23 [scale:0b][index:100b][base:11b]
> >    Displacement:     0x80  (1-byte, as per ModRM.mod = 1b)
> > 
> > The %riz 'register' indicates a null index.
> > 
> > In long mode, a REX prefix may be used. When a REX prefix is present,
> > REX.X adds a fourth bit to the register selection of SIB.index. This gives
> > the ability to refer to all the 16 general purpose registers. When REX.X is
> > 1b and SIB.index is 100b, the index is indicated in %r12. In our example,
> > this would look like:
> > 
> >    42 8b 4c 23 80    mov -0x80(%rbx,%r12,1),%rcx
> >    REX:              0x42 [W:0b][R:0b][X:1b][B:0b]
> >    ModRM:            0x4c [mod:1b][reg:1b][rm:100b]
> >    SIB:              0x23 [scale:0b][.X: 1b, index:100b][.B:0b, base:11b]
> >    Displacement:     0x80  (1-byte, as per ModRM.mod = 1b)
> > 
> > Cc: Borislav Petkov <bp@xxxxxxx>
> > Cc: Andy Lutomirski <luto@xxxxxxxxxx>
> > Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> > Cc: Adam Buchbinder <adam.buchbinder@xxxxxxxxx>
> > Cc: Colin Ian King <colin.king@xxxxxxxxxxxxx>
> > Cc: Lorenzo Stoakes <lstoakes@xxxxxxxxx>
> > Cc: Qiaowei Ren <qiaowei.ren@xxxxxxxxx>
> > Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > Cc: Nathan Howard <liverlint@xxxxxxxxx>
> > Cc: Adan Hawthorn <adanhawthorn@xxxxxxxxx>
> > Cc: Joe Perches <joe@xxxxxxxxxxx>
> > Cc: Ravi V. Shankar <ravi.v.shankar@xxxxxxxxx>
> > Cc: x86@xxxxxxxxxx
> > Signed-off-by: Ricardo Neri <ricardo.neri-calderon@xxxxxxxxxxxxxxx>
> > ---
> >  arch/x86/mm/mpx.c | 20 ++++++++++++++++++--
> >  1 file changed, 18 insertions(+), 2 deletions(-)
> > 
> > diff --git a/arch/x86/mm/mpx.c b/arch/x86/mm/mpx.c
> > index ebdead8..7397b81 100644
> > --- a/arch/x86/mm/mpx.c
> > +++ b/arch/x86/mm/mpx.c
> > @@ -110,6 +110,14 @@ static int get_reg_offset(struct insn *insn, struct pt_regs *regs,
> >  		regno = X86_SIB_INDEX(insn->sib.value);
> >  		if (X86_REX_X(insn->rex_prefix.value))
> >  			regno += 8;
> 
> <--- newline.
I will add a new line here.

> 
> > +		/*
> > +		 * If ModRM.mod !=3 and SIB.index (regno=4) the scale*index
> > +		 * portion of the address computation is null. This is
> > +		 * true only if REX.X is 0. In such a case, the SIB index
> > +		 * is used in the address computation.
> > +		 */
> > +		if (X86_MODRM_MOD(insn->modrm.value) != 3 && regno == 4)
> > +			return -EDOM;
> >  		break;
> >  
> >  	case REG_TYPE_BASE:
> > @@ -159,11 +167,19 @@ static void __user *mpx_get_addr_ref(struct insn *insn, struct pt_regs *regs)
> >  				goto out_err;
> >  
> >  			indx_offset = get_reg_offset(insn, regs, REG_TYPE_INDEX);
> > -			if (indx_offset < 0)
> 
> <--- newline.
I will add a new line here.

> 
> > +			/*
> > +			 * A negative offset generally means a error, except
> 
> 							     an
> 
> > +			 * -EDOM, which means that the contents of the register
> > +			 * should not be used as index.
> > +			 */
> > +			if (indx_offset == -EDOM)
> > +				indx = 0;
> > +			else if (indx_offset < 0)
> >  				goto out_err;
> > +			else
> > +				indx = regs_get_register(regs, indx_offset);
> >  
> >  			base = regs_get_register(regs, base_offset);
> > -			indx = regs_get_register(regs, indx_offset);
> >  			eff_addr = base + indx * (1 << X86_SIB_SCALE(sib));
> >  		} else {
> >  			addr_offset = get_reg_offset(insn, regs, REG_TYPE_RM);
> > -- 
> > 2.9.3
> > 
> 
> -- 
> Regards/Gruss,
>     Boris.

Thanks for reviewing!

BR,
Ricardo
> 
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