Re: [PATCH v7 05/26] x86/mpx: Do not use SIB.base if its value is 101b and ModRM.mod = 0

[Ricardo Neri]

On Mon, 2017-05-29 at 15:07 +0200, Borislav Petkov wrote:
> On Fri, May 05, 2017 at 11:17:03AM -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 a SIB byte is used and the
> > base of the SIB byte points is base = 101b and the mod part
> > of the ModRM byte is zero, the base port on the effective address
> > computation is null. In this case, a 32-bit displacement follows the SIB
> > byte. This is obtained when the instruction decoder parses the operands.
> > 
> > To signal this scenario, a -EDOM error is returned to indicate callers that
> > they should ignore the base.
> > 
> > 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 | 27 ++++++++++++++++++++-------
> >  1 file changed, 20 insertions(+), 7 deletions(-)
> > 
> > diff --git a/arch/x86/mm/mpx.c b/arch/x86/mm/mpx.c
> > index 7397b81..30aef92 100644
> > --- a/arch/x86/mm/mpx.c
> > +++ b/arch/x86/mm/mpx.c
> > @@ -122,6 +122,15 @@ static int get_reg_offset(struct insn *insn, struct pt_regs *regs,
> >  
> >  	case REG_TYPE_BASE:
> >  		regno = X86_SIB_BASE(insn->sib.value);
> > +		/*
> > +		 * If ModRM.mod is 0 and SIB.base == 5, the base of the
> > +		 * register-indirect addressing is 0. In this case, a
> > +		 * 32-bit displacement is expected in this case; the
> > +		 * instruction decoder finds such displacement for us.
> 
> That last sentence reads funny. Just say:
> 
> "In this case, a 32-bit displacement follows the SIB byte."

Agreed. I will update the comment to make more sense.

Thanks and BR,
Ricardo