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
2017-05-27T03:36:35Z

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@suse.de>
> > Cc: Andy Lutomirski <luto@kernel.org>
> > 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 | 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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