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

Ricardo Neri
2017-06-15T19:36:06Z

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