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

[Borislav Petkov]

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.
> -
> /*
> -
>  * 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.
> -
> 	/*
> -
> 	 * -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.