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 <[email protected]>
> Cc: Andy Lutomirski <[email protected]>
> Cc: Dave Hansen <[email protected]>
> Cc: Adam Buchbinder <[email protected]>
> Cc: Colin Ian King <[email protected]>
> Cc: Lorenzo Stoakes <[email protected]>
> Cc: Qiaowei Ren <[email protected]>
> Cc: Peter Zijlstra <[email protected]>
> Cc: Nathan Howard <[email protected]>
> Cc: Adan Hawthorn <[email protected]>
> Cc: Joe Perches <[email protected]>
> Cc: Ravi V. Shankar <[email protected]>
> Cc: [email protected]
> Signed-off-by: Ricardo Neri <[email protected]>
> ---
> 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.
> + /*
> + * 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.
> + /*
> + * 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.
SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284
(AG Nürnberg)
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