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

Borislav Petkov
2017-05-29T13:07:09Z

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@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 | 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."

> + */
> + if (!X86_MODRM_MOD(insn->modrm.value) && regno == 5)
> + return -EDOM;
> +
> if (X86_REX_B(insn->rex_prefix.value))
> regno += 8;
> break;

--
Regards/Gruss,
Boris.

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