Re: [v2 2/7] x86/mpx: Fail when implicit zero-displacement is used along with R/EBP

Andy Lutomirski
2016-12-24T01:58:36Z

On Fri, Dec 23, 2016 at 5:37 PM, Ricardo Neri
<ricardo.neri-calderon@linux.intel.com> wrote:
> Section 2.2.1.2 of the Intel 64 and IA-32 Architectures Software
> Developer's Manual volume 2A states that when memory addressing with no
> explicit displacement (i.e, mod part of ModR/M is 0), a SIB byte is used
> and the base of the SIB byte points to (R/EBP) (i.e., base = 5), an
> explicit displacement of 0 must be used.
>
> Make the address decoder to return -EINVAL in such a case.
>
> 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: 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 | 7 +++++++
> 1 file changed, 7 insertions(+)
>
> diff --git a/arch/x86/mm/mpx.c b/arch/x86/mm/mpx.c
> index 6a75a75..71681d0 100644
> --- a/arch/x86/mm/mpx.c
> +++ b/arch/x86/mm/mpx.c
> @@ -120,6 +120,13 @@ static int get_reg_offset(struct insn *insn, struct pt_regs *regs,
>
> case REG_TYPE_BASE:
> regno = X86_SIB_BASE(insn->sib.value);
> + if (regno == 5 && X86_MODRM_RM(insn->modrm.value) == 0) {
> + WARN_ONCE(1, "An explicit displacement is required when %sBP used as SIB base.",
> + (IS_ENABLED(CONFIG_X86_64) && insn->x86_64) ?
> + "R13 or R" : "E");
> + return -EINVAL;
> + }
> +

Now that I've read the cover letter, I see what's going on. This
should not warn -- user code can easily trigger this deliberately.