Re: [v6 PATCH 09/21] x86/insn-eval: Add functions to get default operand and address sizes

[Borislav Petkov]

On Tue, Mar 07, 2017 at 04:32:42PM -0800, Ricardo Neri wrote:
> These functions read the default values of the address and operand sizes
> as specified in the segment descriptor. This information is determined
> from the D and L bits. Hence, it can be used for both IA-32e 64-bit and
> 32-bit legacy modes. For virtual-8086 mode, the default address and
> operand sizes are always 2 bytes.

Yeah, we tend to call that customarily 16-bit :)

> The D bit is only meaningful for code segments. Thus, these functions
> always use the code segment selector contained in regs.
> 
> Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> Cc: Adam Buchbinder <adam.buchbinder@xxxxxxxxx>
> Cc: Colin Ian King <colin.king@xxxxxxxxxxxxx>
> Cc: Lorenzo Stoakes <lstoakes@xxxxxxxxx>
> Cc: Qiaowei Ren <qiaowei.ren@xxxxxxxxx>
> Cc: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
> Cc: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
> Cc: Adrian Hunter <adrian.hunter@xxxxxxxxx>
> Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> Cc: Thomas Garnier <thgarnie@xxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Borislav Petkov <bp@xxxxxxx>
> Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
> Cc: Ravi V. Shankar <ravi.v.shankar@xxxxxxxxx>
> Cc: x86@xxxxxxxxxx
> Signed-off-by: Ricardo Neri <ricardo.neri-calderon@xxxxxxxxxxxxxxx>
> ---
>  arch/x86/include/asm/insn-eval.h |  2 +
>  arch/x86/lib/insn-eval.c         | 80 ++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 82 insertions(+)
> 
> diff --git a/arch/x86/include/asm/insn-eval.h b/arch/x86/include/asm/insn-eval.h
> index b201742..a0d81fc 100644
> --- a/arch/x86/include/asm/insn-eval.h
> +++ b/arch/x86/include/asm/insn-eval.h
> @@ -15,6 +15,8 @@ void __user *insn_get_addr_ref(struct insn *insn, struct pt_regs *regs);
>  int insn_get_reg_offset_modrm_rm(struct insn *insn, struct pt_regs *regs);
>  int insn_get_reg_offset_sib_base(struct insn *insn, struct pt_regs *regs);
>  int insn_get_reg_offset_sib_base(struct insn *insn, struct pt_regs *regs);
> +unsigned char insn_get_seg_default_address_bytes(struct pt_regs *regs);
> +unsigned char insn_get_seg_default_operand_bytes(struct pt_regs *regs);
>  unsigned long insn_get_seg_base(struct pt_regs *regs, struct insn *insn,
>  				int regoff, bool use_default_seg);
>  
> diff --git a/arch/x86/lib/insn-eval.c b/arch/x86/lib/insn-eval.c
> index 383ca83..cda6c71 100644
> --- a/arch/x86/lib/insn-eval.c
> +++ b/arch/x86/lib/insn-eval.c
> @@ -421,6 +421,86 @@ unsigned long insn_get_seg_base(struct pt_regs *regs, struct insn *insn,
>  }
>  
>  /**
> + * insn_get_seg_default_address_bytes - Obtain default address size of segment
> + * @regs:	Set of registers containing the segment selector
> + *
> + * Obtain the default address size as indicated in the segment descriptor
> + * selected in regs' code segment selector. In protected mode, the default
> + * address is determined by inspecting the L and D bits of the segment
> + * descriptor. In virtual-8086 mode, the default is always two bytes.
> + *
> + * Return: Default address size of segment

		0 on error.

> + */
> +unsigned char insn_get_seg_default_address_bytes(struct pt_regs *regs)
> +{
> +	struct desc_struct *desc;
> +	unsigned short seg;
> +	int ret;
> +
> +	if (v8086_mode(regs))
> +		return 2;
> +
> +	seg = (unsigned short)regs->cs;
> +
> +	ret = get_desc(seg, &desc);
> +	if (ret)
> +		return 0;
> +
> +	switch ((desc->l << 1) | desc->d) {
> +	case 0: /* Legacy mode. 16-bit addresses. CS.L=0, CS.D=0 */
> +		return 2;
> +	case 1: /* Legacy mode. 32-bit addresses. CS.L=0, CS.D=1 */
> +		return 4;
> +	case 2: /* IA-32e 64-bit mode. 64-bit addresses. CS.L=1, CS.D=0 */
> +		return 8;
> +	case 3: /* Invalid setting. CS.L=1, CS.D=1 */
> +		/* fall through */
> +	default:
> +		return 0;
> +	}
> +}
> +
> +/**
> + * insn_get_seg_default_operand_bytes - Obtain default operand size of segment
> + * @regs:	Set of registers containing the segment selector
> + *
> + * Obtain the default operand size as indicated in the segment descriptor
> + * selected in regs' code segment selector. In protected mode, the default
> + * operand size is determined by inspecting the L and D bits of the segment
> + * descriptor. In virtual-8086 mode, the default is always two bytes.
> + *
> + * Return: Default operand size of segment
> + */
> +unsigned char insn_get_seg_default_operand_bytes(struct pt_regs *regs)

Right, so default address and operand size always go together so I don't
think you need two separate functions.

So what I'd suggest - provided this pans out (I still haven't reviewed
the whole thing) - is to determine the operating mode of the segment:
long, legacy, etc and then return both address and operand sizes. Patch
17/21 needs them both at the same time AFAICT.

-- 
Regards/Gruss,
    Boris.

SUSE Linux GmbH, GF: Felix ImendÃrffer, Jane Smithard, Graham Norton, HRB 21284 (AG NÃrnberg)
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