Re: [PATCH v7 13/26] x86/insn-eval: Add function to get default params of code segment

[Borislav Petkov]

On Fri, May 05, 2017 at 11:17:11AM -0700, Ricardo Neri wrote:
> This function returns 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.
> 
> 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 |  6 ++++
>  arch/x86/lib/insn-eval.c         | 65 ++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 71 insertions(+)
> 
> diff --git a/arch/x86/include/asm/insn-eval.h b/arch/x86/include/asm/insn-eval.h
> index 7f3c7fe..9ed1c88 100644
> --- a/arch/x86/include/asm/insn-eval.h
> +++ b/arch/x86/include/asm/insn-eval.h
> @@ -11,9 +11,15 @@
>  #include <linux/err.h>
>  #include <asm/ptrace.h>
>  
> +struct insn_code_seg_defaults {

A whole struct for a function which gets called only once?

Bah, that's a bit too much, if you ask me.

So you're returning two small unsigned integers - i.e., you can just as
well return a single u8 and put address and operand sizes in there:

	ret = oper_sz | addr_sz << 4;

No need for special structs for that.

> +	unsigned char address_bytes;
> +	unsigned char operand_bytes;
> +};
> +
>  void __user *insn_get_addr_ref(struct insn *insn, struct pt_regs *regs);
>  int insn_get_modrm_rm_off(struct insn *insn, struct pt_regs *regs);
>  unsigned long insn_get_seg_base(struct pt_regs *regs, struct insn *insn,
>  				int regoff);
> +struct insn_code_seg_defaults insn_get_code_seg_defaults(struct pt_regs *regs);
>  
>  #endif /* _ASM_X86_INSN_EVAL_H */
> diff --git a/arch/x86/lib/insn-eval.c b/arch/x86/lib/insn-eval.c
> index c77ed80..693e5a8 100644
> --- a/arch/x86/lib/insn-eval.c
> +++ b/arch/x86/lib/insn-eval.c
> @@ -603,6 +603,71 @@ static unsigned long get_seg_limit(struct pt_regs *regs, struct insn *insn,
>  }
>  
>  /**
> + * insn_get_code_seg_defaults() - Obtain code segment default parameters
> + * @regs:	Structure with register values as seen when entering kernel mode
> + *
> + * Obtain the default parameters of the code segment: address and operand sizes.
> + * The code segment is obtained from the selector contained in the CS register
> + * in regs. 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 for both address and operand sizes.
> + *
> + * Return: A populated insn_code_seg_defaults structure on success. The
> + * structure contains only zeros on failure.

s/failure/error/

> + */
> +struct insn_code_seg_defaults insn_get_code_seg_defaults(struct pt_regs *regs)
> +{
> +	struct desc_struct *desc;
> +	struct insn_code_seg_defaults defs;
> +	unsigned short sel;
> +	/*
> +	 * The most significant byte of AR_TYPE_MASK determines whether a
> +	 * segment contains data or code.
> +	 */
> +	unsigned int type_mask = AR_TYPE_MASK & (1 << 11);
> +
> +	memset(&defs, 0, sizeof(defs));
> +
> +	if (v8086_mode(regs)) {
> +		defs.address_bytes = 2;
> +		defs.operand_bytes = 2;
> +		return defs;
> +	}
> +
> +	sel = (unsigned short)regs->cs;
> +
> +	desc = get_desc(sel);
> +	if (!desc)
> +		return defs;
> +
> +	/* if data segment, return */
> +	if (!(desc->b & type_mask))
> +		return defs;

So you can simplify that into:

	/* A code segment? */
	if (!(desc->b & BIT(11)))
		return defs;

and remove that type_mask thing.

-- 
Regards/Gruss,
    Boris.

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