Re: [v6 PATCH 08/21] x86/insn-eval: Add utility function to get segment descriptor base address

[Borislav Petkov]

On Tue, Mar 07, 2017 at 04:32:41PM -0800, Ricardo Neri wrote:
> With segmentation, the base address of the segment descriptor is needed
> to compute a linear address. The segment descriptor used in the address
> computation depends on either any segment override prefixes in the in the

s/in the //

> instruction or the default segment determined by the registers involved
> in the address computation. Thus, both the instruction as well as the
> register (specified as the offset from the base of pt_regs) are given as
> inputs, along with a boolean variable to select between override and
> default.
> 
> The segment selector is determined by get_seg_selector with the inputs

Please end function names with parentheses: get_seg_selector().

> described above. Once the selector is known the base address is

					known, ...

> determined. In protected mode, the selector is used to obtain the segment
> descriptor and then its base address. If in 64-bit user mode, the segment =
> base address is zero except when FS or GS are used. In virtual-8086 mode,
> the base address is computed as the value of the segment selector shifted 4
> positions to the left.

Good.

> 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         | 66 ++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 68 insertions(+)
> 
> diff --git a/arch/x86/include/asm/insn-eval.h b/arch/x86/include/asm/insn-eval.h
> index 754211b..b201742 100644
> --- a/arch/x86/include/asm/insn-eval.h
> +++ b/arch/x86/include/asm/insn-eval.h
> @@ -15,5 +15,7 @@ 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 long insn_get_seg_base(struct pt_regs *regs, struct insn *insn,
> +				int regoff, bool use_default_seg);
>  
>  #endif /* _ASM_X86_INSN_EVAL_H */
> diff --git a/arch/x86/lib/insn-eval.c b/arch/x86/lib/insn-eval.c
> index 8608adf..383ca83 100644
> --- a/arch/x86/lib/insn-eval.c
> +++ b/arch/x86/lib/insn-eval.c
> @@ -355,6 +355,72 @@ static int get_desc(unsigned short seg, struct desc_struct **desc)
>  }
>  
>  /**
> + * insn_get_seg_base() - Obtain base address contained in descriptor
> + * @regs:	Set of registers containing the segment selector
> + * @insn:	Instruction structure with selector override prefixes
> + * @regoff:	Operand offset, in pt_regs, of which the selector is needed
> + * @use_default_seg: Use the default segment instead of prefix overrides

I'm wondering whether you really need that bool or you can deduce this
from pt_regs... I guess I'll see...

> + *
> + * Obtain the base address of the segment descriptor as indicated by either
> + * any segment override prefixes contained in insn or the default segment
> + * applicable to the register indicated by regoff. regoff is specified as the
> + * offset in bytes from the base of pt_regs.
> + *
> + * Return: In protected mode, base address of the segment. It may be zero in
> + * certain cases for 64-bit builds and/or 64-bit applications. In virtual-8086
> + * mode, the segment selector shifed 4 positions to the right. -1L in case of

s/shifed/shifted/

> + * error.
> + */
> +unsigned long insn_get_seg_base(struct pt_regs *regs, struct insn *insn,
> +				int regoff, bool use_default_seg)
> +{
> +	struct desc_struct *desc;
> +	unsigned short seg;
> +	enum segment seg_type;
> +	int ret;
> +
> +	seg_type = resolve_seg_selector(insn, regoff, use_default_seg);

<--- error handling.

And that's not really a "seg_type" but simply the "sel"-ector. And that
"enum segment" is not really a segment but an segment override prefixes
enum. Can we please get the nomenclature right first?

> +
> +	seg = get_segment_selector(regs, seg_type);

s/seg/sel/

> +	if (seg < 0)
> +		return -1L;
> +
> +	if (v8086_mode(regs))
> +		/*
> +		 * Base is simply the segment selector shifted 4
> +		 * positions to the right.
> +		 */
> +		return (unsigned long)(seg << 4);
> +
> +#ifdef CONFIG_X86_64
> +	if (user_64bit_mode(regs)) {

	if (IS_ENABLED(CONFIG_X86_64) && user_64bit_mode(regs)) {

> +		/*
> +		 * Only FS or GS will have a base address, the rest of
> +		 * the segments' bases are forced to 0.
> +		 */
> +		unsigned long base;
> +
> +		if (seg_type == SEG_FS)
> +			rdmsrl(MSR_FS_BASE, base);
> +		else if (seg_type == SEG_GS)
> +			/*
> +			 * swapgs was called at the kernel entry point. Thus,
> +			 * MSR_KERNEL_GS_BASE will have the user-space GS base.
> +			 */
> +			rdmsrl(MSR_KERNEL_GS_BASE, base);
> +		else
> +			base = 0;
> +		return base;
> +	}
> +#endif
> +	ret = get_desc(seg, &desc);
> +	if (ret)
> +		return -1L;
> +
> +	return get_desc_base(desc);
> +}
> +
> +/**
>   * insn_get_reg_offset_modrm_rm - Obtain register in r/m part of ModRM byte
>   * @insn:	Instruction structure containing the ModRM byte
>   * @regs:	Set of registers indicated by the ModRM byte
> -- 
> 2.9.3
> 

-- 
Regards/Gruss,
    Boris.

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