Re: [PATCH v7 12/26] x86/insn-eval: Add utility functions to get segment descriptor base address and limit

Borislav Petkov
2017-05-31T16:58:39Z

On Fri, May 05, 2017 at 11:17:10AM -0700, 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
> 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.

...

> diff --git a/arch/x86/lib/insn-eval.c b/arch/x86/lib/insn-eval.c
> index f46cb31..c77ed80 100644
> --- a/arch/x86/lib/insn-eval.c
> +++ b/arch/x86/lib/insn-eval.c
> @@ -476,6 +476,133 @@ static struct desc_struct *get_desc(unsigned short sel)
> }
>
> /**
> + * insn_get_seg_base() - Obtain base address of segment descriptor.
> + * @regs: Structure with register values as seen when entering kernel mode
> + * @insn: Instruction structure with selector override prefixes
> + * @regoff: Operand offset, in pt_regs, of which the selector is needed
> + *
> + * 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. Zero in for long
> + * mode, except when FS or GS are used. In virtual-8086 mode, the segment
> + * selector shifted 4 positions to the right. -1L in case of
> + * error.
> + */
> +unsigned long insn_get_seg_base(struct pt_regs *regs, struct insn *insn,
> + int regoff)
> +{
> + struct desc_struct *desc;
> + unsigned short sel;
> + enum segment_register seg_reg;
> +
> + seg_reg = resolve_seg_register(insn, regs, regoff);
> + if (seg_reg == SEG_REG_INVAL)
> + return -1L;
> +
> + sel = get_segment_selector(regs, seg_reg);
> + if ((short)sel < 0)

I guess it would be better if that function returned a signed short so
you don't have to cast it here. (You're casting it to an unsigned long
below anyway.)

> + return -1L;
> +
> + if (v8086_mode(regs))
> + /*
> + * Base is simply the segment selector shifted 4
> + * positions to the right.
> + */
> + return (unsigned long)(sel << 4);
> +

...

> +static unsigned long get_seg_limit(struct pt_regs *regs, struct insn *insn,
> + int regoff)
> +{
> + struct desc_struct *desc;
> + unsigned short sel;
> + unsigned long limit;
> + enum segment_register seg_reg;
> +
> + seg_reg = resolve_seg_register(insn, regs, regoff);
> + if (seg_reg == SEG_REG_INVAL)
> + return 0;
> +
> + sel = get_segment_selector(regs, seg_reg);
> + if ((short)sel < 0)

Ditto.

> + return 0;
> +
> + if (user_64bit_mode(regs) || v8086_mode(regs))
> + return -1L;
> +
> + if (!sel)
> + return 0;
> +
> + desc = get_desc(sel);
> + if (!desc)
> + return 0;
> +
> + /*
> + * If the granularity bit is set, the limit is given in multiples
> + * of 4096. When the granularity bit is set, the least 12 significant

the 12 least significant bits

> + * bits are not tested when checking the segment limits. In practice,
> + * this means that the segment ends in (limit << 12) + 0xfff.
> + */
> + limit = get_desc_limit(desc);
> + if (desc->g)
> + limit <<= 12 | 0x7;

That 0x7 doesn't look like 0xfff - it shifts limit by 15 instead. You
can simply write it like you mean it:

limit = (limit << 12) + 0xfff;

--
Regards/Gruss,
Boris.

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