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

Borislav Petkov
2017-04-20T08:25:33Z

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@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: Arnaldo Carvalho de Melo <acme@redhat.com>
> Cc: Masami Hiramatsu <mhiramat@kernel.org>
> Cc: Adrian Hunter <adrian.hunter@intel.com>
> Cc: Kees Cook <keescook@chromium.org>
> Cc: Thomas Garnier <thgarnie@google.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Borislav Petkov <bp@suse.de>
> Cc: Dmitry Vyukov <dvyukov@google.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/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 Imendörffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nürnberg)
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