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

Ricardo Neri
2017-04-26T22:37:44Z

On Thu, 2017-04-20 at 10:25 +0200, Borislav Petkov wrote:
> 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 //

I will fix this typo.
>
> > 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().

I will use parentheses.
>
> > described above. Once the selector is known the base address is
>
> known, ...

Will fix.
>
> > 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/

I will correct the typo.
>
> > + * 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.

I will add it.
>
> And that's not really a "seg_type" but simply the "sel"-ector.

I will update the variable names to reflect the fact that they are
segment selectors.

> And that
> "enum segment" is not really a segment but an segment override prefixes
> enum. Can we please get the nomenclature right first?

I need a human-readable way of identifying what segment selector (in
pt_regs, vm86regs or directly reading the segment registers) to use.
Since there is a segment override prefix for all of them, I thought I
could use them. Perhaps I can rename enum segment to enum
segment_selector and comment that the values in the enum are those of
the override prefixes. Would that be reasonable?

>
> > +
> > + seg = get_segment_selector(regs, seg_type);
>
> s/seg/sel/

Will change.

>
> > + 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)) {

I will change it.

Thanks and BR,
Ricardo