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

Ricardo Neri
2017-06-15T19:24:35Z

On Wed, 2017-06-07 at 14:59 +0200, Borislav Petkov wrote:
> 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@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 | 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.

OK. This makes sense. Perhaps I can use a couple of #define's to set and
get the the address and operand sizes in a single u8. This would make
the code more readable.

>
> > + 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/

Will correct.
>
> > + */
> > +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.

Alternatively, I can do desc->type & BIT(3) to avoid using desc-b, which
is less elegant.

Thanks and BR,
Ricardo