Re: [v6 PATCH 09/21] x86/insn-eval: Add functions to get default operand and address sizes

[Ricardo Neri]

On Thu, 2017-04-20 at 15:06 +0200, Borislav Petkov wrote:
> On Tue, Mar 07, 2017 at 04:32:42PM -0800, Ricardo Neri wrote:
> > These functions read 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.
> 
> Yeah, we tend to call that customarily 16-bit :)

I will call it like this.
> 
> > The D bit is only meaningful for code segments. Thus, these functions
> > always use the code segment selector contained in regs.
> > 
> > Cc: Dave Hansen 
> > Cc: Adam Buchbinder 
> > Cc: Colin Ian King 
> > Cc: Lorenzo Stoakes 
> > Cc: Qiaowei Ren 
> > Cc: Arnaldo Carvalho de Melo 
> > Cc: Masami Hiramatsu 
> > Cc: Adrian Hunter 
> > Cc: Kees Cook 
> > Cc: Thomas Garnier 
> > Cc: Peter Zijlstra 
> > Cc: Borislav Petkov 
> > Cc: Dmitry Vyukov 
> > Cc: Ravi V. Shankar 
> > Cc: x...@kernel.org
> > Signed-off-by: Ricardo Neri 
> > ---
> >  arch/x86/include/asm/insn-eval.h |  2 +
> >  arch/x86/lib/insn-eval.c | 80 
> > 
> >  2 files changed, 82 insertions(+)
> > 
> > diff --git a/arch/x86/include/asm/insn-eval.h 
> > b/arch/x86/include/asm/insn-eval.h
> > index b201742..a0d81fc 100644
> > --- a/arch/x86/include/asm/insn-eval.h
> > +++ b/arch/x86/include/asm/insn-eval.h
> > @@ -15,6 +15,8 @@ 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 char insn_get_seg_default_address_bytes(struct pt_regs *regs);
> > +unsigned char insn_get_seg_default_operand_bytes(struct pt_regs *regs);
> >  unsigned long insn_get_seg_base(struct pt_regs *regs, struct insn *insn,
> > int regoff, bool use_default_seg);
> >  
> > diff --git a/arch/x86/lib/insn-eval.c b/arch/x86/lib/insn-eval.c
> > index 383ca83..cda6c71 100644
> > --- a/arch/x86/lib/insn-eval.c
> > +++ b/arch/x86/lib/insn-eval.c
> > @@ -421,6 +421,86 @@ unsigned long insn_get_seg_base(struct pt_regs *regs, 
> > struct insn *insn,
> >  }
> >  
> >  /**
> > + * insn_get_seg_default_address_bytes - Obtain default address size of 
> > segment
> > + * @regs:  Set of registers containing the segment selector
> > + *
> > + * Obtain the default address size as indicated in the segment descriptor
> > + * selected in regs' code segment selector. 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.
> > + *
> > + * Return: Default address size of segment
> 
>   0 on error.
> 
> > + */
> > +unsigned char insn_get_seg_default_address_bytes(struct pt_regs *regs)
> > +{
> > +   struct desc_struct *desc;
> > +   unsigned short seg;
> > +   int ret;
> > +
> > +   if (v8086_mode(regs))
> > +   return 2;
> > +
> > +   seg = (unsigned short)regs->cs;
> > +
> > +   ret = get_desc(seg, );
> > +   if (ret)
> > +   return 0;
> > +
> > +   switch ((desc->l << 1) | desc->d) {
> > +   case 0: /* Legacy mode. 16-bit addresses. CS.L=0, CS.D=0 */
> > +   return 2;
> > +   case 1: /* Legacy mode. 32-bit addresses. CS.L=0, CS.D=1 */
> > +   return 4;
> > +   case 2: /* IA-32e 64-bit mode. 64-bit addresses. CS.L=1, CS.D=0 */
> > +   return 8;
> > +   case 3: /* Invalid setting. CS.L=1, CS.D=1 */
> > +   /* fall through */
> > +   default:
> > +   return 0;
> > +   }
> > +}
> > +
> > +/**
> > + * insn_get_seg_default_operand_bytes - Obtain default operand size of 
> > segment
> > + * @regs:  Set of registers containing the segment selector
> > + *
> > + * Obtain the default operand size as indicated in the segment descriptor
> > + * selected in regs' code segment selector. In protected mode, the default
> > + * operand size is determined by inspecting the L and D bits of the segment
> > + * descriptor. In virtual-8086 mode, the default is always two bytes.
> > + *
> > + * Return: Default operand size of segment
> > + */
> > +unsigned char insn_get_seg_default_operand_bytes(struct pt_regs *regs)
> 
> Right, so default address and operand size always go together so I don't
> think you need two separate functions.
> 
> So what I'd suggest - provided this pans out (I still haven't 

Re: [v6 PATCH 09/21] x86/insn-eval: Add functions to get default operand and address sizes

[Borislav Petkov]

On Tue, Mar 07, 2017 at 04:32:42PM -0800, Ricardo Neri wrote:
> These functions read 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.

Yeah, we tend to call that customarily 16-bit :)

> The D bit is only meaningful for code segments. Thus, these functions
> always use the code segment selector contained in regs.
> 
> Cc: Dave Hansen 
> Cc: Adam Buchbinder 
> Cc: Colin Ian King 
> Cc: Lorenzo Stoakes 
> Cc: Qiaowei Ren 
> Cc: Arnaldo Carvalho de Melo 
> Cc: Masami Hiramatsu 
> Cc: Adrian Hunter 
> Cc: Kees Cook 
> Cc: Thomas Garnier 
> Cc: Peter Zijlstra 
> Cc: Borislav Petkov 
> Cc: Dmitry Vyukov 
> Cc: Ravi V. Shankar 
> Cc: x...@kernel.org
> Signed-off-by: Ricardo Neri 
> ---
>  arch/x86/include/asm/insn-eval.h |  2 +
>  arch/x86/lib/insn-eval.c | 80 
> 
>  2 files changed, 82 insertions(+)
> 
> diff --git a/arch/x86/include/asm/insn-eval.h 
> b/arch/x86/include/asm/insn-eval.h
> index b201742..a0d81fc 100644
> --- a/arch/x86/include/asm/insn-eval.h
> +++ b/arch/x86/include/asm/insn-eval.h
> @@ -15,6 +15,8 @@ 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 char insn_get_seg_default_address_bytes(struct pt_regs *regs);
> +unsigned char insn_get_seg_default_operand_bytes(struct pt_regs *regs);
>  unsigned long insn_get_seg_base(struct pt_regs *regs, struct insn *insn,
>   int regoff, bool use_default_seg);
>  
> diff --git a/arch/x86/lib/insn-eval.c b/arch/x86/lib/insn-eval.c
> index 383ca83..cda6c71 100644
> --- a/arch/x86/lib/insn-eval.c
> +++ b/arch/x86/lib/insn-eval.c
> @@ -421,6 +421,86 @@ unsigned long insn_get_seg_base(struct pt_regs *regs, 
> struct insn *insn,
>  }
>  
>  /**
> + * insn_get_seg_default_address_bytes - Obtain default address size of 
> segment
> + * @regs:Set of registers containing the segment selector
> + *
> + * Obtain the default address size as indicated in the segment descriptor
> + * selected in regs' code segment selector. 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.
> + *
> + * Return: Default address size of segment

0 on error.

> + */
> +unsigned char insn_get_seg_default_address_bytes(struct pt_regs *regs)
> +{
> + struct desc_struct *desc;
> + unsigned short seg;
> + int ret;
> +
> + if (v8086_mode(regs))
> + return 2;
> +
> + seg = (unsigned short)regs->cs;
> +
> + ret = get_desc(seg, );
> + if (ret)
> + return 0;
> +
> + switch ((desc->l << 1) | desc->d) {
> + case 0: /* Legacy mode. 16-bit addresses. CS.L=0, CS.D=0 */
> + return 2;
> + case 1: /* Legacy mode. 32-bit addresses. CS.L=0, CS.D=1 */
> + return 4;
> + case 2: /* IA-32e 64-bit mode. 64-bit addresses. CS.L=1, CS.D=0 */
> + return 8;
> + case 3: /* Invalid setting. CS.L=1, CS.D=1 */
> + /* fall through */
> + default:
> + return 0;
> + }
> +}
> +
> +/**
> + * insn_get_seg_default_operand_bytes - Obtain default operand size of 
> segment
> + * @regs:Set of registers containing the segment selector
> + *
> + * Obtain the default operand size as indicated in the segment descriptor
> + * selected in regs' code segment selector. In protected mode, the default
> + * operand size is determined by inspecting the L and D bits of the segment
> + * descriptor. In virtual-8086 mode, the default is always two bytes.
> + *
> + * Return: Default operand size of segment
> + */
> +unsigned char insn_get_seg_default_operand_bytes(struct pt_regs *regs)

Right, so default address and operand size always go together so I don't
think you need two separate functions.

So what I'd suggest - provided this pans out (I still haven't reviewed
the whole thing) - is to determine the operating mode of the segment:
long, legacy, etc and then return both address and operand sizes. Patch
17/21 needs them both at the same time AFAICT.

-- 
Regards/Gruss,
Boris.

SUSE Linux GmbH, GF: Felix Imendörffer,