Re: [v6 PATCH 10/21] x86/insn-eval: Do not use R/EBP as base if mod in ModRM is zero

[Ricardo Neri]

On Sun, 2017-05-07 at 19:20 +0200, Borislav Petkov wrote:
> On Wed, Apr 26, 2017 at 06:29:59PM -0700, Ricardo Neri wrote:
> > >   if (X86_MODRM_MOD(insn->modrm.value) == 0 &&
> > >   X86_MODRM_RM(insn->modrm.value)  == 5)
> > > 
> > > looks more understandable to me.
> > 
> > Should I go with !(X86_MODRM_MOD(insn->modrm.value)) as you suggested in
> > other patches?
> 
> Ah, yes pls.
> 
 I did this in v7[1].

Thanks and BR,
Ricardo

[1]. https://lkml.org/lkml/2017/5/5/399

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Re: [v6 PATCH 10/21] x86/insn-eval: Do not use R/EBP as base if mod in ModRM is zero

[Borislav Petkov]

On Wed, Apr 26, 2017 at 06:29:59PM -0700, Ricardo Neri wrote:
> > if (X86_MODRM_MOD(insn->modrm.value) == 0 &&
> > X86_MODRM_RM(insn->modrm.value)  == 5)
> > 
> > looks more understandable to me.
> 
> Should I go with !(X86_MODRM_MOD(insn->modrm.value)) as you suggested in
> other patches?

Ah, yes pls.

Thanks.

-- 
Regards/Gruss,
Boris.

SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284 
(AG Nürnberg)
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Re: [v6 PATCH 10/21] x86/insn-eval: Do not use R/EBP as base if mod in ModRM is zero

[Ricardo Neri]

On Fri, 2017-04-21 at 12:52 +0200, Borislav Petkov wrote:
> On Tue, Mar 07, 2017 at 04:32:43PM -0800, Ricardo Neri wrote:
> > Section 2.2.1.3 of the Intel 64 and IA-32 Architectures Software
> > Developer's Manual volume 2A states that when the mod part of the ModRM
> > byte is zero and R/EBP is specified in the R/M part of such bit, the value
> > of the aforementioned register should not be used in the address
> > computation. Instead, a 32-bit displacement is expected. The instruction
> > decoder takes care of setting the displacement to the expected value.
> > Returning -EDOM signals callers that they should ignore the value of such
> > register when computing the address encoded in the instruction operands.
> > 
> > Also, callers should exercise care to correctly interpret this particular
> > case. In IA-32e 64-bit mode, the address is given by the displacement plus
> > the value of the RIP. In IA-32e compatibility mode, the value of EIP is
> > ignored. This correction is done for our insn_get_addr_ref.
> > 
> > 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/lib/insn-eval.c | 25 +++--
> >  1 file changed, 23 insertions(+), 2 deletions(-)
> > 
> > diff --git a/arch/x86/lib/insn-eval.c b/arch/x86/lib/insn-eval.c
> > index cda6c71..ea10b03 100644
> > --- a/arch/x86/lib/insn-eval.c
> > +++ b/arch/x86/lib/insn-eval.c
> > @@ -250,6 +250,14 @@ static int get_reg_offset(struct insn *insn, struct 
> > pt_regs *regs,
> > switch (type) {
> > case REG_TYPE_RM:
> > regno = X86_MODRM_RM(insn->modrm.value);
> > +   /* if mod=0, register R/EBP is not used in the address
> > +* computation. Instead, a 32-bit displacement is expected;
> > +* the instruction decoder takes care of reading such
> > +* displacement. This is true for both R/EBP and R13, as the
> > +* REX.B bit is not decoded.
> > +*/
> 
> I'd simply write here: "ModRM.mod == 0 and ModRM.rm == 5 means a 32-bit
> displacement is following."

I will shorten the comment.
> 
> In addition, kernel comments style is:
> 
>   /*
>* A sentence ending with a full-stop.
>* Another sentence. ...
>* More sentences. ...
>*/

... and use the correct style. I feel bad I missed this one.
> 
> > +   if (regno == 5 && X86_MODRM_MOD(insn->modrm.value) == 0)
> > +   return -EDOM;
> 
>   if (X86_MODRM_MOD(insn->modrm.value) == 0 &&
>   X86_MODRM_RM(insn->modrm.value)  == 5)
> 
> looks more understandable to me.

Should I go with !(X86_MODRM_MOD(insn->modrm.value)) as you suggested in
other patches?

> 
> > if (X86_REX_B(insn->rex_prefix.value))
> > regno += 8;
> > break;
> > @@ -599,9 +607,22 @@ void __user *insn_get_addr_ref(struct insn *insn, 
> > struct pt_regs *regs)
> > eff_addr = base + indx * (1 << X86_SIB_SCALE(sib));
> > } else {
> > addr_offset = get_reg_offset(insn, regs, REG_TYPE_RM);
> > -   if (addr_offset < 0)
> > +   /* -EDOM means that we must ignore the address_offset.
> > +* The only case in which we see this value is when
> > +* R/M points to R/EBP. In such a case, in 64-bit mode
> > +* the effective address is relative to tho RIP.
> 
> s/tho//

Will correct.
> 
> > +*/
> 
> Kernel comments style is:
> 
>   /*
>* A sentence ending with a full-stop.
>* Another sentence. ...
>* More sentences. ...
>*/
> 

Will correct.
> > +   if (addr_offset == -EDOM) {
> > +   eff_addr = 0;
> > +#ifdef CONFIG_X86_64
> > +   if (user_64bit_mode(regs))
> > +   eff_addr = (long)regs->ip;
> 
> Is regs->ip the rIP of the *following* insn?

No this is a bug. This should be regs->ip + insn.length.
> 
> > +#endif
> 
> You can do this in a prepatch and then get rid of the ifdeffery here:
> 
> diff --git a/arch/x86/include/asm/ptrace.h b/arch/x86/include/asm/ptrace.h
> index 2b5d686ea9f3..f6239273c5f1 100644
> --- a/arch/x86/include/asm/ptrace.h
> +++ 

Re: [v6 PATCH 10/21] x86/insn-eval: Do not use R/EBP as base if mod in ModRM is zero

[Borislav Petkov]

On Tue, Mar 07, 2017 at 04:32:43PM -0800, Ricardo Neri wrote:
> Section 2.2.1.3 of the Intel 64 and IA-32 Architectures Software
> Developer's Manual volume 2A states that when the mod part of the ModRM
> byte is zero and R/EBP is specified in the R/M part of such bit, the value
> of the aforementioned register should not be used in the address
> computation. Instead, a 32-bit displacement is expected. The instruction
> decoder takes care of setting the displacement to the expected value.
> Returning -EDOM signals callers that they should ignore the value of such
> register when computing the address encoded in the instruction operands.
> 
> Also, callers should exercise care to correctly interpret this particular
> case. In IA-32e 64-bit mode, the address is given by the displacement plus
> the value of the RIP. In IA-32e compatibility mode, the value of EIP is
> ignored. This correction is done for our insn_get_addr_ref.
> 
> 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/lib/insn-eval.c | 25 +++--
>  1 file changed, 23 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/lib/insn-eval.c b/arch/x86/lib/insn-eval.c
> index cda6c71..ea10b03 100644
> --- a/arch/x86/lib/insn-eval.c
> +++ b/arch/x86/lib/insn-eval.c
> @@ -250,6 +250,14 @@ static int get_reg_offset(struct insn *insn, struct 
> pt_regs *regs,
>   switch (type) {
>   case REG_TYPE_RM:
>   regno = X86_MODRM_RM(insn->modrm.value);
> + /* if mod=0, register R/EBP is not used in the address
> +  * computation. Instead, a 32-bit displacement is expected;
> +  * the instruction decoder takes care of reading such
> +  * displacement. This is true for both R/EBP and R13, as the
> +  * REX.B bit is not decoded.
> +  */

I'd simply write here: "ModRM.mod == 0 and ModRM.rm == 5 means a 32-bit
displacement is following."

In addition, kernel comments style is:

/*
 * A sentence ending with a full-stop.
 * Another sentence. ...
 * More sentences. ...
 */

> + if (regno == 5 && X86_MODRM_MOD(insn->modrm.value) == 0)
> + return -EDOM;

if (X86_MODRM_MOD(insn->modrm.value) == 0 &&
X86_MODRM_RM(insn->modrm.value)  == 5)

looks more understandable to me.

>   if (X86_REX_B(insn->rex_prefix.value))
>   regno += 8;
>   break;
> @@ -599,9 +607,22 @@ void __user *insn_get_addr_ref(struct insn *insn, struct 
> pt_regs *regs)
>   eff_addr = base + indx * (1 << X86_SIB_SCALE(sib));
>   } else {
>   addr_offset = get_reg_offset(insn, regs, REG_TYPE_RM);
> - if (addr_offset < 0)
> + /* -EDOM means that we must ignore the address_offset.
> +  * The only case in which we see this value is when
> +  * R/M points to R/EBP. In such a case, in 64-bit mode
> +  * the effective address is relative to tho RIP.

s/tho//

> +  */

Kernel comments style is:

/*
 * A sentence ending with a full-stop.
 * Another sentence. ...
 * More sentences. ...
 */

> + if (addr_offset == -EDOM) {
> + eff_addr = 0;
> +#ifdef CONFIG_X86_64
> + if (user_64bit_mode(regs))
> + eff_addr = (long)regs->ip;

Is regs->ip the rIP of the *following* insn?

> +#endif

You can do this in a prepatch and then get rid of the ifdeffery here:

diff --git a/arch/x86/include/asm/ptrace.h b/arch/x86/include/asm/ptrace.h
index 2b5d686ea9f3..f6239273c5f1 100644
--- a/arch/x86/include/asm/ptrace.h
+++ b/arch/x86/include/asm/ptrace.h
@@ -115,9 +115,9 @@ static inline int v8086_mode(struct pt_regs *regs)
 #endif
 }
 
-#ifdef CONFIG_X86_64
 static inline bool user_64bit_mode(struct pt_regs *regs)
 {
+#ifdef CONFIG_X86_64
 #ifndef CONFIG_PARAVIRT
/*
 * On non-paravirt systems, this is the only long mode CPL 3
@@ -128,6 +128,9 @@ static inline bool user_64bit_mode(struct pt_regs *regs)
/* Headers are too twisted for this to go in paravirt.h. */
return