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

[Borislav Petkov <bp@suse.de>]

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 <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/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 regs->cs == __USER_CS || regs->cs == pv_info.extra_user_64bit_cs;
 #endif
+#else /* !CONFIG_X86_64 */
+	return false;
+#endif
 }
 
 #define current_user_stack_pointer()	current_pt_regs()->sp
---

-- 
Regards/Gruss,
    Boris.

SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nürnberg)
--