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.han...@linux.intel.com>
> Cc: Adam Buchbinder <adam.buchbin...@gmail.com>
> Cc: Colin Ian King <colin.k...@canonical.com>
> Cc: Lorenzo Stoakes <lstoa...@gmail.com>
> Cc: Qiaowei Ren <qiaowei....@intel.com>
> Cc: Arnaldo Carvalho de Melo <a...@redhat.com>
> Cc: Masami Hiramatsu <mhira...@kernel.org>
> Cc: Adrian Hunter <adrian.hun...@intel.com>
> Cc: Kees Cook <keesc...@chromium.org>
> Cc: Thomas Garnier <thgar...@google.com>
> Cc: Peter Zijlstra <pet...@infradead.org>
> Cc: Borislav Petkov <b...@suse.de>
> Cc: Dmitry Vyukov <dvyu...@google.com>
> Cc: Ravi V. Shankar <ravi.v.shan...@intel.com>
> Cc: x...@kernel.org
> Signed-off-by: Ricardo Neri <ricardo.neri-calde...@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 Imend├Ârffer, Jane Smithard, Graham Norton, HRB 21284 
(AG N├╝rnberg)
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