Re: [v6 PATCH 05/21] x86/insn-eval: Add utility functions to get register offsets

[Borislav Petkov]

On Tue, Mar 07, 2017 at 04:32:38PM -0800, Ricardo Neri wrote:
> The function insn_get_reg_offset takes as argument an enumeration that

Please end function names with parentheses.

And do you mean get_reg_offset(), per chance?

> indicates the type of offset that is returned: the R/M part of the ModRM
> byte, the index of the SIB byte or the base of the SIB byte.

Err, you mean, it returns the offset to the register the argument
specifies.

> Callers of
> this function would need the definition of such enumeration. This is not
> needed. Instead, helper functions can be defined for this purpose can be
> added.

"Instead, add helpers... "

> These functions are useful in cases when, for instance, the caller
> needs to decide whether the operand is a register or a memory location by
> looking at the mod part of the ModRM byte.
> 
> 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 |  3 +++
>  arch/x86/lib/insn-eval.c | 51 
> 
>  2 files changed, 54 insertions(+)
> 
> diff --git a/arch/x86/include/asm/insn-eval.h 
> b/arch/x86/include/asm/insn-eval.h
> index 5cab1b1..754211b 100644
> --- a/arch/x86/include/asm/insn-eval.h
> +++ b/arch/x86/include/asm/insn-eval.h
> @@ -12,5 +12,8 @@
>  #include 
>  
>  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);

Forgotten to edit the copy-paste?

Which means, nothing really needs insn_get_reg_offset_sib_index() and
you can get rid of it?

>  #endif /* _ASM_X86_INSN_EVAL_H */
> diff --git a/arch/x86/lib/insn-eval.c b/arch/x86/lib/insn-eval.c
> index 23cf010..78df1c9 100644
> --- a/arch/x86/lib/insn-eval.c
> +++ b/arch/x86/lib/insn-eval.c
> @@ -98,6 +98,57 @@ static int get_reg_offset(struct insn *insn, struct 
> pt_regs *regs,
>   return regoff[regno];
>  }
>  
> +/**
> + * insn_get_reg_offset_modrm_rm - Obtain register in r/m part of ModRM byte
> + * @insn:Instruction structure containing the ModRM byte
> + * @regs:Set of registers indicated by the ModRM byte

That's simply struct pt_regs - not a set of registers indicated by
ModRM?!?

> + * Obtain the register indicated by the r/m part of the ModRM byte. The
> + * register is obtained as an offset from the base of pt_regs. In specific
> + * cases, the returned value can be -EDOM to indicate that the particular 
> value
> + * of ModRM does not refer to a register.

Put that sentence under the "Return: " paragraph below so that it is
immediately obvious what the retvals are.

> + *
> + * Return: Register indicated by r/m, as an offset within struct pt_regs
> + */
> +int insn_get_reg_offset_modrm_rm(struct insn *insn, struct pt_regs *regs)

That name is too long: insn_get_modrm_rm_off() should be enough.

> +{
> + return get_reg_offset(insn, regs, REG_TYPE_RM);
> +}
> +
> +/**
> + * insn_get_reg_offset_sib_base - Obtain register in base part of SiB byte
> + * @insn:Instruction structure containing the SiB byte
> + * @regs:Set of registers indicated by the SiB byte
> + *
> + * Obtain the register indicated by the base part of the SiB byte. The
> + * register is obtained as an offset from the base of pt_regs. In specific
> + * cases, the returned value can be -EDOM to indicate that the particular 
> value
> + * of SiB does not refer to a register.
> + *
> + * Return: Register indicated by SiB's base, as an offset within struct 
> pt_regs

Let's stick to a single spelling: SIB, all caps.

> + */
> +int insn_get_reg_offset_sib_base(struct insn *insn, struct pt_regs *regs)

insn_get_sib_base_off()

Ditto for the rest of the comments on insn_get_reg_offset_modrm_rm() above.

> +{
> + return get_reg_offset(insn, regs, REG_TYPE_BASE);
> +}
> +
> +/**
> + * insn_get_reg_offset_sib_index - Obtain register in index part of SiB byte
> + * @insn:Instruction structure containing the SiB byte
> + * @regs:Set of registers indicated by the SiB byte
> + *
> + * Obtain the register indicated by the index part of the SiB byte. The
> + * register is obtained as an offset from the index of pt_regs. In specific
> + * 

Re: [v6 PATCH 04/21] x86/mpx, x86/insn: Relocate insn util functions to a new insn-kernel

[Borislav Petkov]

On Tue, Mar 07, 2017 at 04:32:37PM -0800, Ricardo Neri wrote:
> Other kernel submodules can benefit from using the utility functions
> defined in mpx.c to obtain the addresses and values of operands contained
> in the general purpose registers. An instance of this is the emulation code
> used for instructions protected by the Intel User-Mode Instruction
> Prevention feature.
> 
> Thus, these functions are relocated to a new insn-eval.c file. The reason
> to not relocate these utilities into insn.c is that the latter solely
> analyses instructions given by a struct insn without any knowledge of the
> meaning of the values of instruction operands. This new utility insn-
> eval.c aims to be used to resolve effective and userspace linear addresses
> based on the contents of the instruction operands as well as the contents
> of pt_regs structure.
> 
> These utilities come with a separate header. This is to avoid taking insn.c
> out of sync from the instructions decoders under tools/obj and tools/perf.
> This also avoids adding cumbersome #ifdef's for the #include'd files
> required to decode instructions in a kernel context.
> 
> Functions are simply relocated. There are not functional or indentation
> changes.

...

> + case REG_TYPE_BASE:
> + regno = X86_SIB_BASE(insn->sib.value);
> + /*
> +  * If mod is 0 and register R/EBP (regno=5) is indicated in the
> +  * base part of the SIB byte, the value of such register should
> +  * not be used in the address computation. Also, a 32-bit
> +  * displacement is expected in this case; the instruction
> +  * decoder takes care of it. This is true for both R13 and
> +  * R/EBP as REX.B will not be decoded.
> +  */
> + if (regno == 5 && X86_MODRM_MOD(insn->modrm.value) == 0)
> + return -EDOM;
> +
> + if (X86_REX_B(insn->rex_prefix.value))
> + regno += 8;
> + break;
> +
> + default:
> + pr_err("invalid register type");
> + BUG();

WARNING: Avoid crashing the kernel - try using WARN_ON & recovery code rather 
than BUG() or BUG_ON()
#211: FILE: arch/x86/lib/insn-eval.c:90:
+   BUG();

And checkpatch is kinda right. We need to warn here, not explode. Oh and
that function returns negative values on error...

Please change that with a patch ontop of the move.

Thanks.

-- 
Regards/Gruss,
Boris.

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