Re: [PATCH v7 07/26] x86/insn-eval: Do not BUG on invalid register type

[Ricardo Neri]

On Mon, 2017-05-29 at 18:37 +0200, Borislav Petkov wrote:
> On Fri, May 05, 2017 at 11:17:05AM -0700, Ricardo Neri wrote:
> > We are not in a critical failure path. The invalid register type is caused
> > when trying to decode invalid instruction bytes from a user-space program.
> > Thus, simply print an error message. To prevent this warning from being
> > abused from user space programs, use the rate-limited variant of printk.
> > 
> > Cc: Borislav Petkov <bp@xxxxxxx>
> > Cc: Andy Lutomirski <luto@xxxxxxxxxx>
> > Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> > Cc: Adam Buchbinder <adam.buchbinder@xxxxxxxxx>
> > Cc: Colin Ian King <colin.king@xxxxxxxxxxxxx>
> > Cc: Lorenzo Stoakes <lstoakes@xxxxxxxxx>
> > Cc: Qiaowei Ren <qiaowei.ren@xxxxxxxxx>
> > Cc: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
> > Cc: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
> > Cc: Adrian Hunter <adrian.hunter@xxxxxxxxx>
> > Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> > Cc: Thomas Garnier <thgarnie@xxxxxxxxxx>
> > Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
> > Cc: Ravi V. Shankar <ravi.v.shankar@xxxxxxxxx>
> > Cc: x86@xxxxxxxxxx
> > Signed-off-by: Ricardo Neri <ricardo.neri-calderon@xxxxxxxxxxxxxxx>
> > ---
> >  arch/x86/lib/insn-eval.c | 6 +++---
> >  1 file changed, 3 insertions(+), 3 deletions(-)
> > 
> > diff --git a/arch/x86/lib/insn-eval.c b/arch/x86/lib/insn-eval.c
> > index e746a6f..182e2ae 100644
> > --- a/arch/x86/lib/insn-eval.c
> > +++ b/arch/x86/lib/insn-eval.c
> > @@ -5,6 +5,7 @@
> >   */
> >  #include <linux/kernel.h>
> >  #include <linux/string.h>
> > +#include <linux/ratelimit.h>
> >  #include <asm/inat.h>
> >  #include <asm/insn.h>
> >  #include <asm/insn-eval.h>
> > @@ -85,9 +86,8 @@ static int get_reg_offset(struct insn *insn, struct pt_regs *regs,
> >  		break;
> >  
> >  	default:
> > -		pr_err("invalid register type");
> > -		BUG();
> > -		break;
> > +		printk_ratelimited(KERN_ERR "insn-eval: x86: invalid register type");
> 
> You can use pr_err_ratelimited() and define "insn-eval" with pr_fmt.
> Look for examples in the tree.

Will do. I have looked at the examples.
> 
> Btw, "insn-eval" is perhaps not the right name - since we're building
> an instruction decoder, maybe it should be called "insn-dec" or so. I'm
> looking at those other arch/x86/lib/insn.c, arch/x86/include/asm/inat.h
> things and how they're starting to morph into one decoding facility,
> AFAICT.

I agree that insn-eval reads somewhat funny. I did not want to go with
insn-dec.c as insn.c, in my opinion, already decodes the instruction
(i.e., it finds prefixes, opcodes, ModRM, SIB and displacement bytes).
In insn-eval.c I simply take those decoded parameters and evaluate them
to obtain the values they contain (e.g., a specific memory location).
Perhaps, insn-resolve.c could be a better name? Or maybe isnn-operands?

Thanks and BR,
Ricardo