Re: [Xen-devel] [PATCH v2] x86/emul: Poision the stubs with debug traps

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 20.03.17 at 11:58, <andrew.cooper3@xxxxxxxxxx> wrote:
> ...rather than leaving fragments of old instructions in place.  This reduces
> the chances of something going further-wrong (as the debug trap will be 
> caught
> and terminate the guest) in a cascade-failure where we end up executing the
> instruction fragments.
> 
> Before:
>     (XEN) d2v0 exception 6 (ec=0000) in emulation stub (line 6239)
>     (XEN) d2v0 stub: c4 e1 44 77 c3 80 d0 82 ff ff ff d1 90 ec 90
> 
> After:
>     (XEN) d3v0 exception 6 (ec=0000) in emulation stub (line 6239)
>     (XEN) d3v0 stub: c4 e1 44 77 c3 cc cc cc cc cc cc cc cc cc cc
> 
> To make this work, the int3 handler needs to be extended to attempt recovery
> rather than simply returning back to Xen context.  This is a good general
> precaution anyway, as nothing good will happen from letting Xen blindly
> execute over 0xcc's.

I partly disagree: It can be a very useful thing to not have to rebuild
Xen with hard coded INT3 invocations removed/re-added between
runs with or without a debugger attached.

> --- a/xen/arch/x86/traps.c
> +++ b/xen/arch/x86/traps.c
> @@ -1206,9 +1206,21 @@ void do_int3(struct cpu_user_regs *regs)
>  
>      if ( !guest_mode(regs) )
>      {
> -        debugger_trap_fatal(TRAP_int3, regs);
> -        return;
> -    } 
> +        unsigned long fixup;
> +
> +        if ( (fixup = search_exception_table(regs)) != 0 )
> +        {
> +            this_cpu(last_extable_addr) = regs->rip;
> +            regs->rip = fixup;

Should this be accompanied by a dprintk(), like most other such
cases are?

> +            return;
> +        }
> +
> +        if ( debugger_trap_fatal(TRAP_int3, regs) )
> +            return;
> +
> +        show_execution_state(regs);
> +        panic("FATAL TRAP: vector = %d (int3)", TRAP_int3);

Along the lines of what I've said above, I'd like to see this being made
conditional upon NDEBUG. Plus I think you want to call fatal_trap()
here.

Jan


_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
https://lists.xen.org/xen-devel