Re: [PATCH v3 2/2] x86/refcount: Implement fast refcount overflow protection

[Kees Cook]

On Tue, May 9, 2017 at 10:08 AM, Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
> On Mon, May 08, 2017 at 08:58:29PM -0500, Josh Poimboeuf wrote:
>> On Mon, May 08, 2017 at 04:31:11PM -0700, Kees Cook wrote:
>> > On Mon, May 8, 2017 at 3:53 PM, Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
>> > > On Mon, May 08, 2017 at 12:32:52PM -0700, Kees Cook wrote:
>> > >> +#define REFCOUNT_EXCEPTION                           \
>> > >> +     "movl $0x7fffffff, %[counter]\n\t"              \
>> > >> +     "int $"__stringify(X86_REFCOUNT_VECTOR)"\n"     \
>> > >> +     "0:\n\t"                                        \
>> > >> +     _ASM_EXTABLE(0b, 0b)
>> > >
>> > > Despite the objtool warnings going away, this still uses the exception
>> > > table in a new way, which will confuse objtool.  I need to do some more
>> > > thinking about the best way to fix it, either as a change to your patch
>> > > or a change to objtool.
>> >
>> > In that it's not a "true" exception?
>>
>> Right.  And also that it doesn't need the "fixup" since it would return
>> to the same address anyway.
>
> How about the following on top of your patch?  It uses #UD (invalid
> opcode).  Notice it's mostly code deletions :-)

Hah, I wrote this patch almost exactly last night, but hadn't had a
chance to send it out. :)

I ended up defining a new exception handler, which means nothing
special in the generic trap code. I didn't send it out because it was
still using a jns instead of js, and I was pondering if I wanted to
reintroduce the text section jump just to gain the initial benefit of
forward-branch-not-taken optimization...

> diff --git a/arch/x86/include/asm/refcount.h b/arch/x86/include/asm/refcount.h
> index 6e8bbd7..653a985 100644
> --- a/arch/x86/include/asm/refcount.h
> +++ b/arch/x86/include/asm/refcount.h
> @@ -8,15 +8,16 @@
>   */
>  #include <linux/refcount.h>
>  #include <asm/irq_vectors.h>
> +#include <asm/bug.h>
>
>  #define REFCOUNT_EXCEPTION                             \
>         "movl $0x7fffffff, %[counter]\n\t"              \
> -       "int $"__stringify(X86_REFCOUNT_VECTOR)"\n"     \
> -       "0:\n\t"                                        \
> -       _ASM_EXTABLE(0b, 0b)
> +       "1:\t" ASM_UD0 "\n"                             \
> +       "2:\n\t"                                        \
> +       _ASM_EXTABLE(1b, 2b)

I used _ASM_EXTABLE_REFCOUNT(1b, 2b) here, with
arch/x86/include/asm/asm.h adding:

+# define _ASM_EXTABLE_REFCOUNT(from, to)                       \
+       _ASM_EXTABLE_HANDLE(from, to, ex_handler_refcount)

> diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
> index 0b2dbcc..7de95b7 100644
> --- a/arch/x86/kernel/traps.c
> +++ b/arch/x86/kernel/traps.c
> @@ -220,8 +220,8 @@ do_trap_no_signal(struct task_struct *tsk, int trapnr, char *str,
>         if (!user_mode(regs)) {
>                 if (fixup_exception(regs, trapnr)) {
>                         if (IS_ENABLED(CONFIG_FAST_REFCOUNT) &&
> -                           trapnr == X86_REFCOUNT_VECTOR)
> -                               refcount_error_report(regs, str);
> +                           trapnr == X86_TRAP_UD)
> +                               refcount_error_report(regs);
>
>                         return 0;
>                 }

And then I could leave out this hunk, instead adding this to
arch/x86/mm/extable.c:

+bool ex_handler_refcount(const struct exception_table_entry *fixup,
+                        struct pt_regs *regs, int trapnr)
+{
+       regs->ip = ex_fixup_addr(fixup);
+       refcount_error_report(regs, "overflow");
+       return true;
+}
+EXPORT_SYMBOL_GPL(ex_handler_refcount);

After looking at the assembly output, the "movl" instructions can be
various sizes, depending on where %[counter] lives, so I'm also
considering returning to using PaX's "lea", but I'm not sure the
benefit would be very large.

-Kees

-- 
Kees Cook
Pixel Security