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SubjectRe: [PATCH v3 2/2] x86/refcount: Implement fast refcount overflow protection
On Tue, May 09, 2017 at 10:29:16AM -0700, Kees Cook wrote:
> On Tue, May 9, 2017 at 10:08 AM, Josh Poimboeuf <jpoimboe@redhat.com> 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@redhat.com> 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.

Good, your patch sounds better than mine :-)

--
Josh

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