Re: [PATCH v6 2/2] x86/refcount: Implement fast refcount overflow protection

[Kees Cook <keescook@chromium.org>]

On Wed, Jul 19, 2017 at 12:52 PM, Josh Poimboeuf <jpoimboe@redhat.com> wrote:
> On Wed, Jul 19, 2017 at 12:45:19PM -0700, Kees Cook wrote:
>> > diff --git a/arch/x86/include/asm/refcount.h b/arch/x86/include/asm/refcount.h
>> > index 13b91e850a02..e7587db3487c 100644
>> > --- a/arch/x86/include/asm/refcount.h
>> > +++ b/arch/x86/include/asm/refcount.h
>> > @@ -15,6 +15,7 @@
>> >         ".pushsection .text.unlikely\n"                 \
>> >         "111:\tlea %[counter], %%" _ASM_CX "\n"         \
>> >         "112:\t" ASM_UD0 "\n"                           \
>> > +       ASM_UNREACHABLE                                 \
>> >         ".popsection\n"                                 \
>> >         "113:\n"                                        \
>> >         _ASM_EXTABLE_REFCOUNT(112b, 113b)
>> > diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h
>> > index cd4bbe8242bd..85e0b8f42ca0 100644
>> > --- a/include/linux/compiler-gcc.h
>> > +++ b/include/linux/compiler-gcc.h
>> > @@ -202,15 +202,25 @@
>> >  #endif
>> >
>> >  #ifdef CONFIG_STACK_VALIDATION
>> > +
>> >  #define annotate_unreachable() ({                                      \
>> >         asm("%c0:\t\n"                                                  \
>> > -           ".pushsection .discard.unreachable\t\n"                     \
>> > -           ".long %c0b - .\t\n"                                        \
>> > -           ".popsection\t\n" : : "i" (__LINE__));                      \
>> > +           ".pushsection .discard.unreachable\n\t"                     \
>> > +           ".long %c0b - .\n\t"                                        \
>> > +           ".popsection\n\t" : : "i" (__LINE__));                      \
>>
>> Is this just an indentation change?
>
> This was sneaking in a fix to put the tab after the newline instead of
> before it.  I figured it's not worth its own commit.

Ah! Now I see it. Gotcha.

>> >  })
>> > +
>> > +#define ASM_UNREACHABLE                                                        \
>> > +       "999: .pushsection .discard.unreachable\n\t"                    \
>> > +       ".long 999b - .\n\t"                                            \
>> > +       ".popsection\n\t"
>>
>> Just so I understand, we'll get a single byte added for each exception
>> case, but it'll get discarded during final link?
>
> I think it's four bytes actually, but yeah, the section gets stripped at
> vmlinux link time.

Right, yes.

BTW, I think this needs compiler.h coverage instead of the #else in
compiler-gcc.h (since it's different from how annotate_unreachable is
used only in compiler-gcc.h. I'll adjust.

Also, in looking at CONFIG_STACK_VALIDATION, do you want it to just
warn and skip, or do you want to error out the build if validation
isn't available but it's in the .config?

-Kees

-- 
Kees Cook
Pixel Security