Re: [PATCH] locking/refcounts, x86/asm: Use unique .text section for refcount exceptions

Mike Galbraith
2017-09-02T04:03:55Z

On Fri, 2017-09-01 at 13:22 -0700, Kees Cook wrote:
> Using .text.unlikely for refcount exceptions isn't safe because gcc may
> move entire functions into .text.unlikely (e.g. in6_dev_get()), which
> would cause any uses of a protected refcount_t function to stay inline
> with the function, triggering the protection unconditionally:
>
> .section .text.unlikely,"ax",@progbits
> .type in6_dev_get, @function
> in6_dev_getx:
> .LFB4673:
> .loc 2 4128 0
> .cfi_startproc
> ...
> lock; incl 480(%rbx)
> js 111f
> .pushsection .text.unlikely
> 111: lea 480(%rbx), %rcx
> 112: .byte 0x0f, 0xff
> .popsection
> 113:
>
> This creates a unique .text section and adds an additional test to the
> exception handler to WARN in the case of having none of OF, SF, nor ZF
> set so we can see things like this more easily in the future.

Closure: gcc-4.8.5 now builds a functional kernel as well, so that
aspect of this bug was just a larger a dose of the same toxin.

Question below.

diff --git a/arch/x86/include/asm/refcount.h
b/arch/x86/include/asm/refcount.h
> index ff871210b9f2..4e44250e7d0d 100644
> --- a/arch/x86/include/asm/refcount.h
> +++ b/arch/x86/include/asm/refcount.h
> @@ -15,7 +15,7 @@
> * back to the regular execution flow in .text.
> */
> #define _REFCOUNT_EXCEPTION \
> - ".pushsection .text.unlikely\n" \
> + ".pushsection .text..refcount\n" \

Why two dots? (.text.refcount_ex?)

-Mike