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

Kees Cook
2017-09-02T02:58:14Z

On Fri, Sep 1, 2017 at 2:43 PM, Ard Biesheuvel
<ard.biesheuvel@linaro.org> wrote:
> On 1 September 2017 at 21:22, Kees Cook <keescook@chromium.org> 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.
>>
>> Reported-by: Mike Galbraith <efault@gmx.de>
>> Fixes: 7a46ec0e2f48 ("locking/refcounts, x86/asm: Implement fast refcount overflow protection")
>> Signed-off-by: Kees Cook <keescook@chromium.org>
>> ---
>> arch/x86/Kconfig | 2 +-
>> arch/x86/include/asm/refcount.h | 2 +-
>> arch/x86/mm/extable.c | 7 ++++++-
>> include/asm-generic/vmlinux.lds.h | 1 +
>> 4 files changed, 9 insertions(+), 3 deletions(-)
>>
>> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
>> index eaa8ff41f424..c6acdcdb3fc6 100644
>> --- a/arch/x86/Kconfig
>> +++ b/arch/x86/Kconfig
>> @@ -56,7 +56,7 @@ config X86
>> select ARCH_HAS_MMIO_FLUSH
>> select ARCH_HAS_PMEM_API if X86_64
>> # Causing hangs/crashes, see the commit that added this change for details.
>> - select ARCH_HAS_REFCOUNT if BROKEN
>> + select ARCH_HAS_REFCOUNT
>> select ARCH_HAS_UACCESS_FLUSHCACHE if X86_64
>> select ARCH_HAS_SET_MEMORY
>> select ARCH_HAS_SG_CHAIN
>> 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" \
>
> You could also use a .subsection here: those are always emitted out of
> line, but into the same section.

I considered it (especially after looking to see how you handled it in
the arm64 port) but decided against it in favor of collecting them all
at the end with .text.unlikely.

-Kees