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

From: Kees Cook
Date: Mon May 08 2017 - 19:31:29 EST


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?

>> diff --git a/include/asm-generic/sections.h b/include/asm-generic/sections.h
>> index 532372c6cf15..0590f384f234 100644
>> --- a/include/asm-generic/sections.h
>> +++ b/include/asm-generic/sections.h
>> @@ -20,6 +20,8 @@
>> * may be out of this range on some architectures.
>> * [_sinittext, _einittext]: contains .init.text.* sections
>> * [__bss_start, __bss_stop]: contains BSS sections
>> + * [__refcount_overflow/underflow_start, ..._end]: contains .text sections
>> + * for refcount error handling.
>> *
>> * Following global variables are optional and may be unavailable on some
>> * architectures and/or kernel configurations.
>> @@ -39,6 +41,8 @@ extern char __per_cpu_load[], __per_cpu_start[], __per_cpu_end[];
>> extern char __kprobes_text_start[], __kprobes_text_end[];
>> extern char __entry_text_start[], __entry_text_end[];
>> extern char __start_rodata[], __end_rodata[];
>> +extern char __refcount_overflow_start[], __refcount_overflow_end[];
>> +extern char __refcount_underflow_start[], __refcount_underflow_end[];
>
> I think this part is no longer needed, since you got rid of the new
> sections?

Oh whoops, thanks. I thought I'd removed those, but clearly I didn't. :) Thanks!

-Kees

--
Kees Cook
Pixel Security