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

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Date:   Sat, 2 Sep 2017 12:51:57 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     Ingo Molnar <mingo@...e.hu>, "x86@...nel.org" <x86@...nel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Mike Galbraith <efault@....de>,
        LKML <linux-kernel@...r.kernel.org>,
        "Reshetova, Elena" <elena.reshetova@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>
Subject: Re: [PATCH] locking/refcounts, x86/asm: Use unique .text section for
 refcount exceptions

On Sat, Sep 2, 2017 at 3:29 AM, Ingo Molnar <mingo@...nel.org> wrote:
>
> * Kees Cook <keescook@...omium.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@....de>
>> Fixes: 7a46ec0e2f48 ("locking/refcounts, x86/asm: Implement fast refcount overflow protection")
>> Signed-off-by: Kees Cook <keescook@...omium.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(-)
>
> Could you please split this into two patches: one that fixes the .unlikely bug,
> the other that re-enables the optimized version?
>
> Should there be any other problem with refcounts this would make any bisection
> result more clear-cut.

Certainly!

-Kees

-- 
Kees Cook
Pixel Security