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

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Message-ID: <CAGXu5j+J=o7hHceQn6pFtCmQcfUtSrtZ9uP8fLo0gkDWNdetAA@mail.gmail.com>
Date:   Mon, 8 May 2017 16:31:11 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        PaX Team <pageexec@...email.hu>, Jann Horn <jannh@...gle.com>,
        Eric Biggers <ebiggers3@...il.com>,
        Christoph Hellwig <hch@...radead.org>,
        "axboe@...nel.dk" <axboe@...nel.dk>,
        James Bottomley <James.Bottomley@...senpartnership.com>,
        Elena Reshetova <elena.reshetova@...el.com>,
        Hans Liljestrand <ishkamiel@...il.com>,
        David Windsor <dwindsor@...il.com>,
        "x86@...nel.org" <x86@...nel.org>, Ingo Molnar <mingo@...nel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "David S. Miller" <davem@...emloft.net>,
        Rik van Riel <riel@...hat.com>,
        linux-arch <linux-arch@...r.kernel.org>,
        "kernel-hardening@...ts.openwall.com" 
        <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH v3 2/2] x86/refcount: Implement fast refcount overflow protection

On Mon, May 8, 2017 at 3:53 PM, Josh Poimboeuf <jpoimboe@...hat.com> 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