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Subject: Re: [PATCH 2/3] ubsan: Split "bounds" checker from other options
To:     Kees Cook <keescook@chromium.org>
Cc:     Elena Petrova <lenaptr@google.com>,
        Alexander Potapenko <glider@google.com>,
        Dmitry Vyukov <dvyukov@google.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Dan Carpenter <dan.carpenter@oracle.com>,
        "Gustavo A. R. Silva" <gustavo@embeddedor.com>,
        Arnd Bergmann <arnd@arndb.de>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        kasan-dev@googlegroups.com, linux-kernel@vger.kernel.org,
        kernel-hardening@lists.openwall.com
References: <20191120010636.27368-1-keescook@chromium.org>
 <20191120010636.27368-3-keescook@chromium.org>
From:   Andrey Ryabinin <aryabinin@virtuozzo.com>
Message-ID: <06a84afd-bc97-d2b5-3129-d23473f7acb5@virtuozzo.com>
Date:   Thu, 21 Nov 2019 15:54:44 +0300
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On 11/20/19 4:06 AM, Kees Cook wrote:
> In order to do kernel builds with the bounds checker individually
> available, introduce CONFIG_UBSAN_BOUNDS, with the remaining options
> under CONFIG_UBSAN_MISC.
> 
> For example, using this, we can start to expand the coverage syzkaller is
> providing. Right now, all of UBSan is disabled for syzbot builds because
> taken as a whole, it is too noisy. This will let us focus on one feature
> at a time.
> 
> For the bounds checker specifically, this provides a mechanism to
> eliminate an entire class of array overflows with close to zero
> performance overhead (I cannot measure a difference). In my (mostly)
> defconfig, enabling bounds checking adds ~4200 checks to the kernel.
> Performance changes are in the noise, likely due to the branch predictors
> optimizing for the non-fail path.
> 
> Some notes on the bounds checker:
> 
> - it does not instrument {mem,str}*()-family functions, it only
>   instruments direct indexed accesses (e.g. "foo[i]"). Dealing with
>   the {mem,str}*()-family functions is a work-in-progress around
>   CONFIG_FORTIFY_SOURCE[1].
> 
> - it ignores flexible array members, including the very old single
>   byte (e.g. "int foo[1];") declarations. (Note that GCC's
>   implementation appears to ignore _all_ trailing arrays, but Clang only
>   ignores empty, 0, and 1 byte arrays[2].)
> 
> [1] https://github.com/KSPP/linux/issues/6
> [2] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92589
> 
> Suggested-by: Elena Petrova <lenaptr@google.com>
> Signed-off-by: Kees Cook <keescook@chromium.org>

Reviewed-by: Andrey Ryabinin <aryabinin@virtuozzo.com>