Received-SPF: pass (google.com: domain of linux-kernel+bounces-35390-linux.lists.archive=gmail.com@vger.kernel.org designates 2604:1380:4601:e00::3 as permitted sender) client-ip=2604:1380:4601:e00::3;
Message-ID: <c915b476-784c-4d64-8063-f18837350568@helsinkinet.fi>
Date: Tue, 23 Jan 2024 15:29:55 +0200
Precedence: bulk
MIME-Version: 1.0
User-Agent: Mozilla Thunderbird
Subject: Re: [PATCH 27/82] m68k: Refactor intentional wrap-around calculation
Content-Language: en-US
To: Geert Uytterhoeven <geert@linux-m68k.org>,
 Kees Cook <keescook@chromium.org>
Cc: linux-hardening@vger.kernel.org, Andrew Morton
 <akpm@linux-foundation.org>, Arnd Bergmann <arnd@arndb.de>,
 Liam Howlett <liam.howlett@oracle.com>,
 "Matthew Wilcox (Oracle)" <willy@infradead.org>,
 Hugh Dickins <hughd@google.com>, linux-m68k@lists.linux-m68k.org,
 "Gustavo A. R. Silva" <gustavoars@kernel.org>,
 Bill Wendling <morbo@google.com>, Justin Stitt <justinstitt@google.com>,
 linux-kernel@vger.kernel.org
References: <20240122235208.work.748-kees@kernel.org>
 <20240123002814.1396804-27-keescook@chromium.org>
 <CAMuHMdWa9tXQRHkkX-W+5n8kuCvEFZdsfStPtYGA0MZpTGX79Q@mail.gmail.com>
From: Eero Tamminen <oak@helsinkinet.fi>
In-Reply-To: <CAMuHMdWa9tXQRHkkX-W+5n8kuCvEFZdsfStPtYGA0MZpTGX79Q@mail.gmail.com>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 8bit

Hi,

On 23.1.2024 10.13, Geert Uytterhoeven wrote:
> On Tue, Jan 23, 2024 at 1:35 AM Kees Cook <keescook@chromium.org> wrote:
>> In an effort to separate intentional arithmetic wrap-around from
>> unexpected wrap-around, we need to refactor places that depend on this
>> kind of math. One of the most common code patterns of this is:
>>
>>          VAR + value < VAR
>>
>> Notably, this is considered "undefined behavior" for signed and pointer
>> types, which the kernel works around by using the -fno-strict-overflow
>> option in the build[1] (which used to just be -fwrapv). Regardless, we
>> want to get the kernel source to the position where we can meaningfully
>> instrument arithmetic wrap-around conditions and catch them when they
>> are unexpected, regardless of whether they are signed[2], unsigned[3],
>> or pointer[4] types.
>>
>> Refactor open-coded unsigned wrap-around addition test to use
>> check_add_overflow(), retaining the result for later usage (which removes
>> the redundant open-coded addition). This paves the way to enabling the
>> unsigned wrap-around sanitizer[2] in the future.
>>
>> Link: https://git.kernel.org/linus/68df3755e383e6fecf2354a67b08f92f18536594 [1]
>> Link: https://github.com/KSPP/linux/issues/26 [2]
>> Link: https://github.com/KSPP/linux/issues/27 [3]
>> Link: https://github.com/KSPP/linux/issues/344 [4]
>> Cc: Geert Uytterhoeven <geert@linux-m68k.org>
>> Cc: Andrew Morton <akpm@linux-foundation.org>
>> Cc: Arnd Bergmann <arnd@arndb.de>
>> Cc: Liam Howlett <liam.howlett@oracle.com>
>> Cc: "Matthew Wilcox (Oracle)" <willy@infradead.org>
>> Cc: Hugh Dickins <hughd@google.com>
>> Cc: linux-m68k@lists.linux-m68k.org
>> Signed-off-by: Kees Cook <keescook@chromium.org>
> 
> Thanks for your patch!
> 
>> --- a/arch/m68k/kernel/sys_m68k.c
>> +++ b/arch/m68k/kernel/sys_m68k.c
>> @@ -391,10 +391,11 @@ sys_cacheflush (unsigned long addr, int scope, int cache, unsigned long len)
>>
>>                  mmap_read_lock(current->mm);
>>          } else {
>> +               unsigned long sum;
> 
> "sum" sounds like this is a dummy variable, to please the third
> parameter of check_add_overflow()...
> 
>>                  struct vm_area_struct *vma;
>>
>>                  /* Check for overflow.  */
> 
> I agree with Liam: please drop the comment.
> 
>> -               if (addr + len < addr)
>> +               if (check_add_overflow(addr, len, &sum))
>>                          goto out;
>>
>>                  /*
>> @@ -403,7 +404,7 @@ sys_cacheflush (unsigned long addr, int scope, int cache, unsigned long len)
>>                   */
>>                  mmap_read_lock(current->mm);
>>                  vma = vma_lookup(current->mm, addr);
>> -               if (!vma || addr + len > vma->vm_end)
>> +               if (!vma || sum > vma->vm_end)
> 
> ... Oh, it is actually used. What about renaming it to "end" instead?

IMHO this is more descriptive:
+               if (check_add_overflow(addr, len, &sum))

than this:
+               if (check_add_overflow(addr, len, &end))

"sum" is IMHO quite obviously sum of the preceding args, whereas I do 
not know what "end" would be.


	- Eero

>>                          goto out_unlock;
>>          }
> 
> With the above fixed:
> 
> Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
> Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
> 
> If you want me to take this through the m68k tree (for v6.9), please
> let me know.
> Thanks!
> 
> Gr{oetje,eeting}s,
> 
>                          Geert
>