linux-kernel - Re: [PATCH 27/82] m68k: Refactor intentional wrap-around calculation

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Date: Tue, 23 Jan 2024 15:29:55 +0200
From: Eero Tamminen <oak@...sinkinet.fi>
To: Geert Uytterhoeven <geert@...ux-m68k.org>,
 Kees Cook <keescook@...omium.org>
Cc: linux-hardening@...r.kernel.org, Andrew Morton
 <akpm@...ux-foundation.org>, Arnd Bergmann <arnd@...db.de>,
 Liam Howlett <liam.howlett@...cle.com>,
 "Matthew Wilcox (Oracle)" <willy@...radead.org>,
 Hugh Dickins <hughd@...gle.com>, linux-m68k@...ts.linux-m68k.org,
 "Gustavo A. R. Silva" <gustavoars@...nel.org>,
 Bill Wendling <morbo@...gle.com>, Justin Stitt <justinstitt@...gle.com>,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH 27/82] m68k: Refactor intentional wrap-around calculation

Hi,

On 23.1.2024 10.13, Geert Uytterhoeven wrote:
> On Tue, Jan 23, 2024 at 1:35 AM Kees Cook <keescook@...omium.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@...ux-m68k.org>
>> Cc: Andrew Morton <akpm@...ux-foundation.org>
>> Cc: Arnd Bergmann <arnd@...db.de>
>> Cc: Liam Howlett <liam.howlett@...cle.com>
>> Cc: "Matthew Wilcox (Oracle)" <willy@...radead.org>
>> Cc: Hugh Dickins <hughd@...gle.com>
>> Cc: linux-m68k@...ts.linux-m68k.org
>> Signed-off-by: Kees Cook <keescook@...omium.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@...ux-m68k.org>
> Acked-by: Geert Uytterhoeven <geert@...ux-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
>