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

[Geert Uytterhoeven]

Hi Kees,

On Tue, Jan 23, 2024 at 1:35 AM Kees Cook <keescook@xxxxxxxxxxxx> 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@xxxxxxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Arnd Bergmann <arnd@xxxxxxxx>
> Cc: Liam Howlett <liam.howlett@xxxxxxxxxx>
> Cc: "Matthew Wilcox (Oracle)" <willy@xxxxxxxxxxxxx>
> Cc: Hugh Dickins <hughd@xxxxxxxxxx>
> Cc: linux-m68k@xxxxxxxxxxxxxxxxxxxx
> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>

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?

>                         goto out_unlock;
>         }

With the above fixed:

Reviewed-by: Geert Uytterhoeven <geert@xxxxxxxxxxxxxx>
Acked-by: Geert Uytterhoeven <geert@xxxxxxxxxxxxxx>

If you want me to take this through the m68k tree (for v6.9), please
let me know.
Thanks!

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68korg

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds