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

[Eero Tamminen]

Hi,

On 23.1.2024 10.13, Geert Uytterhoeven wrote:
> 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?

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@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