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

[Geert Uytterhoeven]

Hi Eero,

On Tue, Jan 23, 2024 at 2:30 PM Eero Tamminen <oak@xxxxxxxxxxxxxx> wrote:
> 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.

"end" is the end of the block of size "len" pointed to by "addr".

IMHO "if (sum > vma->vm_end)" is less descriptive...

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