Re: [PATCH 32/82] vringh: Refactor intentional wrap-around calculation

[Eugenio Perez Martin]

On Tue, Jan 23, 2024 at 2:42 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: "Michael S. Tsirkin" <mst@xxxxxxxxxx>
> Cc: Jason Wang <jasowang@xxxxxxxxxx>
> Cc: kvm@xxxxxxxxxxxxxxx
> Cc: virtualization@xxxxxxxxxxxxxxx
> Cc: netdev@xxxxxxxxxxxxxxx
> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> ---
>  drivers/vhost/vringh.c | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/vhost/vringh.c b/drivers/vhost/vringh.c
> index 7b8fd977f71c..07442f0a52bd 100644
> --- a/drivers/vhost/vringh.c
> +++ b/drivers/vhost/vringh.c
> @@ -145,6 +145,8 @@ static inline bool range_check(struct vringh *vrh, u64 addr, size_t *len,
>                                bool (*getrange)(struct vringh *,
>                                                 u64, struct vringh_range *))
>  {
> +       u64 sum;

I understand this is part of a bulk change so little time to think
about names :). But what about "end" or similar?

Either way,
Acked-by: Eugenio Pérez <eperezma@xxxxxxxxxx>

> +
>         if (addr < range->start || addr > range->end_incl) {
>                 if (!getrange(vrh, addr, range))
>                         return false;
> @@ -152,20 +154,20 @@ static inline bool range_check(struct vringh *vrh, u64 addr, size_t *len,
>         BUG_ON(addr < range->start || addr > range->end_incl);
>
>         /* To end of memory? */
> -       if (unlikely(addr + *len == 0)) {
> +       if (unlikely(U64_MAX - addr == *len)) {
>                 if (range->end_incl == -1ULL)
>                         return true;
>                 goto truncate;
>         }
>
>         /* Otherwise, don't wrap. */
> -       if (addr + *len < addr) {
> +       if (check_add_overflow(addr, *len, &sum)) {
>                 vringh_bad("Wrapping descriptor %zu@0x%llx",
>                            *len, (unsigned long long)addr);
>                 return false;
>         }
>
> -       if (unlikely(addr + *len - 1 > range->end_incl))
> +       if (unlikely(sum - 1 > range->end_incl))
>                 goto truncate;
>         return true;
>
> --
> 2.34.1
>
>