Re: [PATCH 78/82] mm/vmalloc: Refactor intentional wrap-around test

[Lorenzo Stoakes]

On Mon, Jan 22, 2024 at 04:27:53PM -0800, Kees Cook 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 wrap-around addition test to use add_would_overflow().
> This paves the way to enabling the wrap-around sanitizers 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: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Uladzislau Rezki <urezki@xxxxxxxxx>
> Cc: Christoph Hellwig <hch@xxxxxxxxxxxxx>
> Cc: Lorenzo Stoakes <lstoakes@xxxxxxxxx>
> Cc: linux-mm@xxxxxxxxx
> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> ---
>  mm/vmalloc.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index 7932ac99e9d3..3d73f2ac6957 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -3750,7 +3750,7 @@ long vread_iter(struct iov_iter *iter, const char *addr, size_t count)
>  	addr = kasan_reset_tag(addr);
>
>  	/* Don't allow overflow */
> -	if ((unsigned long) addr + count < count)
> +	if (add_would_overflow(count, (unsigned long)addr))
>  		count = -(unsigned long) addr;
>
>  	remains = count;
> --
> 2.34.1
>

Looks good to me,

Reviewed-by: Lorenzo Stoakes <lstoakes@xxxxxxxxx>