Re: [PATCH 33/82] mm/vmalloc: Refactor intentional wrap-around calculation

[Lorenzo Stoakes]

On Mon, Jan 22, 2024 at 04:27:08PM -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 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: 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 | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index d12a17fc0c17..7932ac99e9d3 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -1223,6 +1223,7 @@ is_within_this_va(struct vmap_area *va, unsigned long size,
>  	unsigned long align, unsigned long vstart)
>  {
>  	unsigned long nva_start_addr;
> +	unsigned long sum;
>
>  	if (va->va_start > vstart)
>  		nva_start_addr = ALIGN(va->va_start, align);
> @@ -1230,11 +1231,11 @@ is_within_this_va(struct vmap_area *va, unsigned long size,
>  		nva_start_addr = ALIGN(vstart, align);
>
>  	/* Can be overflowed due to big size or alignment. */
> -	if (nva_start_addr + size < nva_start_addr ||
> +	if (check_add_overflow(nva_start_addr, size, &sum) ||
>  			nva_start_addr < vstart)
>  		return false;
>
> -	return (nva_start_addr + size <= va->va_end);
> +	return (sum <= va->va_end);
>  }
>
>  /*
> --
> 2.34.1
>

Looks good to me,

Reviewed-by: Lorenzo Stoakes <lstoakes@xxxxxxxxx>