netdev - Re: [PATCH net-next 1/3] net, neigh: Add build-time assertion to avoid neigh->flags overflow

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Date:   Wed, 13 Oct 2021 21:10:49 -0600
From:   David Ahern <dsahern@...il.com>
To:     Daniel Borkmann <daniel@...earbox.net>, davem@...emloft.net,
        kuba@...nel.org
Cc:     roopa@...dia.com, dsahern@...nel.org, m@...bda.lt,
        john.fastabend@...il.com, netdev@...r.kernel.org,
        bpf@...r.kernel.org
Subject: Re: [PATCH net-next 1/3] net, neigh: Add build-time assertion to
 avoid neigh->flags overflow

On 10/13/21 7:21 AM, Daniel Borkmann wrote:
> Currently, NDA_FLAGS_EXT flags allow a maximum of 24 bits to be used for
> extended neighbor flags. These are eventually fed into neigh->flags by
> shifting with NTF_EXT_SHIFT as per commit 2c611ad97a82 ("net, neigh:
> Extend neigh->flags to 32 bit to allow for extensions").
> 
> If really ever needed in future, the full 32 bits from NDA_FLAGS_EXT can
> be used, it would only require to move neigh->flags from u32 to u64 inside
> the kernel.
> 
> Add a build-time assertion such that when extending the NTF_EXT_MASK with
> new bits, we'll trigger an error once we surpass the 24th bit. This assumes
> that no bit holes in new NTF_EXT_* flags will slip in from UAPI, but I
> think this is reasonable to assume.
> 
> Suggested-by: David Ahern <dsahern@...nel.org>
> Signed-off-by: Daniel Borkmann <daniel@...earbox.net>
> ---
>  net/core/neighbour.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/net/core/neighbour.c b/net/core/neighbour.c
> index eae73efa9245..4fc601f9cd06 100644
> --- a/net/core/neighbour.c
> +++ b/net/core/neighbour.c
> @@ -1940,6 +1940,9 @@ static int neigh_add(struct sk_buff *skb, struct nlmsghdr *nlh,
>  			NL_SET_ERR_MSG(extack, "Invalid extended flags");
>  			goto out;
>  		}
> +		BUILD_BUG_ON(sizeof(neigh->flags) * BITS_PER_BYTE <
> +			     (sizeof(ndm->ndm_flags) * BITS_PER_BYTE +
> +			      hweight32(NTF_EXT_MASK)));
>  		ndm_flags |= (ext << NTF_EXT_SHIFT);
>  	}
>  	if (ndm->ndm_ifindex) {
> 

Reviewed-by: David Ahern <dsahern@...nel.org>