Re: [PATCH net 1/2] net: do not keep the dst cache when uncloning an skb dst and its metadata

[Antoine Tenart <atenart@kernel.org>]

Quoting Antoine Tenart (2022-02-02 12:01:36)
> When uncloning an skb dst and its associated metadata a new dst+metadata
> is allocated and the tunnel information from the old metadata is copied
> over there.
> 
> The issue is the tunnel metadata has references to cached dst, which are
> copied along the way. When a dst+metadata refcount drops to 0 the
> metadata is freed including the cached dst entries. As they are also
> referenced in the initial dst+metadata, this ends up in UaFs.
> 
> In practice the above did not happen because of another issue, the
> dst+metadata was never freed because its refcount never dropped to 0
> (this will be fixed in a subsequent patch).
> 
> Fix this by initializing the dst cache after copying the tunnel
> information from the old metadata to also unshare the dst cache.
> 
> Fixes: d71785ffc7e7 ("net: add dst_cache to ovs vxlan lwtunnel")
> Cc: Paolo Abeni <pabeni@redhat.com>
> Reported-by: Vlad Buslov <vladbu@nvidia.com>
> Tested-by: Vlad Buslov <vladbu@nvidia.com>
> Signed-off-by: Antoine Tenart <atenart@kernel.org>
> ---
>  include/net/dst_metadata.h | 13 ++++++++++++-
>  1 file changed, 12 insertions(+), 1 deletion(-)
> 
> diff --git a/include/net/dst_metadata.h b/include/net/dst_metadata.h
> index 14efa0ded75d..c8f8b7b56bba 100644
> --- a/include/net/dst_metadata.h
> +++ b/include/net/dst_metadata.h
> @@ -110,8 +110,8 @@ static inline struct metadata_dst *tun_rx_dst(int md_size)
>  static inline struct metadata_dst *tun_dst_unclone(struct sk_buff *skb)
>  {
>         struct metadata_dst *md_dst = skb_metadata_dst(skb);
> -       int md_size;
>         struct metadata_dst *new_md;
> +       int md_size, ret;

Hmmm ret should probably be defined inside a CONFIG_DST_CACHE section.

>         if (!md_dst || md_dst->type != METADATA_IP_TUNNEL)
>                 return ERR_PTR(-EINVAL);
> @@ -123,6 +123,17 @@ static inline struct metadata_dst *tun_dst_unclone(struct sk_buff *skb)
>  
>         memcpy(&new_md->u.tun_info, &md_dst->u.tun_info,
>                sizeof(struct ip_tunnel_info) + md_size);
> +#ifdef CONFIG_DST_CACHE
> +       ret = dst_cache_init(&new_md->u.tun_info.dst_cache, GFP_ATOMIC);
> +       if (ret) {
> +               /* We can't call metadata_dst_free directly as the still shared
> +                * dst cache would be released.
> +                */
> +               kfree(new_md);
> +               return ERR_PTR(ret);
> +       }
> +#endif
> +
>         skb_dst_drop(skb);
>         dst_hold(&new_md->dst);
>         skb_dst_set(skb, &new_md->dst);
> -- 
> 2.34.1
>