Re: [PATCH v2] sock: Use unsafe_memcpy() for sock_copy()

[Kuniyuki Iwashima]

From: Kees Cook <keescook@xxxxxxxxxxxx>
Date: Fri, 16 Feb 2024 15:22:23 -0800
> While testing for places where zero-sized destinations were still showing
> up in the kernel, sock_copy() and inet_reqsk_clone() were found, which
> are using very specific memcpy() offsets for both avoiding a portion of
> struct sock, and copying beyond the end of it (since struct sock is really
> just a common header before the protocol-specific allocation). Instead
> of trying to unravel this historical lack of container_of(), just switch
> to unsafe_memcpy(), since that's effectively what was happening already
> (memcpy() wasn't checking 0-sized destinations while the code base was
> being converted away from fake flexible arrays).
> 
> Avoid the following false positive warning with future changes to
> CONFIG_FORTIFY_SOURCE:
> 
>   memcpy: detected field-spanning write (size 3068) of destination "&nsk->__sk_common.skc_dontcopy_end" at net/core/sock.c:2057 (size 0)
> 
> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> ---
> Cc: Jakub Kicinski <kuba@xxxxxxxxxx>
> Cc: "David S. Miller" <davem@xxxxxxxxxxxxx>
> Cc: Eric Dumazet <edumazet@xxxxxxxxxx>
> Cc: Paolo Abeni <pabeni@xxxxxxxxxx>
> Cc: netdev@xxxxxxxxxxxxxxx
> 
> v2- add inet_reqsk_clone() instance too
> v1- https://lore.kernel.org/lkml/20240216204423.work.066-kees@xxxxxxxxxx/
> ---
>  net/core/sock.c                 | 5 +++--
>  net/ipv4/inet_connection_sock.c | 5 +++--
>  2 files changed, 6 insertions(+), 4 deletions(-)
> 
> diff --git a/net/core/sock.c b/net/core/sock.c
> index 0a7f46c37f0c..b7ea358eb18f 100644
> --- a/net/core/sock.c
> +++ b/net/core/sock.c
> @@ -2053,8 +2053,9 @@ static void sock_copy(struct sock *nsk, const struct sock *osk)
>  
>  	memcpy(nsk, osk, offsetof(struct sock, sk_dontcopy_begin));
>  
> -	memcpy(&nsk->sk_dontcopy_end, &osk->sk_dontcopy_end,
> -	       prot->obj_size - offsetof(struct sock, sk_dontcopy_end));
> +	unsafe_memcpy(&nsk->sk_dontcopy_end, &osk->sk_dontcopy_end,
> +		      prot->obj_size - offsetof(struct sock, sk_dontcopy_end),
> +		      /* alloc is larger than struct, see sk_prot_alloc() */);
>  
>  #ifdef CONFIG_SECURITY_NETWORK
>  	nsk->sk_security = sptr;
> diff --git a/net/ipv4/inet_connection_sock.c b/net/ipv4/inet_connection_sock.c
> index 459af1f89739..4a1d96ba3ad1 100644
> --- a/net/ipv4/inet_connection_sock.c
> +++ b/net/ipv4/inet_connection_sock.c
> @@ -906,8 +906,9 @@ static struct request_sock *inet_reqsk_clone(struct request_sock *req,
>  
>  	memcpy(nreq_sk, req_sk,
>  	       offsetof(struct sock, sk_dontcopy_begin));
> -	memcpy(&nreq_sk->sk_dontcopy_end, &req_sk->sk_dontcopy_end,
> -	       req->rsk_ops->obj_size - offsetof(struct sock, sk_dontcopy_end));
> +	unsafe_memcpy(&nreq_sk->sk_dontcopy_end, &req_sk->sk_dontcopy_end,
> +		      req->rsk_ops->obj_size - offsetof(struct sock, sk_dontcopy_end),
> +		      /* alloc is larger than struct, see sk_prot_alloc() */);

nit: reqsk is allocated in inet_reqsk_clone().


>  
>  	sk_node_init(&nreq_sk->sk_node);
>  	nreq_sk->sk_tx_queue_mapping = req_sk->sk_tx_queue_mapping;
> -- 
> 2.34.1
>