While testing for places where zero-sized destinations were still
showing up in the kernel, sock_copy() was found, which is 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@chromium.org>
---
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Paolo Abeni <pabeni@redhat.com>
Cc: netdev@vger.kernel.org
---
 net/core/sock.c | 5 +++--
 1 file changed, 3 insertions(+), 2 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;
-- 
2.34.1

From: Kees Cook <keescook@chromium.org>
Date: Fri, 16 Feb 2024 12:44:24 -0800
> While testing for places where zero-sized destinations were still
> showing up in the kernel, sock_copy() was found, which is 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@chromium.org>

I confirmed unsafe_memcpy() is just memcpy() without fortified checks.

Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>


> ---
> Cc: Jakub Kicinski <kuba@kernel.org>
> Cc: "David S. Miller" <davem@davemloft.net>
> Cc: Eric Dumazet <edumazet@google.com>
> Cc: Paolo Abeni <pabeni@redhat.com>
> Cc: netdev@vger.kernel.org
> ---
>  net/core/sock.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 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;
> -- 
> 2.34.1