The struct xt_entry_target fake flexible array has not be converted to a
true flexible array, which is mainly blocked by it being both UAPI and
used in the middle of other structures. In order to properly check for
0-sized destinations in memcpy(), an exception must be made for the one
place where it is still a destination. Since memcpy() was already
skipping checks for 0-sized destinations, using unsafe_memcpy() is no
change in behavior.

Signed-off-by: Kees Cook <keescook@chromium.org>
---
Cc: Pablo Neira Ayuso <pablo@netfilter.org>
Cc: Jozsef Kadlecsik <kadlec@netfilter.org>
Cc: Florian Westphal <fw@strlen.de>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Paolo Abeni <pabeni@redhat.com>
Cc: Gustavo A. R. Silva <gustavo@embeddedor.com>
Cc: netfilter-devel@vger.kernel.org
Cc: coreteam@netfilter.org
Cc: netdev@vger.kernel.org
---
 net/netfilter/x_tables.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/net/netfilter/x_tables.c b/net/netfilter/x_tables.c
index 21624d68314f..da5d929c7c85 100644
--- a/net/netfilter/x_tables.c
+++ b/net/netfilter/x_tables.c
@@ -1142,7 +1142,8 @@ void xt_compat_target_from_user(struct xt_entry_target *t, void **dstptr,
 	if (target->compat_from_user)
 		target->compat_from_user(t->data, ct->data);
 	else
-		memcpy(t->data, ct->data, tsize - sizeof(*ct));
+		unsafe_memcpy(t->data, ct->data, tsize - sizeof(*ct),
+			      /* UAPI 0-sized destination */);
 
 	tsize += off;
 	t->u.user.target_size = tsize;
-- 
2.34.1

On Fri, Feb 16, 2024 at 03:31:32PM -0800, Kees Cook wrote:
> The struct xt_entry_target fake flexible array has not be converted to a
> true flexible array, which is mainly blocked by it being both UAPI and
> used in the middle of other structures. In order to properly check for
> 0-sized destinations in memcpy(), an exception must be made for the one
> place where it is still a destination. Since memcpy() was already
> skipping checks for 0-sized destinations, using unsafe_memcpy() is no
> change in behavior.
> 
> Signed-off-by: Kees Cook <keescook@chromium.org>

Reviewed-by: Simon Horman <horms@kernel.org>