crypto: algif - add and use sock_kzfree_s() instead of memzero_explicit()

Commit e1bd95bf7c25 ("crypto: algif - zeroize IV buffer") and
2a6af25befd0 ("crypto: algif - zeroize message digest buffer")
added memzero_explicit() calls on buffers that are later on
passed back to sock_kfree_s().

This is a discussed follow-up that, instead, extends the sock
API and adds sock_kzfree_s(), which internally uses kzfree()
instead of kfree() for passing the buffers back to slab.

Having sock_kzfree_s() allows to keep the changes more minimal
by just having a drop-in replacement instead of adding
memzero_explicit() calls everywhere before sock_kfree_s().

In kzfree(), the compiler is not allowed to optimize the memset()
away and thus there's no need for memzero_explicit(). Both,
sock_kfree_s() and sock_kzfree_s() are wrappers for
__sock_kfree_s() and call into kfree() resp. kzfree(); here,
__sock_kfree_s() needs to be explicitly inlined as we want the
compiler to optimize the call and condition away and thus it
produces e.g. on x86_64 the _same_ assembler output for
sock_kfree_s() before and after, and thus also allows for
avoiding code duplication.

Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

1 files changed, 20 insertions, 4 deletions

diff --git a/net/core/sock.c b/net/core/sock.c
index 15e0c67b1069..04ce26a996bd 100644
--- a/net/core/sock.c
+++ b/net/core/sock.c
@@ -1713,18 +1713,34 @@ void *sock_kmalloc(struct sock *sk, int size, gfp_t priority)
 }
 EXPORT_SYMBOL(sock_kmalloc);
 
-/*
- * Free an option memory block.
+/* Free an option memory block. Note, we actually want the inline
+ * here as this allows gcc to detect the nullify and fold away the
+ * condition entirely.
  */
-void sock_kfree_s(struct sock *sk, void *mem, int size)
+static inline void __sock_kfree_s(struct sock *sk, void *mem, int size,
+                                  const bool nullify)
 {
         if (WARN_ON_ONCE(!mem))
                 return;
-        kfree(mem);
+        if (nullify)
+                kzfree(mem);
+        else
+                kfree(mem);
         atomic_sub(size, &sk->sk_omem_alloc);
 }
+
+void sock_kfree_s(struct sock *sk, void *mem, int size)
+{
+        __sock_kfree_s(sk, mem, size, false);
+}
 EXPORT_SYMBOL(sock_kfree_s);
 
+void sock_kzfree_s(struct sock *sk, void *mem, int size)
+{
+        __sock_kfree_s(sk, mem, size, true);
+}
+EXPORT_SYMBOL(sock_kzfree_s);
+
 /* It is almost wait_for_tcp_memory minus release_sock/lock_sock.
    I think, these locks should be removed for datagram sockets.
  */