mm: add kvfree_sensitive() for freeing sensitive data objects

For kvmalloc'ed data object that contains sensitive information like
cryptographic keys, we need to make sure that the buffer is always cleared
before freeing it.  Using memset() alone for buffer clearing may not
provide certainty as the compiler may compile it away.  To be sure, the
special memzero_explicit() has to be used.

This patch introduces a new kvfree_sensitive() for freeing those sensitive
data objects allocated by kvmalloc().  The relevant places where
kvfree_sensitive() can be used are modified to use it.

Link: http://lkml.kernel.org/r/20200407200318.11711-1-longman@redhat.com
Fixes: 4f08824 ("KEYS: Avoid false positive ENOMEM error on key read")
Signed-off-by: Waiman Long <longman@redhat.com>
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Reviewed-by: Eric Biggers <ebiggers@google.com>
Acked-by: David Howells <dhowells@redhat.com>
Cc: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
Cc: James Morris <jmorris@namei.org>
Cc: "Serge E. Hallyn" <serge@hallyn.com>
Cc: Joe Perches <joe@perches.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: David Rientjes <rientjes@google.com>
Cc: Uladzislau Rezki <urezki@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>

Author: Waiman-Long

include/linux/mm.h

@@ -780,6 +780,7 @@ static inline void *kvcalloc(size_t n, size_t size, gfp_t flags)
 }
 
 extern void kvfree(const void *addr);
+extern void kvfree_sensitive(const void *addr, size_t len);
 
 /*
  * Mapcount of compound page as a whole, does not include mapped sub-pages.

mm/util.c

@@ -604,6 +604,24 @@ void kvfree(const void *addr)
 }
 EXPORT_SYMBOL(kvfree);
 
+/**
+ * kvfree_sensitive - Free a data object containing sensitive information.
+ * @addr: address of the data object to be freed.
+ * @len: length of the data object.
+ *
+ * Use the special memzero_explicit() function to clear the content of a
+ * kvmalloc'ed object containing sensitive data to make sure that the
+ * compiler won't optimize out the data clearing.
+ */
+void kvfree_sensitive(const void *addr, size_t len)
+{
+	if (likely(!ZERO_OR_NULL_PTR(addr))) {
+		memzero_explicit((void *)addr, len);
+		kvfree(addr);
+	}
+}
+EXPORT_SYMBOL(kvfree_sensitive);
+
 static inline void *__page_rmapping(struct page *page)
 {
 	unsigned long mapping;

security/keys/internal.h

@@ -350,15 +350,4 @@ static inline void key_check(const struct key *key)
 #define key_check(key) do {} while(0)
 
 #endif
-
-/*
- * Helper function to clear and free a kvmalloc'ed memory object.
- */
-static inline void __kvzfree(const void *addr, size_t len)
-{
-	if (addr) {
-		memset((void *)addr, 0, len);
-		kvfree(addr);
-	}
-}
 #endif /* _INTERNAL_H */

security/keys/keyctl.c

@@ -142,10 +142,7 @@ SYSCALL_DEFINE5(add_key, const char __user *, _type,
 
 	key_ref_put(keyring_ref);
  error3:
-	if (payload) {
-		memzero_explicit(payload, plen);
-		kvfree(payload);
-	}
+	kvfree_sensitive(payload, plen);
  error2:
 	kfree(description);
  error:
@@ -360,7 +357,7 @@ long keyctl_update_key(key_serial_t id,
 
 	key_ref_put(key_ref);
 error2:
-	__kvzfree(payload, plen);
+	kvfree_sensitive(payload, plen);
 error:
 	return ret;
 }
@@ -914,7 +911,7 @@ long keyctl_read_key(key_serial_t keyid, char __user *buffer, size_t buflen)
 		 */
 		if (ret > key_data_len) {
 			if (unlikely(key_data))
-				__kvzfree(key_data, key_data_len);
+				kvfree_sensitive(key_data, key_data_len);
 			key_data_len = ret;
 			continue;	/* Allocate buffer */
 		}
@@ -923,7 +920,7 @@ long keyctl_read_key(key_serial_t keyid, char __user *buffer, size_t buflen)
 			ret = -EFAULT;
 		break;
 	}
-	__kvzfree(key_data, key_data_len);
+	kvfree_sensitive(key_data, key_data_len);
 
 key_put_out:
 	key_put(key);
@@ -1225,10 +1222,7 @@ long keyctl_instantiate_key_common(key_serial_t id,
 		keyctl_change_reqkey_auth(NULL);
 
 error2:
-	if (payload) {
-		memzero_explicit(payload, plen);
-		kvfree(payload);
-	}
+	kvfree_sensitive(payload, plen);
 error:
 	return ret;
 }