[PATCH v2] KEYS: encrypted: Replace deprecated strcpy and improve get_derived_key

[Thorsten Blum <thorsten.blum@linux.dev>]

strcpy() is deprecated; use the safer strscpy() and use its return
value, the number of bytes copied, instead of calling strlen() on the
destination buffer again. String truncation can be ignored since
'derived_buf' is guaranteed to be large enough.

Link: https://github.com/KSPP/linux/issues/88
Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
---
Changes in v2:
- Revert some changes to include the trailing '\0' even if key_type == 0
  since this would change the bytes passed to sha256() (Eric Biggers)
- Use strscpy() and its return value instead
- Link to v1: https://lore.kernel.org/lkml/20251113135831.98587-1-thorsten.blum@linux.dev/
---
 security/keys/encrypted-keys/encrypted.c | 21 ++++++++-------------
 1 file changed, 8 insertions(+), 13 deletions(-)

diff --git a/security/keys/encrypted-keys/encrypted.c b/security/keys/encrypted-keys/encrypted.c
index 15841466b5d4..94408909f1dd 100644
--- a/security/keys/encrypted-keys/encrypted.c
+++ b/security/keys/encrypted-keys/encrypted.c
@@ -12,6 +12,7 @@
  */
 
 #include <linux/uaccess.h>
+#include <linux/minmax.h>
 #include <linux/module.h>
 #include <linux/init.h>
 #include <linux/slab.h>
@@ -330,23 +331,17 @@ static int get_derived_key(u8 *derived_key, enum derived_key_type key_type,
 			   const u8 *master_key, size_t master_keylen)
 {
 	u8 *derived_buf;
-	unsigned int derived_buf_len;
-
-	derived_buf_len = strlen("AUTH_KEY") + 1 + master_keylen;
-	if (derived_buf_len < HASH_SIZE)
-		derived_buf_len = HASH_SIZE;
+	size_t derived_buf_len;
+	const char *key_name;
+	ssize_t len;
 
+	derived_buf_len = max(strlen("AUTH_KEY") + 1 + master_keylen, HASH_SIZE);
 	derived_buf = kzalloc(derived_buf_len, GFP_KERNEL);
 	if (!derived_buf)
 		return -ENOMEM;
-
-	if (key_type)
-		strcpy(derived_buf, "AUTH_KEY");
-	else
-		strcpy(derived_buf, "ENC_KEY");
-
-	memcpy(derived_buf + strlen(derived_buf) + 1, master_key,
-	       master_keylen);
+	key_name = key_type ? "AUTH_KEY" : "ENC_KEY";
+	len = strscpy(derived_buf, key_name, derived_buf_len);
+	memcpy(derived_buf + len + 1, master_key, master_keylen);
 	sha256(derived_buf, derived_buf_len, derived_key);
 	kfree_sensitive(derived_buf);
 	return 0;
-- 
2.51.1