KEYS: encrypted: fix integer overflow of datablob_len

encrypted_key_alloc() stores datablob_len in a u16. It is computed from
multiple string and payload lengths. If the result exceeds U16_MAX, the
assignment truncates the allocation size. KASAN reports a 32760-byte
slab-out-of-bounds write when __ekey_init() copies the master key
description into the undersized buffer.

The total payload length stored in key->datalen is also a u16. Use
check_add_overflow() to reject values that do not fit either destination,
and use kzalloc_flex() for the flexible-array allocation.

Fixes: 7e70cb4978 ("keys: add new key-type encrypted")
Cc: [email protected]
Assisted-by: GitHub-Copilot:claude-opus-4.6
Signed-off-by: Cen Zhang <[email protected]>
Signed-off-by: Francis Perron <[email protected]>
Reviewed-by: Jarkko Sakkinen <[email protected]>
Tested-by: R Nageswara Sastry <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jarkko Sakkinen <[email protected]>

security/keys/encrypted-keys/encrypted.c

@@ -19,6 +19,7 @@
 #include <linux/parser.h>
 #include <linux/string.h>
 #include <linux/err.h>
+#include <linux/overflow.h>
 #include <keys/user-type.h>
 #include <keys/trusted-type.h>
 #include <keys/encrypted-type.h>
@@ -579,6 +580,7 @@ static struct encrypted_key_payload *encrypted_key_alloc(struct key *key,
 {
 	struct encrypted_key_payload *epayload = NULL;
 	unsigned short datablob_len;
+	unsigned short payload_totallen;
 	unsigned short decrypted_datalen;
 	unsigned short payload_datalen;
 	unsigned int encrypted_datalen;
@@ -632,16 +634,22 @@ static struct encrypted_key_payload *encrypted_key_alloc(struct key *key,
 
 	encrypted_datalen = roundup(decrypted_datalen, blksize);
 
-	datablob_len = format_len + 1 + strlen(master_desc) + 1
-	    + strlen(datalen) + 1 + ivsize + 1 + encrypted_datalen;
+	if (check_add_overflow(format_len + 1 + strlen(master_desc) + 1
+			       + strlen(datalen) + 1 + ivsize + 1,
+			       encrypted_datalen, &datablob_len))
+		return ERR_PTR(-EINVAL);
 
-	ret = key_payload_reserve(key, payload_datalen + datablob_len
-				  + HASH_SIZE + 1);
+	if (check_add_overflow(datablob_len,
+			       payload_datalen + HASH_SIZE + 1,
+			       &payload_totallen))
+		return ERR_PTR(-EINVAL);
+
+	ret = key_payload_reserve(key, payload_totallen);
 	if (ret < 0)
 		return ERR_PTR(ret);
 
-	epayload = kzalloc(sizeof(*epayload) + payload_datalen +
-			   datablob_len + HASH_SIZE + 1, GFP_KERNEL);
+	epayload = kzalloc_flex(*epayload, payload_data, payload_totallen,
+				GFP_KERNEL);
 	if (!epayload)
 		return ERR_PTR(-ENOMEM);