[PATCH v2] KEYS: encrypted: fix integer overflow of datablob_len

["Cen Zhang (Microsoft Security FORGE Labs)" <blbllhy@gmail.com>]

From: "Cen Zhang (Microsoft Security FORGE Labs)" <cenzhang@microsoft.com>

The datablob_len field in struct encrypted_key_payload and the local
variable in encrypted_key_alloc() are declared as unsigned short, which
has a maximum value of 65535. The datablob_len is computed as:

  format_len + 1 + strlen(master_desc) + 1 + strlen(datalen) + 1
  + ivsize + 1 + encrypted_datalen

An attacker can create an encrypted key with a very long datalen string
(e.g., 32756 characters of leading zeros followed by "4096", which
kstrtol() happily parses as 4096), and then update it with a very long
master_desc string (~32760 characters). The combined lengths exceed
65535, causing the unsigned short to silently wrap around. This results
in a grossly undersized kzalloc() allocation, and the subsequent
memcpy() in __ekey_init() writes ~32KB past the end of the buffer,
corrupting adjacent slab objects.

Fix this by:
1. Using check_add_overflow() to calculate datablob_len directly into
   its existing unsigned short destination.
2. Checking the total payload length the same way before passing it to
   key_payload_reserve(), since key->datalen is also unsigned short.
3. Using kzalloc_flex() to allocate encrypted_key_payload together with
   its trailing payload_data[] array.

Fixes: 7e70cb497850 ("keys: add new key-type encrypted")
Cc: stable@vger.kernel.org
Assisted-by: GitHub-Copilot:claude-opus-4.6
Signed-off-by: Cen Zhang (Microsoft Security FORGE Labs) <cenzhang@microsoft.com>
Signed-off-by: Francis Perron (Akrites SIRT) <francis@akrites.dev>
---
Changes in v2:
- Keep datablob_len unchanged and check both unsigned short bounds with
  check_add_overflow().
- Use kzalloc_flex() for the trailing payload_data[] array.
- Correct the attribution and sign-off trailers.

The initial version was discussed off-list and is not publicly archived.

 security/keys/encrypted-keys/encrypted.c | 20 ++++++++++++++------
 1 file changed, 14 insertions(+), 6 deletions(-)

diff --git a/security/keys/encrypted-keys/encrypted.c b/security/keys/encrypted-keys/encrypted.c
index 59cb77b237b3..e07092ea301a 100644
--- a/security/keys/encrypted-keys/encrypted.c
+++ b/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);
+
+	if (check_add_overflow(datablob_len,
+			       payload_datalen + HASH_SIZE + 1,
+			       &payload_totallen))
+		return ERR_PTR(-EINVAL);
 
-	ret = key_payload_reserve(key, payload_datalen + datablob_len
-				  + HASH_SIZE + 1);
+	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);
 
-- 
2.55.0