[PATCH v2 1/2] crypto: asymmetric_keys - prevent overflow in asymmetric_key_generate_id

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

Use check_add_overflow() to guard against a potential integer overflow
when adding the binary blob lengths in asymmetric_key_generate_id() and
return -EOVERFLOW accordingly. This prevents a possible buffer overflow
when copying data from potentially malicious X.509 fields that can be
arbitrarily large, such as ASN.1 INTEGER serial numbers, issuer names,
etc.

Also use struct_size() to calculate the number of bytes to allocate for
the new asymmetric key id.

Cc: stable@vger.kernel.org
Fixes: 7901c1a8effb ("KEYS: Implement binary asymmetric key ID handling")
Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
---
Changes in v2:
- Use check_add_overflow() and error out as suggested by Lukas
- Update patch description
- Add Fixes: tag and @stable for backporting
- Link to v1: https://lore.kernel.org/lkml/20251007185220.234611-2-thorsten.blum@linux.dev/
---
 crypto/asymmetric_keys/asymmetric_type.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/crypto/asymmetric_keys/asymmetric_type.c b/crypto/asymmetric_keys/asymmetric_type.c
index ba2d9d1ea235..bd96f799757d 100644
--- a/crypto/asymmetric_keys/asymmetric_type.c
+++ b/crypto/asymmetric_keys/asymmetric_type.c
@@ -11,6 +11,7 @@
 #include <crypto/public_key.h>
 #include <linux/seq_file.h>
 #include <linux/module.h>
+#include <linux/overflow.h>
 #include <linux/slab.h>
 #include <linux/ctype.h>
 #include <keys/system_keyring.h>
@@ -141,12 +142,14 @@ struct asymmetric_key_id *asymmetric_key_generate_id(const void *val_1,
 						     size_t len_2)
 {
 	struct asymmetric_key_id *kid;
+	size_t len;
 
-	kid = kmalloc(sizeof(struct asymmetric_key_id) + len_1 + len_2,
-		      GFP_KERNEL);
+	if (check_add_overflow(len_1, len_2, &len))
+		return ERR_PTR(-EOVERFLOW);
+	kid = kmalloc(struct_size(kid, data, len), GFP_KERNEL);
 	if (!kid)
 		return ERR_PTR(-ENOMEM);
-	kid->len = len_1 + len_2;
+	kid->len = len;
 	memcpy(kid->data, val_1, len_1);
 	memcpy(kid->data + len_1, val_2, len_2);
 	return kid;
-- 
2.51.0