[PATCH v3] crypto: asymmetric_keys - prevent overflow in asymmetric_key_generate_id

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

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

Fixes: 7901c1a8effb ("KEYS: Implement binary asymmetric key ID handling")
Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
---
Changes in v3:
- Also use check_add_overflow() for adding the struct size itself as
  suggested by Lukas
- Drop struct_size()
- Update patch description
- Drop patch 2/2
- Link to v2: https://lore.kernel.org/lkml/20251012203841.60230-1-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 | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/crypto/asymmetric_keys/asymmetric_type.c b/crypto/asymmetric_keys/asymmetric_type.c
index ba2d9d1ea235..348966ea2175 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 +142,17 @@ struct asymmetric_key_id *asymmetric_key_generate_id(const void *val_1,
 						     size_t len_2)
 {
 	struct asymmetric_key_id *kid;
-
-	kid = kmalloc(sizeof(struct asymmetric_key_id) + len_1 + len_2,
-		      GFP_KERNEL);
+	size_t kid_sz;
+	size_t len;
+
+	if (check_add_overflow(len_1, len_2, &len))
+		return ERR_PTR(-EOVERFLOW);
+	if (check_add_overflow(sizeof(struct asymmetric_key_id), len, &kid_sz))
+		return ERR_PTR(-EOVERFLOW);
+	kid = kmalloc(kid_sz, 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