KEYS: trusted: dcp: fix key_len validation and calc_blob_len() return type

Two correctness and type-hygiene issues exist in the DCP trusted keys
implementation.

First, trusted_dcp_unseal() reads p->key_len from a user-supplied blob
without checking if it exceeds MAX_KEY_SIZE.  If a crafted blob provides a
payload_len larger than 128, the subsequent do_aead_crypto() call writes
past the end of the p->key array into the adjacent p->blob buffer within
the same struct trusted_key_payload -- the caller's own input, not
unrelated kernel memory.  While not exploitable, this violates strict array
bounds and triggers static analyzers.  Fix this by adding a validation
check against MIN_KEY_SIZE and MAX_KEY_SIZE immediately after reading the
length, matching the checks already done in trusted_core.c.

Second, calc_blob_len() calculates a sum in size_t that truncates to
unsigned int on 64-bit platforms.  Because the DCP hardware is only present
on 32-bit i.MX SoC platforms, size_t and unsigned int are functionally
equivalent in production, making this truncation harmless in practice.
Nevertheless, updating the return type to size_t (and subsequently updating
'blen' in the seal/unseal paths) resolves type-narrowing warnings and
improves overall code hygiene.

Fixes: 2e8a0f40a39c ("KEYS: trusted: Introduce NXP DCP-backed trusted keys")
Signed-off-by: Fabrice Derepas <fabrice.derepas@canonical.com>
Reviewed-by: David Gstir <david@sigma-star.at>
Reviewed-by: Richard Weinberger <richard@nod.at>
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Tested-by: Jarkko Sakkinen <jarkko@kernel.org>
Link: https://lore.kernel.org/r/20260719163939.3624767-1-fabrice.derepas@canonical.com
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>

1 files changed, 11 insertions, 4 deletions

diff --git a/security/keys/trusted-keys/trusted_dcp.c b/security/keys/trusted-keys/trusted_dcp.c
index 7b6eb655df0c..c078adebe190 100644
--- a/security/keys/trusted-keys/trusted_dcp.c
+++ b/security/keys/trusted-keys/trusted_dcp.c
@@ -69,7 +69,7 @@ static bool skip_zk_test;
 module_param_named(dcp_skip_zk_test, skip_zk_test, bool, 0);
 MODULE_PARM_DESC(dcp_skip_zk_test, "Don't test whether device keys are zero'ed");
 
-static unsigned int calc_blob_len(unsigned int payload_len)
+static size_t calc_blob_len(unsigned int payload_len)
 {
 	return sizeof(struct dcp_blob_fmt) + payload_len + DCP_BLOB_AUTHLEN;
 }
@@ -200,7 +200,8 @@ static int encrypt_blob_key(u8 *plain_key, u8 *encrypted_key)
 static int trusted_dcp_seal(struct trusted_key_payload *p, char *datablob)
 {
 	struct dcp_blob_fmt *b = (struct dcp_blob_fmt *)p->blob;
-	int blen, ret;
+	size_t blen;
+	int ret;
 	u8 *plain_blob_key;
 
 	blen = calc_blob_len(p->key_len);
@@ -242,7 +243,8 @@ out:
 static int trusted_dcp_unseal(struct trusted_key_payload *p, char *datablob)
 {
 	struct dcp_blob_fmt *b = (struct dcp_blob_fmt *)p->blob;
-	int blen, ret;
+	size_t blen;
+	int ret;
 	u8 *plain_blob_key = NULL;
 
 	if (b->fmt_version != DCP_BLOB_VERSION) {
@@ -253,9 +255,14 @@ static int trusted_dcp_unseal(struct trusted_key_payload *p, char *datablob)
 	}
 
 	p->key_len = le32_to_cpu(b->payload_len);
+	if (p->key_len < MIN_KEY_SIZE || p->key_len > MAX_KEY_SIZE) {
+		ret = -EINVAL;
+		goto out;
+	}
+
 	blen = calc_blob_len(p->key_len);
 	if (blen != p->blob_len) {
-		pr_err("DCP blob has bad length: %i != %i\n", blen,
+		pr_err("DCP blob has bad length: %zu != %u\n", blen,
 		       p->blob_len);
 		ret = -EINVAL;
 		goto out;