[PATCH] KEYS: trusted: Fix blob allocation size in tpm2_key_decode()

[Stefano Garzarella <sgarzare@redhat.com>]

From: Stefano Garzarella <sgarzare@redhat.com>

tpm2_key_decode() allocates 4 bytes more than needed. The ASN.1
callbacks tpm2_key_priv() and tpm2_key_pub() provide the lengths
of TPM2B_PRIVATE and TPM2B_PUBLIC, so ctx.priv_len and ctx.pub_len
already account for the 2-byte `size` field each of those structures
starts with.

I noticed this while reviewing commit 114f00d738f1 ("KEYS: trusted:
Fix tpm2_load_cmd() boundary check"), which correctly reports
ctx.priv_len + ctx.pub_len as the decoded blob size [1].

Let's allocate exactly that amount, matching the data copied into
the blob.

[1] https://lore.kernel.org/linux-integrity/apfoKo-BdwaLXtkT@sgarzare-redhat/

Fixes: f2219745250f ("security: keys: trusted: use ASN.1 TPM2 key format for the blobs")
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
---
 security/keys/trusted-keys/trusted_tpm2.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/security/keys/trusted-keys/trusted_tpm2.c b/security/keys/trusted-keys/trusted_tpm2.c
index 01f18bb37047..a9b8a31a637c 100644
--- a/security/keys/trusted-keys/trusted_tpm2.c
+++ b/security/keys/trusted-keys/trusted_tpm2.c
@@ -115,7 +115,7 @@ static int tpm2_key_decode(struct trusted_key_payload *payload,
 	if (ctx.priv_len + ctx.pub_len > MAX_BLOB_SIZE)
 		return -EINVAL;
 
-	blob = kmalloc(ctx.priv_len + ctx.pub_len + 4, GFP_KERNEL);
+	blob = kmalloc(ctx.priv_len + ctx.pub_len, GFP_KERNEL);
 	if (!blob)
 		return -ENOMEM;
 
-- 
2.55.0