[PATCH v2] KEYS: trusted: Reject short TPM2 public areas

[Daehyeon Ko]

tpm2_load_cmd() reads TPMA_OBJECT with get_unaligned_be32(pub + 4), but
does not require public_len to cover that field.

A TPM 2.0 ASN.1 key blob with public_len zero makes the read begin at the
end of the B + 4-byte decoded allocation, causing a four-byte heap
out-of-bounds read before the TPM command is transmitted. This is reachable
from an unprivileged add_key() call when TPM trusted keys and a TPM2 device
are available.

This affects v5.13-rc1 and later kernels built with CONFIG_TRUSTED_KEYS=y
and CONFIG_TRUSTED_KEYS_TPM=y when a TPM2 device is present. A KASAN run
as UID 1000 with no effective capabilities reported:

BUG: KASAN: slab-out-of-bounds in tpm2_unseal_trusted
Read of size 4 at addr ffff888106273b48 by task exploit/160
CPU: 1 UID: 1000 PID: 160 Comm: exploit
The buggy address belongs to the object at ffff888106273b40
 which belongs to the cache kmalloc-8 of size 8
The buggy address is located 0 bytes to the right of
 allocated 8-byte region [ffff888106273b40, ffff888106273b48)

TPMT_PUBLIC starts with the two-byte type, two-byte nameAlg and four-byte
objectAttributes fields. Require public_len to cover all eight bytes before
reading the attributes.

The exact input produced the KASAN read in 3/3 fresh boots. The fixed build
rejected it with -EINVAL and no KASAN report in 3/3 boots; valid TPM 2.0
ASN.1 and legacy trusted-key loads continued to succeed.

The issue was found with an LLM-assisted source audit and validated with an
end-to-end QEMU and swtpm reproducer.

Fixes: e5fb5d2c5a03 ("security: keys: trusted: Make sealed key properly interoperable")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Daehyeon Ko <4ncienth@xxxxxxxxx>
---
Changes in v2:
- Describe the wrapper as the TPM 2.0 ASN.1 key format.
- Explain the eight-byte minimum next to the check.
- Return -EINVAL for a malformed TPMT_PUBLIC instead of -E2BIG.
- Replace the obsolete Assisted-by trailer with changelog disclosure.

v1: https://lore.kernel.org/r/20260925095308.3248297-1-4ncienth@xxxxxxxxx

 security/keys/trusted-keys/trusted_tpm2.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/security/keys/trusted-keys/trusted_tpm2.c b/security/keys/trusted-keys/trusted_tpm2.c
index 30188a0ec6766..fb45795b53327 100644
--- a/security/keys/trusted-keys/trusted_tpm2.c
+++ b/security/keys/trusted-keys/trusted_tpm2.c
@@ -425,6 +425,13 @@ static int tpm2_load_cmd(struct tpm_chip *chip,
 	if (private_len + 2 + public_len + 2 > blob_len)
 		return -E2BIG;
 
+	/*
+	 * TPMT_PUBLIC starts with type (2), nameAlg (2), and
+	 * objectAttributes (4), which is read below.
+	 */
+	if (public_len < 8)
+		return -EINVAL;
+
 	pub = blob + 2 + private_len + 2;
 	/* key attributes are always at offset 4 */
 	attrs = get_unaligned_be32(pub + 4);

base-commit: 722658cac48a6ba49f5506fd74d010ad9dbb2f05
-- 
2.55.0