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

[Daehyeon Ko <4ncienth@gmail.com>]

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

A new-format 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 -E2BIG and no KASAN report in 3/3 boots; valid new- and
old-format trusted-key loads continued to succeed.

Fixes: e5fb5d2c5a03 ("security: keys: trusted: Make sealed key properly interoperable")
Cc: stable@vger.kernel.org
Assisted-by: LLM
Signed-off-by: Daehyeon Ko <4ncienth@gmail.com>
---
Tested with QEMU tpm-tis and swtpm: vulnerable 3/3 KASAN reports,
fixed 3/3 clean -E2BIG rejections, with public_len 7/8 and new/old-format
controls passing. Stable 5.15+ requires 114f00d738f1 first; both patches
apply cleanly in that order.

The source reproducer and full logs are available privately on request.

 security/keys/trusted-keys/trusted_tpm2.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/security/keys/trusted-keys/trusted_tpm2.c b/security/keys/trusted-keys/trusted_tpm2.c
index c2a69bcf381d7..b3109e0a924f5 100644
--- a/security/keys/trusted-keys/trusted_tpm2.c
+++ b/security/keys/trusted-keys/trusted_tpm2.c
@@ -418,6 +418,8 @@ static int tpm2_load_cmd(struct tpm_chip *chip,
 	public_len = get_unaligned_be16(blob + 2 + private_len);
 	if (private_len + 2 + public_len + 2 > blob_len)
 		return -E2BIG;
+	if (public_len < 8)
+		return -E2BIG;
 
 	pub = blob + 2 + private_len + 2;
 	/* key attributes are always at offset 4 */

base-commit: 27d14d3b15d5691bcbf0683883a2ea12469edbea