Linux TPM driver keeps serving commands after null key name mismatch

In the Linux kernel, the following vulnerability has been resolved:

tpm: Disable TPM on null key name mismatch

The null key name check exists to protect against TPM reset attacks, so a mismatch should stop the device from serving further requests. Currently it does not disable the chip when it finds a mismatch.

The mismatch is logged:

tpm tpm0: null key integrity check failed

but the chip keeps serving commands:

/ # tpm2_getcap -c properties-fixed
TPM_PT_FAMILY_INDICATOR:
  as UINT32:                0x08322e3000
  as string:                "2.0"
...

tpm2_load_null() where the null key name check is run sets the chip as disabled only if the rc is non zero. When the mismatch is seen, rc is zero at that point and it returns success. The other issue is that the caller expects the null key handle to be populated when the function returns 0 which it does here without writing the handle and proceeds assuming the null key handle is valid.

During the test, I noticed that tpm2_start_auth_session() uses the uninitialized stack value as the key handle since tpm2_load_null() returns 0 despite the integrity failure and proceeds with TPM2_CC_START_AUTH_SESS with this value as salt key handle.

Set rc to -ENODEV on the mismatch. The error path then disables the chip and returns the correct code to the caller.

Tested in QEMU with swtpm and CONFIG_TCG_TPM2_HMAC=y by making TPM2_CC_CONTEXT_LOAD fail with TPM2_RC_INTEGRITY and changing the name of the re-created null key. The chip is now disabled on the mismatch.