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virt/sev-guest: Prevent IV reuse in the SNP guest driver

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The AMD Secure Processor (ASP) and an SNP guest use a series of
AES-GCM keys called VMPCKs to communicate securely with each other.
The IV to this scheme is a sequence number that both the ASP and the
guest track.

Currently, this sequence number in a guest request must exactly match
the sequence number tracked by the ASP. This means that if the guest
sees an error from the host during a request it can only retry that
exact request or disable the VMPCK to prevent an IV reuse. AES-GCM
cannot tolerate IV reuse, see: "Authentication Failures in NIST version
of GCM" - Antoine Joux et al.

In order to address this, make handle_guest_request() delete the VMPCK
on any non successful return. To allow userspace querying the cert_data
length make handle_guest_request() save the number of pages required by
the host, then have handle_guest_request() retry the request without
requesting the extended data, then return the number of pages required
back to userspace.

  [ bp: Massage, incorporate Tom's review comments. ]

Fixes: fce96cf044 ("virt: Add SEV-SNP guest driver")
Reported-by: Peter Gonda <pgonda@google.com>
Signed-off-by: Peter Gonda <pgonda@google.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
Cc: stable@kernel.org
Link: https://lore.kernel.org/r/20221116175558.2373112-1-pgonda@google.com
This commit is contained in:
Peter Gonda 2022-11-16 09:55:58 -08:00 committed by Borislav Petkov
parent eb7081409f
commit 47894e0fa6
1 changed files with 70 additions and 14 deletions
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84
drivers/virt/coco/sev-guest/sev-guest.c
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@ -67,8 +67,27 @@ static bool is_vmpck_empty(struct snp_guest_dev *snp_dev)
return true;
}
/*
* If an error is received from the host or AMD Secure Processor (ASP) there
* are two options. Either retry the exact same encrypted request or discontinue
* using the VMPCK.
*
* This is because in the current encryption scheme GHCB v2 uses AES-GCM to
* encrypt the requests. The IV for this scheme is the sequence number. GCM
* cannot tolerate IV reuse.
*
* The ASP FW v1.51 only increments the sequence numbers on a successful
* guest<->ASP back and forth and only accepts messages at its exact sequence
* number.
*
* So if the sequence number were to be reused the encryption scheme is
* vulnerable. If the sequence number were incremented for a fresh IV the ASP
* will reject the request.
*/
static void snp_disable_vmpck(struct snp_guest_dev *snp_dev)
{
dev_alert(snp_dev->dev, "Disabling vmpck_id %d to prevent IV reuse.\n",
vmpck_id);
memzero_explicit(snp_dev->vmpck, VMPCK_KEY_LEN);
snp_dev->vmpck = NULL;
}
@ -321,34 +340,71 @@ static int handle_guest_request(struct snp_guest_dev *snp_dev, u64 exit_code, in
if (rc)
return rc;
/* Call firmware to process the request */
/*
* Call firmware to process the request. In this function the encrypted
* message enters shared memory with the host. So after this call the
* sequence number must be incremented or the VMPCK must be deleted to
* prevent reuse of the IV.
*/
rc = snp_issue_guest_request(exit_code, &snp_dev->input, &err);
/*
* If the extended guest request fails due to having too small of a
* certificate data buffer, retry the same guest request without the
* extended data request in order to increment the sequence number
* and thus avoid IV reuse.
*/
if (exit_code == SVM_VMGEXIT_EXT_GUEST_REQUEST &&
err == SNP_GUEST_REQ_INVALID_LEN) {
const unsigned int certs_npages = snp_dev->input.data_npages;
exit_code = SVM_VMGEXIT_GUEST_REQUEST;
/*
* If this call to the firmware succeeds, the sequence number can
* be incremented allowing for continued use of the VMPCK. If
* there is an error reflected in the return value, this value
* is checked further down and the result will be the deletion
* of the VMPCK and the error code being propagated back to the
* user as an ioctl() return code.
*/
rc = snp_issue_guest_request(exit_code, &snp_dev->input, &err);
/*
* Override the error to inform callers the given extended
* request buffer size was too small and give the caller the
* required buffer size.
*/
err = SNP_GUEST_REQ_INVALID_LEN;
snp_dev->input.data_npages = certs_npages;
}
if (fw_err)
*fw_err = err;
if (rc)
return rc;
if (rc) {
dev_alert(snp_dev->dev,
"Detected error from ASP request. rc: %d, fw_err: %llu\n",
rc, *fw_err);
goto disable_vmpck;
}
/*
* The verify_and_dec_payload() will fail only if the hypervisor is
* actively modifying the message header or corrupting the encrypted payload.
* This hints that hypervisor is acting in a bad faith. Disable the VMPCK so that
* the key cannot be used for any communication. The key is disabled to ensure
* that AES-GCM does not use the same IV while encrypting the request payload.
*/
rc = verify_and_dec_payload(snp_dev, resp_buf, resp_sz);
if (rc) {
dev_alert(snp_dev->dev,
"Detected unexpected decode failure, disabling the vmpck_id %d\n",
vmpck_id);
snp_disable_vmpck(snp_dev);
return rc;
"Detected unexpected decode failure from ASP. rc: %d\n",
rc);
goto disable_vmpck;
}
/* Increment to new message sequence after payload decryption was successful. */
snp_inc_msg_seqno(snp_dev);
return 0;
disable_vmpck:
snp_disable_vmpck(snp_dev);
return rc;
}
static int get_report(struct snp_guest_dev *snp_dev, struct snp_guest_request_ioctl *arg)