KVM: SEV: Do cache maintenance on the source VM during intra-host migrationgrafted

Manually perform cache maintenance on the source VM during intra-host
migration to ensure no stale data is left in CPU caches after the VM is
destroyed.  Because the source VM is "converted" to a non-SEV VM, KVM's
memory reclaim flows won't trigger cache maintenance, e.g. when all guest
memory is reclaimed in response to detaching from the mmu_notifier.

Note, relying on the destination VM to do cache maintenance isn't an option
as KVM doesn't require identical guest memory configurations, i.e. the
source VM may have access to memory that the destination VM does not.
Enforcing equivalent memory configurations is infeasible, as it would
require a *deep* comparison of memslots, e.g. to verify that not only are
the memslot identical, but what the memslots point at is also identical.

Fixes: b56639318bb2 ("KVM: SEV: Add support for SEV intra host migration")
Cc: stable@vger.kernel.org
Reported-by: Stefan Teodorescu <fane@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-ID: <20260923163721.1584779-3-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 244 insertions, 0 deletions

diff --git a/lib/digsig.c b/lib/digsig.c
new file mode 100644
index 000000000..9dd319c12
--- /dev/null
+++ b/lib/digsig.c
@@ -0,0 +1,244 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Copyright (C) 2011 Nokia Corporation
+ * Copyright (C) 2011 Intel Corporation
+ *
+ * Author:
+ * Dmitry Kasatkin <dmitry.kasatkin@nokia.com>
+ *                 <dmitry.kasatkin@intel.com>
+ *
+ * File: sign.c
+ *	implements signature (RSA) verification
+ *	pkcs decoding is based on LibTomCrypt code
+ */
+
+#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
+
+#include <linux/err.h>
+#include <linux/module.h>
+#include <linux/slab.h>
+#include <linux/key.h>
+#include <crypto/sha1.h>
+#include <keys/user-type.h>
+#include <linux/mpi.h>
+#include <linux/digsig.h>
+
+static const char *pkcs_1_v1_5_decode_emsa(const unsigned char *msg,
+						unsigned long  msglen,
+						unsigned long  modulus_bitlen,
+						unsigned long *outlen)
+{
+	unsigned long modulus_len, ps_len, i;
+
+	modulus_len = (modulus_bitlen >> 3) + (modulus_bitlen & 7 ? 1 : 0);
+
+	/* test message size */
+	if ((msglen > modulus_len) || (modulus_len < 11))
+		return NULL;
+
+	/* separate encoded message */
+	if (msg[0] != 0x00 || msg[1] != 0x01)
+		return NULL;
+
+	for (i = 2; i < modulus_len - 1; i++)
+		if (msg[i] != 0xFF)
+			break;
+
+	/* separator check */
+	if (msg[i] != 0)
+		/* There was no octet with hexadecimal value 0x00
+		to separate ps from m. */
+		return NULL;
+
+	ps_len = i - 2;
+
+	*outlen = (msglen - (2 + ps_len + 1));
+
+	return msg + 2 + ps_len + 1;
+}
+
+/*
+ * RSA Signature verification with public key
+ */
+static int digsig_verify_rsa(struct key *key,
+		    const char *sig, int siglen,
+		       const char *h, int hlen)
+{
+	int err = -EINVAL;
+	unsigned long len;
+	unsigned long mlen, mblen;
+	unsigned nret, l;
+	int head, i;
+	unsigned char *out1 = NULL;
+	const char *m;
+	MPI in = NULL, res = NULL, pkey[2];
+	uint8_t *p, *datap;
+	const uint8_t *endp;
+	const struct user_key_payload *ukp;
+	struct pubkey_hdr *pkh;
+
+	down_read(&key->sem);
+	ukp = user_key_payload_locked(key);
+
+	if (!ukp) {
+		/* key was revoked before we acquired its semaphore */
+		err = -EKEYREVOKED;
+		goto err1;
+	}
+
+	if (ukp->datalen < sizeof(*pkh))
+		goto err1;
+
+	pkh = (struct pubkey_hdr *)ukp->data;
+
+	if (pkh->version != 1)
+		goto err1;
+
+	if (pkh->algo != PUBKEY_ALGO_RSA)
+		goto err1;
+
+	if (pkh->nmpi != 2)
+		goto err1;
+
+	datap = pkh->mpi;
+	endp = ukp->data + ukp->datalen;
+
+	for (i = 0; i < pkh->nmpi; i++) {
+		unsigned int remaining = endp - datap;
+		pkey[i] = mpi_read_from_buffer(datap, &remaining);
+		if (IS_ERR(pkey[i])) {
+			err = PTR_ERR(pkey[i]);
+			goto err;
+		}
+		datap += remaining;
+	}
+
+	mblen = mpi_get_nbits(pkey[0]);
+	mlen = DIV_ROUND_UP(mblen, 8);
+
+	if (mlen == 0) {
+		err = -EINVAL;
+		goto err;
+	}
+
+	err = -ENOMEM;
+
+	out1 = kzalloc(mlen, GFP_KERNEL);
+	if (!out1)
+		goto err;
+
+	nret = siglen;
+	in = mpi_read_from_buffer(sig, &nret);
+	if (IS_ERR(in)) {
+		err = PTR_ERR(in);
+		goto err;
+	}
+
+	res = mpi_alloc(mpi_get_nlimbs(in) * 2);
+	if (!res)
+		goto err;
+
+	err = mpi_powm(res, in, pkey[1], pkey[0]);
+	if (err)
+		goto err;
+
+	if (mpi_get_nlimbs(res) * BYTES_PER_MPI_LIMB > mlen) {
+		err = -EINVAL;
+		goto err;
+	}
+
+	p = mpi_get_buffer(res, &l, NULL);
+	if (!p) {
+		err = -EINVAL;
+		goto err;
+	}
+
+	len = mlen;
+	head = len - l;
+	memcpy(out1 + head, p, l);
+
+	kfree(p);
+
+	m = pkcs_1_v1_5_decode_emsa(out1, len, mblen, &len);
+
+	if (!m || len != hlen || memcmp(m, h, hlen))
+		err = -EINVAL;
+
+err:
+	mpi_free(in);
+	mpi_free(res);
+	kfree(out1);
+	while (--i >= 0)
+		mpi_free(pkey[i]);
+err1:
+	up_read(&key->sem);
+
+	return err;
+}
+
+/**
+ * digsig_verify() - digital signature verification with public key
+ * @keyring:	keyring to search key in
+ * @sig:	digital signature
+ * @siglen:	length of the signature
+ * @data:	data
+ * @datalen:	length of the data
+ *
+ * Returns 0 on success, -EINVAL otherwise
+ *
+ * Verifies data integrity against digital signature.
+ * Currently only RSA is supported.
+ * Normally hash of the content is used as a data for this function.
+ *
+ */
+int digsig_verify(struct key *keyring, const char *sig, int siglen,
+						const char *data, int datalen)
+{
+	struct signature_hdr *sh = (struct signature_hdr *)sig;
+	struct sha1_ctx ctx;
+	unsigned char hash[SHA1_DIGEST_SIZE];
+	struct key *key;
+	char name[20];
+	int err;
+
+	if (siglen < sizeof(*sh) + 2)
+		return -EINVAL;
+
+	if (sh->algo != PUBKEY_ALGO_RSA)
+		return -ENOTSUPP;
+
+	sprintf(name, "%llX", __be64_to_cpup((uint64_t *)sh->keyid));
+
+	if (keyring) {
+		/* search in specific keyring */
+		key_ref_t kref;
+		kref = keyring_search(make_key_ref(keyring, 1UL),
+				      &key_type_user, name, true);
+		if (IS_ERR(kref))
+			key = ERR_CAST(kref);
+		else
+			key = key_ref_to_ptr(kref);
+	} else {
+		key = request_key(&key_type_user, name, NULL);
+	}
+	if (IS_ERR(key)) {
+		pr_err("key not found, id: %s\n", name);
+		return PTR_ERR(key);
+	}
+
+	sha1_init(&ctx);
+	sha1_update(&ctx, data, datalen);
+	sha1_update(&ctx, sig, sizeof(*sh));
+	sha1_final(&ctx, hash);
+
+	/* pass signature mpis address */
+	err = digsig_verify_rsa(key, sig + sizeof(*sh), siglen - sizeof(*sh),
+			     hash, sizeof(hash));
+
+	key_put(key);
+
+	return err ? -EINVAL : 0;
+}
+EXPORT_SYMBOL_GPL(digsig_verify);
+
+MODULE_LICENSE("GPL");