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, 70 insertions, 0 deletions

diff --git a/lib/crypto/chacha.c b/lib/crypto/chacha.c
new file mode 100644
index 000000000..86e5d382a
--- /dev/null
+++ b/lib/crypto/chacha.c
@@ -0,0 +1,70 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * The ChaCha stream cipher (RFC7539)
+ *
+ * Copyright (C) 2015 Martin Willi
+ */
+
+#include <crypto/chacha.h>
+#include <crypto/utils.h>
+#include <linux/export.h>
+#include <linux/kernel.h>
+#include <linux/module.h>
+
+static void __maybe_unused
+chacha_crypt_generic(struct chacha_state *state, u8 *dst, const u8 *src,
+		     unsigned int bytes, int nrounds)
+{
+	/* aligned to potentially speed up crypto_xor() */
+	u8 stream[CHACHA_BLOCK_SIZE] __aligned(sizeof(long));
+
+	while (bytes >= CHACHA_BLOCK_SIZE) {
+		chacha_block_generic(state, stream, nrounds);
+		crypto_xor_cpy(dst, src, stream, CHACHA_BLOCK_SIZE);
+		bytes -= CHACHA_BLOCK_SIZE;
+		dst += CHACHA_BLOCK_SIZE;
+		src += CHACHA_BLOCK_SIZE;
+	}
+	if (bytes) {
+		chacha_block_generic(state, stream, nrounds);
+		crypto_xor_cpy(dst, src, stream, bytes);
+	}
+}
+
+#ifdef CONFIG_CRYPTO_LIB_CHACHA_ARCH
+#include "chacha.h" /* $(SRCARCH)/chacha.h */
+#else
+#define chacha_crypt_arch chacha_crypt_generic
+#define hchacha_block_arch hchacha_block_generic
+#endif
+
+void chacha_crypt(struct chacha_state *state, u8 *dst, const u8 *src,
+		  unsigned int bytes, int nrounds)
+{
+	chacha_crypt_arch(state, dst, src, bytes, nrounds);
+}
+EXPORT_SYMBOL_GPL(chacha_crypt);
+
+void hchacha_block(const struct chacha_state *state,
+		   u32 out[HCHACHA_OUT_WORDS], int nrounds)
+{
+	hchacha_block_arch(state, out, nrounds);
+}
+EXPORT_SYMBOL_GPL(hchacha_block);
+
+#ifdef chacha_mod_init_arch
+static int __init chacha_mod_init(void)
+{
+	chacha_mod_init_arch();
+	return 0;
+}
+subsys_initcall(chacha_mod_init);
+
+static void __exit chacha_mod_exit(void)
+{
+}
+module_exit(chacha_mod_exit);
+#endif
+
+MODULE_DESCRIPTION("ChaCha stream cipher (RFC7539)");
+MODULE_LICENSE("GPL");