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, 73 insertions, 0 deletions

diff --git a/include/crypto/utils.h b/include/crypto/utils.h
new file mode 100644
index 000000000..2594f4577
--- /dev/null
+++ b/include/crypto/utils.h
@@ -0,0 +1,73 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later */
+/*
+ * Cryptographic utilities
+ *
+ * Copyright (c) 2023 Herbert Xu <herbert@gondor.apana.org.au>
+ */
+#ifndef _CRYPTO_UTILS_H
+#define _CRYPTO_UTILS_H
+
+#include <linux/unaligned.h>
+#include <linux/compiler_attributes.h>
+#include <linux/types.h>
+
+void __crypto_xor(u8 *dst, const u8 *src1, const u8 *src2, unsigned int size);
+
+static inline void crypto_xor(u8 *dst, const u8 *src, unsigned int size)
+{
+	if (IS_ENABLED(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS) &&
+	    __builtin_constant_p(size) &&
+	    (size % sizeof(unsigned long)) == 0) {
+		unsigned long *d = (unsigned long *)dst;
+		unsigned long *s = (unsigned long *)src;
+		unsigned long l;
+
+		while (size > 0) {
+			l = get_unaligned(d) ^ get_unaligned(s++);
+			put_unaligned(l, d++);
+			size -= sizeof(unsigned long);
+		}
+	} else {
+		__crypto_xor(dst, dst, src, size);
+	}
+}
+
+static inline void crypto_xor_cpy(u8 *dst, const u8 *src1, const u8 *src2,
+				  unsigned int size)
+{
+	if (IS_ENABLED(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS) &&
+	    __builtin_constant_p(size) &&
+	    (size % sizeof(unsigned long)) == 0) {
+		unsigned long *d = (unsigned long *)dst;
+		unsigned long *s1 = (unsigned long *)src1;
+		unsigned long *s2 = (unsigned long *)src2;
+		unsigned long l;
+
+		while (size > 0) {
+			l = get_unaligned(s1++) ^ get_unaligned(s2++);
+			put_unaligned(l, d++);
+			size -= sizeof(unsigned long);
+		}
+	} else {
+		__crypto_xor(dst, src1, src2, size);
+	}
+}
+
+noinline unsigned long __crypto_memneq(const void *a, const void *b, size_t size);
+
+/**
+ * crypto_memneq - Compare two areas of memory without leaking
+ *		   timing information.
+ *
+ * @a: One area of memory
+ * @b: Another area of memory
+ * @size: The size of the area.
+ *
+ * Returns 0 when data is equal, 1 otherwise.
+ */
+static inline int crypto_memneq(const void *a, const void *b, size_t size)
+{
+	return __crypto_memneq(a, b, size) != 0UL ? 1 : 0;
+}
+
+#endif	/* _CRYPTO_UTILS_H */