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, 66 insertions, 0 deletions

diff --git a/fs/nullfs.c b/fs/nullfs.c
new file mode 100644
index 000000000..e06352c7b
--- /dev/null
+++ b/fs/nullfs.c
@@ -0,0 +1,66 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/* Copyright (c) 2026 Christian Brauner <brauner@kernel.org> */
+#include <linux/fs/super_types.h>
+#include <linux/fs_context.h>
+#include <linux/magic.h>
+
+#include "mount.h"
+
+static const struct super_operations nullfs_super_operations = {
+	.statfs	= simple_statfs,
+};
+
+static int nullfs_fs_fill_super(struct super_block *s, struct fs_context *fc)
+{
+	struct inode *inode;
+
+	s->s_maxbytes		= MAX_LFS_FILESIZE;
+	s->s_blocksize		= PAGE_SIZE;
+	s->s_blocksize_bits	= PAGE_SHIFT;
+	s->s_magic		= NULL_FS_MAGIC;
+	s->s_op			= &nullfs_super_operations;
+	s->s_export_op		= NULL;
+	s->s_xattr		= NULL;
+	s->s_time_gran		= 1;
+	s->s_d_flags		= 0;
+
+	inode = new_inode(s);
+	if (!inode)
+		return -ENOMEM;
+
+	/* nullfs is permanently empty... */
+	make_empty_dir_inode(inode);
+	simple_inode_init_ts(inode);
+	inode->i_ino	= 1;
+	/* ... and immutable. */
+	inode->i_flags |= S_IMMUTABLE;
+
+	s->s_root = d_make_root(inode);
+	if (!s->s_root)
+		return -ENOMEM;
+
+	return 0;
+}
+
+static int nullfs_fs_get_tree(struct fs_context *fc)
+{
+	return get_tree_nodev(fc, nullfs_fs_fill_super);
+}
+
+static const struct fs_context_operations nullfs_fs_context_ops = {
+	.get_tree	= nullfs_fs_get_tree,
+};
+
+static int nullfs_init_fs_context(struct fs_context *fc)
+{
+	fc->ops		= &nullfs_fs_context_ops;
+	fc->sb_flags	|= SB_NOUSER;
+	fc->s_iflags	|= SB_I_NOEXEC | SB_I_NODEV;
+	return 0;
+}
+
+struct file_system_type nullfs_fs_type = {
+	.name			= "nullfs",
+	.init_fs_context	= nullfs_init_fs_context,
+	.kill_sb		= kill_anon_super,
+};