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, 80 insertions, 0 deletions

diff --git a/drivers/mtd/mtdblock_ro.c b/drivers/mtd/mtdblock_ro.c
new file mode 100644
index 000000000..e564521d6
--- /dev/null
+++ b/drivers/mtd/mtdblock_ro.c
@@ -0,0 +1,80 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * Simple read-only (writable only for RAM) mtdblock driver
+ *
+ * Copyright © 2001-2010 David Woodhouse <dwmw2@infradead.org>
+ */
+
+#include <linux/init.h>
+#include <linux/slab.h>
+#include <linux/mtd/mtd.h>
+#include <linux/mtd/blktrans.h>
+#include <linux/module.h>
+#include <linux/major.h>
+
+static int mtdblock_readsect(struct mtd_blktrans_dev *dev,
+			      unsigned long block, char *buf)
+{
+	size_t retlen;
+	int err;
+
+	err = mtd_read(dev->mtd, (block * 512), 512, &retlen, buf);
+	if (err && !mtd_is_bitflip(err))
+		return 1;
+	return 0;
+}
+
+static int mtdblock_writesect(struct mtd_blktrans_dev *dev,
+			      unsigned long block, char *buf)
+{
+	size_t retlen;
+
+	if (mtd_write(dev->mtd, (block * 512), 512, &retlen, buf))
+		return 1;
+	return 0;
+}
+
+static void mtdblock_add_mtd(struct mtd_blktrans_ops *tr, struct mtd_info *mtd)
+{
+	struct mtd_blktrans_dev *dev = kzalloc_obj(*dev);
+
+	if (!dev)
+		return;
+
+	dev->mtd = mtd;
+	dev->devnum = mtd->index;
+
+	dev->size = mtd->size >> 9;
+	dev->tr = tr;
+	dev->readonly = 1;
+
+	if (mtd_type_is_nand(mtd))
+		pr_warn_ratelimited("%s: MTD device '%s' is NAND, please consider using UBI block devices instead.\n",
+			tr->name, mtd->name);
+
+	if (add_mtd_blktrans_dev(dev))
+		kfree(dev);
+}
+
+static void mtdblock_remove_dev(struct mtd_blktrans_dev *dev)
+{
+	del_mtd_blktrans_dev(dev);
+}
+
+static struct mtd_blktrans_ops mtdblock_tr = {
+	.name		= "mtdblock",
+	.major		= MTD_BLOCK_MAJOR,
+	.part_bits	= 0,
+	.blksize 	= 512,
+	.readsect	= mtdblock_readsect,
+	.writesect	= mtdblock_writesect,
+	.add_mtd	= mtdblock_add_mtd,
+	.remove_dev	= mtdblock_remove_dev,
+	.owner		= THIS_MODULE,
+};
+
+module_mtd_blktrans(mtdblock_tr);
+
+MODULE_LICENSE("GPL");
+MODULE_AUTHOR("David Woodhouse <dwmw2@infradead.org>");
+MODULE_DESCRIPTION("Simple read-only block device emulation access to MTD devices");