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, 120 insertions, 0 deletions

diff --git a/Documentation/usb/usbdevfs-drop-permissions.c b/Documentation/usb/usbdevfs-drop-permissions.c
new file mode 100644
index 000000000..6b8da6ef0
--- /dev/null
+++ b/Documentation/usb/usbdevfs-drop-permissions.c
@@ -0,0 +1,120 @@
+#include <sys/ioctl.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <fcntl.h>
+#include <stdio.h>
+#include <errno.h>
+#include <string.h>
+#include <inttypes.h>
+#include <unistd.h>
+
+#include <linux/usbdevice_fs.h>
+
+/* For building without an updated set of headers */
+#ifndef USBDEVFS_DROP_PRIVILEGES
+#define USBDEVFS_DROP_PRIVILEGES		_IOW('U', 30, __u32)
+#define USBDEVFS_CAP_DROP_PRIVILEGES		0x40
+#endif
+
+void drop_privileges(int fd, uint32_t mask)
+{
+	int res;
+
+	res = ioctl(fd, USBDEVFS_DROP_PRIVILEGES, &mask);
+	if (res)
+		printf("ERROR: USBDEVFS_DROP_PRIVILEGES returned %d\n", res);
+	else
+		printf("OK: privileges dropped!\n");
+}
+
+void reset_device(int fd)
+{
+	int res;
+
+	res = ioctl(fd, USBDEVFS_RESET);
+	if (!res)
+		printf("OK: USBDEVFS_RESET succeeded\n");
+	else
+		printf("ERROR: reset failed! (%d - %s)\n",
+		       -res, strerror(-res));
+}
+
+void claim_some_intf(int fd)
+{
+	int i, res;
+
+	for (i = 0; i < 4; i++) {
+		res = ioctl(fd, USBDEVFS_CLAIMINTERFACE, &i);
+		if (!res)
+			printf("OK: claimed if %d\n", i);
+		else
+			printf("ERROR claiming if %d (%d - %s)\n",
+			       i, -res, strerror(-res));
+	}
+}
+
+int main(int argc, char *argv[])
+{
+	uint32_t mask, caps;
+	int c, fd;
+
+	fd = open(argv[1], O_RDWR);
+	if (fd < 0) {
+		printf("Failed to open file\n");
+		goto err_fd;
+	}
+
+	/*
+	 * check if dropping privileges is supported,
+	 * bail on systems where the capability is not present
+	 */
+	ioctl(fd, USBDEVFS_GET_CAPABILITIES, &caps);
+	if (!(caps & USBDEVFS_CAP_DROP_PRIVILEGES)) {
+		printf("DROP_PRIVILEGES not supported\n");
+		goto err;
+	}
+
+	/*
+	 * Drop privileges but keep the ability to claim all
+	 * free interfaces (i.e., those not used by kernel drivers)
+	 */
+	drop_privileges(fd, -1U);
+
+	printf("Available options:\n"
+		"[0] Exit now\n"
+		"[1] Reset device. Should fail if device is in use\n"
+		"[2] Claim 4 interfaces. Should succeed where not in use\n"
+		"[3] Narrow interface permission mask\n"
+		"Which option shall I run?: ");
+
+	while (scanf("%d", &c) == 1) {
+		switch (c) {
+		case 0:
+			goto exit;
+		case 1:
+			reset_device(fd);
+			break;
+		case 2:
+			claim_some_intf(fd);
+			break;
+		case 3:
+			printf("Insert new mask: ");
+			scanf("%x", &mask);
+			drop_privileges(fd, mask);
+			break;
+		default:
+			printf("I don't recognize that\n");
+		}
+
+		printf("Which test shall I run next?: ");
+	}
+
+exit:
+	close(fd);
+	return 0;
+
+err:
+	close(fd);
+err_fd:
+	return 1;
+}