thunderbolt: Fix NULL dereference in tb_remove_work()grafted

There is a slight race between tb_remove_work() and tb_domain_remove()
which leads to dereferencing a NULL tb->root_switch pointer inside
tb_free_unplugged_xdomains():

        Thread A                            Thread B

  tb_remove_work()
                                    tb_domain_remove()
                                      mutex_lock(&tb->lock)
                                      tb_stop()
                                        /* doesn't cancel a running callback */
                                        cancel_delayed_work(&tcm->remove_work)
                                        ...
                                        tb_switch_remove(tb->root_switch)
                                        tb->root_switch = NULL
                                      mutex_unlock(&tb->lock)
    mutex_lock(&tb->lock)
    ...
    /* without checking ->root_switch */
    tb_free_unplugged_xdomains(tb->root_switch)
    mutex_unlock(&tb->lock)

Commit a8937f35cf39 ("thunderbolt: Remove XDomain from the bus without
holding tb->lock") doesn't seem right to move tb_free_unplugged_xdomains()
out of the &tb->lock section and the check for tb->root_switch, in
particular.  It states:

  For this reason separate removing the XDomain from the topology data
  structures (where we need the lock) from unregistering the device from
  the bus (where remove callbacks of the drivers are being called).

tb_free_unplugged_xdomains() belongs to the former group of functions
requiring the lock.  And it also calls tb_xdomain_remove() which should
only be called with &tb->lock held.

Found by Linux Verification Center (linuxtesting.org) with Svace static
analysis tool.

Fixes: a8937f35cf39 ("thunderbolt: Remove XDomain from the bus without holding tb->lock")
Cc: stable@vger.kernel.org
Signed-off-by: Fedor Pchelkin <pchelkin@ispras.ru>
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.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;
+}