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>

8 files changed, 336 insertions, 0 deletions

diff --git a/samples/check-exec/.gitignore b/samples/check-exec/.gitignore
new file mode 100644
index 000000000..cd759a19d
--- /dev/null
+++ b/samples/check-exec/.gitignore
@@ -0,0 +1,2 @@
+/inc
+/set-exec
diff --git a/samples/check-exec/Makefile b/samples/check-exec/Makefile
new file mode 100644
index 000000000..c4f08ad0f
--- /dev/null
+++ b/samples/check-exec/Makefile
@@ -0,0 +1,15 @@
+# SPDX-License-Identifier: BSD-3-Clause
+
+userprogs-always-y := \
+	inc \
+	set-exec
+
+userccflags += -I usr/include
+
+.PHONY: all clean
+
+all:
+	$(MAKE) -C ../.. samples/check-exec/
+
+clean:
+	$(MAKE) -C ../.. M=samples/check-exec/ clean
diff --git a/samples/check-exec/inc.c b/samples/check-exec/inc.c
new file mode 100644
index 000000000..7f6ef06a2
--- /dev/null
+++ b/samples/check-exec/inc.c
@@ -0,0 +1,212 @@
+// SPDX-License-Identifier: BSD-3-Clause
+/*
+ * Very simple script interpreter that can evaluate two different commands (one
+ * per line):
+ * - "?" to initialize a counter from user's input;
+ * - "+" to increment the counter (which is set to 0 by default).
+ *
+ * See tools/testing/selftests/exec/check-exec-tests.sh and
+ * Documentation/userspace-api/check_exec.rst
+ *
+ * Copyright © 2024 Microsoft Corporation
+ */
+
+#define _GNU_SOURCE
+#include <errno.h>
+#include <linux/fcntl.h>
+#include <linux/prctl.h>
+#include <linux/securebits.h>
+#include <stdbool.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/prctl.h>
+#include <sys/syscall.h>
+#include <unistd.h>
+
+static int sys_execveat(int dirfd, const char *pathname, char *const argv[],
+			char *const envp[], int flags)
+{
+	return syscall(__NR_execveat, dirfd, pathname, argv, envp, flags);
+}
+
+/* Returns 1 on error, 0 otherwise. */
+static int interpret_buffer(char *buffer, size_t buffer_size)
+{
+	char *line, *saveptr = NULL;
+	long long number = 0;
+
+	/* Each command is the first character of a line. */
+	saveptr = NULL;
+	line = strtok_r(buffer, "\n", &saveptr);
+	while (line) {
+		if (*line != '#' && strlen(line) != 1) {
+			fprintf(stderr, "# ERROR: Unknown string\n");
+			return 1;
+		}
+		switch (*line) {
+		case '#':
+			/* Skips shebang and comments. */
+			break;
+		case '+':
+			/* Increments and prints the number. */
+			number++;
+			printf("%lld\n", number);
+			break;
+		case '?':
+			/* Reads integer from stdin. */
+			fprintf(stderr, "> Enter new number: \n");
+			if (scanf("%lld", &number) != 1) {
+				fprintf(stderr,
+					"# WARNING: Failed to read number from stdin\n");
+			}
+			break;
+		default:
+			fprintf(stderr, "# ERROR: Unknown character '%c'\n",
+				*line);
+			return 1;
+		}
+		line = strtok_r(NULL, "\n", &saveptr);
+	}
+	return 0;
+}
+
+/* Returns 1 on error, 0 otherwise. */
+static int interpret_stream(FILE *script, char *const script_name,
+			    char *const *const envp, const bool restrict_stream)
+{
+	int err;
+	char *const script_argv[] = { script_name, NULL };
+	char buf[128] = {};
+	size_t buf_size = sizeof(buf);
+
+	/*
+	 * We pass a valid argv and envp to the kernel to emulate a native
+	 * script execution.  We must use the script file descriptor instead of
+	 * the script path name to avoid race conditions.
+	 */
+	err = sys_execveat(fileno(script), "", script_argv, envp,
+			   AT_EMPTY_PATH | AT_EXECVE_CHECK);
+	if (err && restrict_stream) {
+		perror("ERROR: Script execution check");
+		return 1;
+	}
+
+	/* Reads script. */
+	buf_size = fread(buf, 1, buf_size - 1, script);
+	return interpret_buffer(buf, buf_size);
+}
+
+static void print_usage(const char *argv0)
+{
+	fprintf(stderr, "usage: %s <script.inc> | -i | -c <command>\n\n",
+		argv0);
+	fprintf(stderr, "Example:\n");
+	fprintf(stderr, "  ./set-exec -fi -- ./inc -i < script-exec.inc\n");
+}
+
+int main(const int argc, char *const argv[], char *const *const envp)
+{
+	int opt;
+	char *cmd = NULL;
+	char *script_name = NULL;
+	bool interpret_stdin = false;
+	FILE *script_file = NULL;
+	int secbits;
+	bool deny_interactive, restrict_file;
+	size_t arg_nb;
+
+	secbits = prctl(PR_GET_SECUREBITS);
+	if (secbits == -1) {
+		/*
+		 * This should never happen, except with a buggy seccomp
+		 * filter.
+		 */
+		perror("ERROR: Failed to get securebits");
+		return 1;
+	}
+
+	deny_interactive = !!(secbits & SECBIT_EXEC_DENY_INTERACTIVE);
+	restrict_file = !!(secbits & SECBIT_EXEC_RESTRICT_FILE);
+
+	while ((opt = getopt(argc, argv, "c:i")) != -1) {
+		switch (opt) {
+		case 'c':
+			if (cmd) {
+				fprintf(stderr, "ERROR: Command already set");
+				return 1;
+			}
+			cmd = optarg;
+			break;
+		case 'i':
+			interpret_stdin = true;
+			break;
+		default:
+			print_usage(argv[0]);
+			return 1;
+		}
+	}
+
+	/* Checks that only one argument is used, or read stdin. */
+	arg_nb = !!cmd + !!interpret_stdin;
+	if (arg_nb == 0 && argc == 2) {
+		script_name = argv[1];
+	} else if (arg_nb != 1) {
+		print_usage(argv[0]);
+		return 1;
+	}
+
+	if (cmd) {
+		/*
+		 * Other kind of interactive interpretations should be denied
+		 * as well (e.g. CLI arguments passing script snippets,
+		 * environment variables interpreted as script).  However, any
+		 * way to pass script files should only be restricted according
+		 * to restrict_file.
+		 */
+		if (deny_interactive) {
+			fprintf(stderr,
+				"ERROR: Interactive interpretation denied.\n");
+			return 1;
+		}
+
+		return interpret_buffer(cmd, strlen(cmd));
+	}
+
+	if (interpret_stdin && !script_name) {
+		script_file = stdin;
+		/*
+		 * As for any execve(2) call, this path may be logged by the
+		 * kernel.
+		 */
+		script_name = "/proc/self/fd/0";
+		/*
+		 * When stdin is used, it can point to a regular file or a
+		 * pipe.  Restrict stdin execution according to
+		 * SECBIT_EXEC_DENY_INTERACTIVE but always allow executable
+		 * files (which are not considered as interactive inputs).
+		 */
+		return interpret_stream(script_file, script_name, envp,
+					deny_interactive);
+	} else if (script_name && !interpret_stdin) {
+		/*
+		 * In this sample, we don't pass any argument to scripts, but
+		 * otherwise we would have to forge an argv with such
+		 * arguments.
+		 */
+		script_file = fopen(script_name, "r");
+		if (!script_file) {
+			perror("ERROR: Failed to open script");
+			return 1;
+		}
+		/*
+		 * Restricts file execution according to
+		 * SECBIT_EXEC_RESTRICT_FILE.
+		 */
+		return interpret_stream(script_file, script_name, envp,
+					restrict_file);
+	}
+
+	print_usage(argv[0]);
+	return 1;
+}
diff --git a/samples/check-exec/run-script-ask.sh b/samples/check-exec/run-script-ask.sh
new file mode 100755
index 000000000..8ef0fdc37
--- /dev/null
+++ b/samples/check-exec/run-script-ask.sh
@@ -0,0 +1,9 @@
+#!/usr/bin/env sh
+# SPDX-License-Identifier: BSD-3-Clause
+
+DIR="$(dirname -- "$0")"
+
+PATH="${PATH}:${DIR}"
+
+set -x
+"${DIR}/script-ask.inc"
diff --git a/samples/check-exec/script-ask.inc b/samples/check-exec/script-ask.inc
new file mode 100755
index 000000000..720a8e649
--- /dev/null
+++ b/samples/check-exec/script-ask.inc
@@ -0,0 +1,5 @@
+#!/usr/bin/env inc
+# SPDX-License-Identifier: BSD-3-Clause
+
+?
++
diff --git a/samples/check-exec/script-exec.inc b/samples/check-exec/script-exec.inc
new file mode 100755
index 000000000..3245cb9d8
--- /dev/null
+++ b/samples/check-exec/script-exec.inc
@@ -0,0 +1,4 @@
+#!/usr/bin/env inc
+# SPDX-License-Identifier: BSD-3-Clause
+
++
diff --git a/samples/check-exec/script-noexec.inc b/samples/check-exec/script-noexec.inc
new file mode 100644
index 000000000..3245cb9d8
--- /dev/null
+++ b/samples/check-exec/script-noexec.inc
@@ -0,0 +1,4 @@
+#!/usr/bin/env inc
+# SPDX-License-Identifier: BSD-3-Clause
+
++
diff --git a/samples/check-exec/set-exec.c b/samples/check-exec/set-exec.c
new file mode 100644
index 000000000..ba86a60a2
--- /dev/null
+++ b/samples/check-exec/set-exec.c
@@ -0,0 +1,85 @@
+// SPDX-License-Identifier: BSD-3-Clause
+/*
+ * Simple tool to set SECBIT_EXEC_RESTRICT_FILE, SECBIT_EXEC_DENY_INTERACTIVE,
+ * before executing a command.
+ *
+ * Copyright © 2024 Microsoft Corporation
+ */
+
+#define _GNU_SOURCE
+#define __SANE_USERSPACE_TYPES__
+#include <errno.h>
+#include <linux/prctl.h>
+#include <linux/securebits.h>
+#include <stdbool.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/prctl.h>
+#include <unistd.h>
+
+static void print_usage(const char *argv0)
+{
+	fprintf(stderr, "usage: %s -f|-i -- <cmd> [args]...\n\n", argv0);
+	fprintf(stderr, "Execute a command with\n");
+	fprintf(stderr, "- SECBIT_EXEC_RESTRICT_FILE set: -f\n");
+	fprintf(stderr, "- SECBIT_EXEC_DENY_INTERACTIVE set: -i\n");
+}
+
+int main(const int argc, char *const argv[], char *const *const envp)
+{
+	const char *cmd_path;
+	char *const *cmd_argv;
+	int opt, secbits_cur, secbits_new;
+	bool has_policy = false;
+
+	secbits_cur = prctl(PR_GET_SECUREBITS);
+	if (secbits_cur == -1) {
+		/*
+		 * This should never happen, except with a buggy seccomp
+		 * filter.
+		 */
+		perror("ERROR: Failed to get securebits");
+		return 1;
+	}
+
+	secbits_new = secbits_cur;
+	while ((opt = getopt(argc, argv, "fi")) != -1) {
+		switch (opt) {
+		case 'f':
+			secbits_new |= SECBIT_EXEC_RESTRICT_FILE |
+				       SECBIT_EXEC_RESTRICT_FILE_LOCKED;
+			has_policy = true;
+			break;
+		case 'i':
+			secbits_new |= SECBIT_EXEC_DENY_INTERACTIVE |
+				       SECBIT_EXEC_DENY_INTERACTIVE_LOCKED;
+			has_policy = true;
+			break;
+		default:
+			print_usage(argv[0]);
+			return 1;
+		}
+	}
+
+	if (!argv[optind] || !has_policy) {
+		print_usage(argv[0]);
+		return 1;
+	}
+
+	if (secbits_cur != secbits_new &&
+	    prctl(PR_SET_SECUREBITS, secbits_new)) {
+		perror("Failed to set secure bit(s).");
+		fprintf(stderr,
+			"Hint: The running kernel may not support this feature.\n");
+		return 1;
+	}
+
+	cmd_path = argv[optind];
+	cmd_argv = argv + optind;
+	fprintf(stderr, "Executing command...\n");
+	execvpe(cmd_path, cmd_argv, envp);
+	fprintf(stderr, "Failed to execute \"%s\": %s\n", cmd_path,
+		strerror(errno));
+	return 1;
+}