ptrace: make ptrace() fail if the tracee changed its pid unexpectedly

From: Oleg Nesterov <[email protected]>

[ Upstream commit dbb5afad100a828c97e012c6106566d99f041db6 ]

Suppose we have 2 threads, the group-leader L and a sub-theread T, both parked in ptrace_stop(). Debugger tries to resume both threads and does

	ptrace(PTRACE_CONT, T);
	ptrace(PTRACE_CONT, L);

If the sub-thread T execs in between, the 2nd PTRACE_CONT doesn not resume the old leader L, it resumes the post-exec thread T which was actually now stopped in PTHREAD_EVENT_EXEC. In this case the PTHREAD_EVENT_EXEC event is lost, and the tracer can't know that the tracee changed its pid.

This patch makes ptrace() fail in this case until debugger does wait() and consumes PTHREAD_EVENT_EXEC which reports old_pid. This affects all ptrace requests except the "asynchronous" PTRACE_INTERRUPT/KILL.

The patch doesn't add the new PTRACE_ option to not complicate the API, and I _hope_ this won't cause any noticeable regression:

- If debugger uses PTRACE_O_TRACEEXEC and the thread did an exec and the tracer does a ptrace request without having consumed the exec event, it's 100% sure that the thread the ptracer thinks it is targeting does not exist anymore, or isn't the same as the one it thinks it is targeting.

- To some degree this patch adds nothing new. In the scenario above ptrace(L) can fail with -ESRCH if it is called after the execing sub-thread wakes the leader up and before it "steals" the leader's pid.

Test-case:

	#include <stdio.h>
	#include <unistd.h>
	#include <signal.h>
	#include <sys/ptrace.h>
	#include <sys/wait.h>
	#include <errno.h>
	#include <pthread.h>
	#include <assert.h>

	void *tf(void *arg)
	{
		execve("/usr/bin/true", NULL, NULL);
		assert(0);

		return NULL;
	}

	int main(void)
	{
		int leader = fork();
		if (!leader) {
			kill(getpid(), SIGSTOP);

			pthread_t th;
			pthread_create(&th, NULL, tf, NULL);
			for (;;)
				pause();

			return 0;
		}

		waitpid(leader, NULL, WSTOPPED);

		ptrace(PTRACE_SEIZE, leader, 0,
				PTRACE_O_TRACECLONE | PTRACE_O_TRACEEXEC);
		waitpid(leader, NULL, 0);

		ptrace(PTRACE_CONT, leader, 0,0);
		waitpid(leader, NULL, 0);

		int status, thread = waitpid(-1, &status, 0);
		assert(thread > 0 && thread != leader);
		assert(status == 0x80137f);

		ptrace(PTRACE_CONT, thread, 0,0);
		/*
		 * waitid() because waitpid(leader, &status, WNOWAIT) does not
		 * report status. Why ????
		 *
		 * Why WEXITED? because we have another kernel problem connected
		 * to mt-exec.
		 */
		siginfo_t info;
		assert(waitid(P_PID, leader, &info, WSTOPPED|WEXITED|WNOWAIT) == 0);
		assert(info.si_pid == leader && info.si_status == 0x0405);

		/* OK, it sleeps in ptrace(PTRACE_EVENT_EXEC == 0x04) */
		assert(ptrace(PTRACE_CONT, leader, 0,0) == -1);
		assert(errno == ESRCH);

		assert(leader == waitpid(leader, &status, WNOHANG));
		assert(status == 0x04057f);

		assert(ptrace(PTRACE_CONT, leader, 0,0) == 0);

		return 0;
	}

---
 kernel/ptrace.c | 18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

index 79de1294f8eb..eb4d04cb3aaf 100644
@@ -169,6 +169,21 @@ void __ptrace_unlink(struct task_struct *child)
 	spin_unlock(&child->sighand->siglock);
 }
 
+static bool looks_like_a_spurious_pid(struct task_struct *task)
+{
+	if (task->exit_code != ((PTRACE_EVENT_EXEC << 8) | SIGTRAP))
+		return false;
+
+	if (task_pid_vnr(task) == task->ptrace_message)
+		return false;
+	/*
+	 * The tracee changed its pid but the PTRACE_EVENT_EXEC event
+	 * was not wait()'ed, most probably debugger targets the old
+	 * leader which was destroyed in de_thread().
+	 */
+	return true;
+}
+
 /* Ensure that nothing can wake it up, even SIGKILL */
 static bool ptrace_freeze_traced(struct task_struct *task)
 {

@@ -179,7 +194,8 @@ static bool ptrace_freeze_traced(struct task_struct *task)
 		return ret;
 
 	spin_lock_irq(&task->sighand->siglock);
-	if (task_is_traced(task) && !__fatal_signal_pending(task)) {
+	if (task_is_traced(task) && !looks_like_a_spurious_pid(task) &&
+	    !__fatal_signal_pending(task)) {
 		task->state = __TASK_TRACED;
 		ret = true;
 	}

2.30.2

From: Daniel Wagner <[email protected]>

[ Upstream commit 85428beac80dbcace5b146b218697c73e367dcf5 ]

Reset the ns->file value to NULL also in the error case in nvmet_file_ns_enable().

The ns->file variable points either to file object or contains the error code after the filp_open() call. This can lead to following problem:

When the user first setups an invalid file backend and tries to enable the ns, it will fail. Then the user switches over to a bdev backend and enables successfully the ns. The first received I/O will crash the system because the IO backend is chosen based on the ns->file value:

static u16 nvmet_parse_io_cmd(struct nvmet_req *req)

{
	[...]

	if (req->ns->file)
		return nvmet_file_parse_io_cmd(req);

	return nvmet_bdev_parse_io_cmd(req);
}

---
 drivers/nvme/target/io-cmd-file.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

index 05453f5d1448..6ca17a0babae 100644
@@ -38,9 +38,11 @@ int nvmet_file_ns_enable(struct nvmet_ns *ns)
 
 	ns->file = filp_open(ns->device_path, flags, 0);
 	if (IS_ERR(ns->file)) {
-		pr_err("failed to open file %s: (%ld)\n",
-				ns->device_path, PTR_ERR(ns->file));
-		return PTR_ERR(ns->file);
+		ret = PTR_ERR(ns->file);
+		pr_err("failed to open file %s: (%d)\n",
+			ns->device_path, ret);
+		ns->file = NULL;
+		return ret;
 	}
 
 	ret = vfs_getattr(&ns->file->f_path,

2.30.2

Sasha,

I think the patch subject line is being worked on since it needs to be reset and not seset.

Not sure how we can go about fixing that.

Hi Chaitanya,

This ship has sailed, as the commit already hit mainline. Fixing the typo in the back ports is surely possible but I assume it's better not do change the subject line. seset forever!

Thanks,
Daniel