[PATCH 5.12 029/127] ptrace: make ptrace() fail if the tracee changed its pid unexpectedly

From: Greg Kroah-Hartman
Date: Mon May 24 2021 - 12:02:38 EST

From: Oleg Nesterov <oleg@xxxxxxxxxx>

[ 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.


#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);

return NULL;

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

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

return 0;

waitpid(leader, NULL, WSTOPPED);

ptrace(PTRACE_SEIZE, leader, 0,
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;

Signed-off-by: Oleg Nesterov <oleg@xxxxxxxxxx>
Reported-by: Simon Marchi <simon.marchi@xxxxxxxxxxxx>
Acked-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
Acked-by: Pedro Alves <palves@xxxxxxxxxx>
Acked-by: Simon Marchi <simon.marchi@xxxxxxxxxxxx>
Acked-by: Jan Kratochvil <jan.kratochvil@xxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
kernel/ptrace.c | 18 +++++++++++++++++-
1 file changed, 17 insertions(+), 1 deletion(-)

diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index 61db50f7ca86..5f50fdd1d855 100644
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -169,6 +169,21 @@ void __ptrace_unlink(struct task_struct *child)

+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;

- 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;