This is a note to let you know that I've just added the patch titled ptrace: make ptrace() fail if the tracee changed its pid unexpectedly to the 4.4-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary The filename of the patch is: ptrace-make-ptrace-fail-if-the-tracee-changed-its-pi.patch and it can be found in the queue-4.4 subdirectory. If you, or anyone else, feels it should not be added to the stable tree, please let <stable@xxxxxxxxxxxxxxx> know about it. commit 3667f03d0cf74926d5ca45d1ba6dc8100dd5e773 Author: Oleg Nesterov <oleg@xxxxxxxxxx> Date: Wed May 12 15:33:08 2021 +0200 ptrace: make ptrace() fail if the tracee changed its pid unexpectedly [ 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; } 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> diff --git a/kernel/ptrace.c b/kernel/ptrace.c index da8c358930fb..5a1d8cc7ef4e 100644 --- a/kernel/ptrace.c +++ b/kernel/ptrace.c @@ -129,6 +129,21 @@ 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) { @@ -139,7 +154,8 @@ 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; }