From: Oleg Nesterov oleg@redhat.com

[ 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@redhat.com Reported-by: Simon Marchi simon.marchi@efficios.com Acked-by: "Eric W. Biederman" ebiederm@xmission.com Acked-by: Pedro Alves palves@redhat.com Acked-by: Simon Marchi simon.marchi@efficios.com Acked-by: Jan Kratochvil jan.kratochvil@redhat.com Signed-off-by: Linus Torvalds torvalds@linux-foundation.org Signed-off-by: Sasha Levin sashal@kernel.org --- kernel/ptrace.c | 18 +++++++++++++++++- 1 file changed, 17 insertions(+), 1 deletion(-)

diff --git a/kernel/ptrace.c b/kernel/ptrace.c index ea3370e205fb..4f10223bc7b0 100644 --- a/kernel/ptrace.c +++ b/kernel/ptrace.c @@ -159,6 +159,21 @@ 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) { @@ -169,7 +184,8 @@ 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; }