• Oleg Nesterov's avatar
    ptrace: fix race between ptrace_resume() and wait_task_stopped() · ddb56eac
    Oleg Nesterov authored
    commit b72c186999e689cb0b055ab1c7b3cd8fffbeb5ed upstream.
    
    ptrace_resume() is called when the tracee is still __TASK_TRACED.  We set
    tracee->exit_code and then wake_up_state() changes tracee->state.  If the
    tracer's sub-thread does wait() in between, task_stopped_code(ptrace => T)
    wrongly looks like another report from tracee.
    
    This confuses debugger, and since wait_task_stopped() clears ->exit_code
    the tracee can miss a signal.
    
    Test-case:
    
    	#include <stdio.h>
    	#include <unistd.h>
    	#include <sys/wait.h>
    	#include <sys/ptrace.h>
    	#include <pthread.h>
    	#include <assert.h>
    
    	int pid;
    
    	void *waiter(void *arg)
    	{
    		int stat;
    
    		for (;;) {
    			assert(pid == wait(&stat));
    			assert(WIFSTOPPED(stat));
    			if (WSTOPSIG(stat) == SIGHUP)
    				continue;
    
    			assert(WSTOPSIG(stat) == SIGCONT);
    			printf("ERR! extra/wrong report:%x\n", stat);
    		}
    	}
    
    	int main(void)
    	{
    		pthread_t thread;
    
    		pid = fork();
    		if (!pid) {
    			assert(ptrace(PTRACE_TRACEME, 0,0,0) == 0);
    			for (;;)
    				kill(getpid(), SIGHUP);
    		}
    
    		assert(pthread_create(&thread, NULL, waiter, NULL) == 0);
    
    		for (;;)
    			ptrace(PTRACE_CONT, pid, 0, SIGCONT);
    
    		return 0;
    	}
    
    Note for stable: the bug is very old, but without 9899d11f "ptrace:
    ensure arch_ptrace/ptrace_request can never race with SIGKILL" the fix
    should use lock_task_sighand(child).
    Signed-off-by: 's avatarOleg Nesterov <oleg@redhat.com>
    Reported-by: 's avatarPavel Labath <labath@google.com>
    Tested-by: 's avatarPavel Labath <labath@google.com>
    Signed-off-by: 's avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: 's avatarLinus Torvalds <torvalds@linux-foundation.org>
    Signed-off-by: 's avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
    ddb56eac
ptrace.c 30.2 KB