diff options
authorOleg Nesterov <oleg@redhat.com>2009-08-26 14:29:24 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2009-08-26 20:06:52 -0700
commit4ab6c08336535f8c8e42cf45d7adeda882eff06e (patch)
parent03ef83af528899aa339e42d8024b37e2f434fba4 (diff)
clone(): fix race between copy_process() and de_thread()
Spotted by Hiroshi Shimamoto who also provided the test-case below. copy_process() uses signal->count as a reference counter, but it is not. This test case #include <sys/types.h> #include <sys/wait.h> #include <unistd.h> #include <stdio.h> #include <errno.h> #include <pthread.h> void *null_thread(void *p) { for (;;) sleep(1); return NULL; } void *exec_thread(void *p) { execl("/bin/true", "/bin/true", NULL); return null_thread(p); } int main(int argc, char **argv) { for (;;) { pid_t pid; int ret, status; pid = fork(); if (pid < 0) break; if (!pid) { pthread_t tid; pthread_create(&tid, NULL, exec_thread, NULL); for (;;) pthread_create(&tid, NULL, null_thread, NULL); } do { ret = waitpid(pid, &status, 0); } while (ret == -1 && errno == EINTR); } return 0; } quickly creates an unkillable task. If copy_process(CLONE_THREAD) races with de_thread() copy_signal()->atomic(signal->count) breaks the signal->notify_count logic, and the execing thread can hang forever in kernel space. Change copy_process() to increment count/live only when we know for sure we can't fail. In this case the forked thread will take care of its reference to signal correctly. If copy_process() fails, check CLONE_THREAD flag. If it it set - do nothing, the counters were not changed and current belongs to the same thread group. If it is not set, ->signal must be released in any case (and ->count must be == 1), the forked child is the only thread in the thread group. We need more cleanups here, in particular signal->count should not be used by de_thread/__exit_signal at all. This patch only fixes the bug. Reported-by: Hiroshi Shimamoto <h-shimamoto@ct.jp.nec.com> Tested-by: Hiroshi Shimamoto <h-shimamoto@ct.jp.nec.com> Signed-off-by: Oleg Nesterov <oleg@redhat.com> Acked-by: Roland McGrath <roland@redhat.com> Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> Cc: <stable@kernel.org> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
1 files changed, 5 insertions, 15 deletions
diff --git a/kernel/fork.c b/kernel/fork.c
index 144326b7af5..e6c04d462ab 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -815,11 +815,8 @@ static int copy_signal(unsigned long clone_flags, struct task_struct *tsk)
struct signal_struct *sig;
- if (clone_flags & CLONE_THREAD) {
- atomic_inc(&current->signal->count);
- atomic_inc(&current->signal->live);
+ if (clone_flags & CLONE_THREAD)
return 0;
- }
sig = kmem_cache_alloc(signal_cachep, GFP_KERNEL);
tsk->signal = sig;
@@ -877,16 +874,6 @@ void __cleanup_signal(struct signal_struct *sig)
kmem_cache_free(signal_cachep, sig);
-static void cleanup_signal(struct task_struct *tsk)
- struct signal_struct *sig = tsk->signal;
- atomic_dec(&sig->live);
- if (atomic_dec_and_test(&sig->count))
- __cleanup_signal(sig);
static void copy_flags(unsigned long clone_flags, struct task_struct *p)
unsigned long new_flags = p->flags;
@@ -1239,6 +1226,8 @@ static struct task_struct *copy_process(unsigned long clone_flags,
if (clone_flags & CLONE_THREAD) {
+ atomic_inc(&current->signal->count);
+ atomic_inc(&current->signal->live);
p->group_leader = current->group_leader;
list_add_tail_rcu(&p->thread_group, &p->group_leader->thread_group);
@@ -1282,7 +1271,8 @@ bad_fork_cleanup_mm:
if (p->mm)
- cleanup_signal(p);
+ if (!(clone_flags & CLONE_THREAD))
+ __cleanup_signal(p->signal);