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authorOleg Nesterov <oleg@redhat.com>2010-05-26 14:43:03 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2010-05-27 09:12:45 -0700
commit7d64224217b53b23243fd2aa428001486f2a5da5 (patch)
tree23219e8d6d81e891fe2571e10be6feab7f81cd49
parent363da4022c2753bf4b4b54a1bb9e4527fdcb9d5d (diff)
downloadlinux-7d64224217b53b23243fd2aa428001486f2a5da5.tar.bz2
wait_for_helper: SIGCHLD from user-space can lead to use-after-free
1. wait_for_helper() calls allow_signal(SIGCHLD) to ensure the child can't autoreap itself. However, this means that a spurious SIGCHILD from user-space can set TIF_SIGPENDING and: - kernel_thread() or sys_wait4() can fail due to signal_pending() - worse, wait4() can fail before ____call_usermodehelper() execs or exits. In this case the caller may kfree(subprocess_info) while the child still uses this memory. Change the code to use SIG_DFL instead of magic "(void __user *)2" set by allow_signal(). This means that SIGCHLD won't be delivered, yet the child won't autoreap itsefl. The problem is minor, only root can send a signal to this kthread. 2. If sys_wait4(&ret) fails it doesn't populate "ret", in this case wait_for_helper() reports a random value from uninitialized var. With this patch sys_wait4() should never fail, but still it makes sense to initialize ret = -ECHILD so that the caller can notice the problem. Signed-off-by: Oleg Nesterov <oleg@redhat.com> Acked-by: Neil Horman <nhorman@tuxdriver.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
-rw-r--r--kernel/kmod.c10
1 files changed, 5 insertions, 5 deletions
diff --git a/kernel/kmod.c b/kernel/kmod.c
index 7efba6f8e3ec..48daed4c51ba 100644
--- a/kernel/kmod.c
+++ b/kernel/kmod.c
@@ -175,16 +175,16 @@ static int wait_for_helper(void *data)
struct subprocess_info *sub_info = data;
pid_t pid;
- /* Install a handler: if SIGCLD isn't handled sys_wait4 won't
- * populate the status, but will return -ECHILD. */
- allow_signal(SIGCHLD);
+ /* If SIGCLD is ignored sys_wait4 won't populate the status. */
+ spin_lock_irq(&current->sighand->siglock);
+ current->sighand->action[SIGCHLD-1].sa.sa_handler = SIG_DFL;
+ spin_unlock_irq(&current->sighand->siglock);
pid = kernel_thread(____call_usermodehelper, sub_info, SIGCHLD);
if (pid < 0) {
sub_info->retval = pid;
} else {
- int ret;
-
+ int ret = -ECHILD;
/*
* Normally it is bogus to call wait4() from in-kernel because
* wait4() wants to write the exit code to a userspace address.