Messages in this thread Patch in this message | | | Date | Mon, 15 Mar 2010 20:48:24 +0100 | From | Oleg Nesterov <> | Subject | [PATCH 4/6] wait_for_helper: SIGCHLD from user-space can lead to use-after-free |
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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> ---
kernel/kmod.c | 10 +++++----- 1 file changed, 5 insertions(+), 5 deletions(-)
--- 34-rc1/kernel/kmod.c~4_SIGCHLD_RACE 2010-03-15 20:17:47.000000000 +0100 +++ 34-rc1/kernel/kmod.c 2010-03-15 20:28:31.000000000 +0100 @@ -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(¤t->sighand->siglock); + current->sighand->action[SIGCHLD-1].sa.sa_handler = SIG_DFL; + spin_unlock_irq(¤t->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.
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