unblock signals in exec_usermodehelper

Linus,

Shuu Yamaguchi has found an interesting problem. When 8139too is managed by /sbin/hotplug, 8139too's kernel thread becomes unkillable. Attempts to close the interface cause ifconfig to get stuck in D state, etc...

This is because keventd blocks all signals except SIGCHLD.

keventd parents /sbin/hotplug, which parents ifconfig, which parents the 8139too kernel thread. The net result is that the 8139too kernel thread blocks SIGTERM via its current->blocked. SIGTERM is supposed to stop it.

Obviously, 8139too should be more defensive about its environment and I will address that in a separate patch.

However I suggest that the kernel should be running usermode processes in a more predictable environment, so this patch simply unblocks all signals and calls recalc_sigpending() just prior to doing the execve(). The rest of this patch is just the standard optimisation of caching `current' in a local variable.

This affects modprobe as well. Currently, modprobe is executed with basically a random current->blocked. It gets whatever its parent gives it, and its parent can be anyone who autoloads a module.

Generally, the recent proliferation of kernel threads and signal handlers in the kernel is causing us some trouble. I suggest that we soon move to a scheme where kernel threads are _always_ parented by keventd. This is very simple to do.

For example, I have a bad feeling that if user Fred has CAP_NET_ADMIN and he opens the 8139too device, the 8139too thread will run with uid=Fred. If user Bill also has CAP_NET_ADMIN, Bill won't be able to close the device if he can't send SIGTERM to Fred's process.

For another example, I suspect that if a process opens the 8139too device and then, for some other reason does a waitpid(), and then someone closes the device, the original opener will receive notification of the exit of a child which it didn't know it had.

@@ -11,6 +11,9 @@
 	Limit the concurrent number of kmod modprobes to catch loops from
 	"modprobe needs a service that is in a module".
 	Keith Owens <[email protected]> December 1999
+
+	Unblock all signals when we exec a usermode process.
+	Shuu Yamaguchi <[email protected]> December 2000
 */
 
 #define __KERNEL_SYSCALLS__

@@ -83,9 +86,10 @@
 int exec_usermodehelper(char *program_path, char *argv[], char *envp[])
 {
 	int i;
+	struct task_struct *curtask = current;
 
-	current->session = 1;
-	current->pgrp = 1;
+	curtask->session = 1;
+	curtask->pgrp = 1;
 
 	use_init_fs_context();
 

@@ -95,19 +99,21 @@
 	   as the super user right after the execve fails if you time
 	   the signal just right.
 	*/
-	spin_lock_irq(&current->sigmask_lock);
-	flush_signals(current);
-	flush_signal_handlers(current);
-	spin_unlock_irq(&current->sigmask_lock);
+	spin_lock_irq(&curtask->sigmask_lock);
+	sigemptyset(&curtask->blocked);
+	flush_signals(curtask);
+	flush_signal_handlers(curtask);
+	recalc_sigpending(curtask);
+	spin_unlock_irq(&curtask->sigmask_lock);
 
-	for (i = 0; i < current->files->max_fds; i++ ) {
-		if (current->files->fd[i]) close(i);
+	for (i = 0; i < curtask->files->max_fds; i++ ) {
+		if (curtask->files->fd[i]) close(i);
 	}
 
 	/* Drop the "current user" thing */
 	{
-		struct user_struct *user = current->user;
-		current->user = INIT_USER;
+		struct user_struct *user = curtask->user;
+		curtask->user = INIT_USER;
 		atomic_inc(&INIT_USER->__count);
 		atomic_inc(&INIT_USER->processes);
 		atomic_dec(&user->processes);

@@ -115,9 +121,9 @@
 	}
 
 	/* Give kmod all effective privileges.. */
-	current->euid = current->fsuid = 0;
-	current->egid = current->fsgid = 0;
-	cap_set_full(current->cap_effective);
+	curtask->euid = curtask->fsuid = 0;
+	curtask->egid = curtask->fsgid = 0;
+	cap_set_full(curtask->cap_effective);
 
 	/* Allow execve args to be in kernel space. */
 	set_fs(KERNEL_DS);

-
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