Re: forkbombing Linux distributions

["Jacek Łuczak" <difrost@pin.if.uz.zgora.pl>]

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Hi

I made some tests and almost all Linux distros brings down while freebsd 
survive!Forkbombing is a big problem but i don't think that something like

max_threads = mempages / (16 * THREAD_SIZE / PAGE_SIZE);

is good solution!!!
How about add max_user_threads to the kernel? It could be tunable via 
proc filesystem. Limit is set only for users.
I made a fast:) patch - see below - and test it on 2.6.11, 
2.6.11ac4,2.6.12rc1...works great!!!New forks are stoped in 
copy_process() before dup_task_struct() and EAGAIN is returned. System 
works without any problems and root can killall -9 forkbomb.

Regards,
	Jacek Luczak

[-- Attachment #2: user_threads_limit.patch --]
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--- linux-2.6.12-rc1/kernel/fork.c	2005-03-29 00:53:37.000000000 +0200
+++ linux/kernel/fork.c	2005-03-29 00:54:19.000000000 +0200
@@ -57,6 +57,8 @@
 
 int max_threads;		/* tunable limit on nr_threads */
 
+int max_user_threads;		/* tunable limit on nr_threads per user */
+
 DEFINE_PER_CPU(unsigned long, process_counts) = 0;
 
  __cacheline_aligned DEFINE_RWLOCK(tasklist_lock);  /* outer */
@@ -146,6 +148,21 @@
 	if(max_threads < 20)
 		max_threads = 20;
 
+	/*
+	 * The default maximum number of threads per user.
+	 * 
+	 * FIXME: this value is based on my experiments and is 
+	 * rather good on desktop system; it should be fixed to
+	 * the more universal value.
+	 */
+	max_user_threads = 300;
+	
+	/*
+	 * default value is too high - set to max_threads 
+	 */
+	if (max_threads < max_user_threads)
+		max_user_threads = max_threads;
+	
 	init_task.signal->rlim[RLIMIT_NPROC].rlim_cur = max_threads/2;
 	init_task.signal->rlim[RLIMIT_NPROC].rlim_max = max_threads/2;
 	init_task.signal->rlim[RLIMIT_SIGPENDING] =
@@ -179,6 +196,16 @@
 	return tsk;
 }
 
+/*
+ * This is used to get number of user processes
+ * from current running task.
+ */
+static inline int get_user_processes(void)
+{
+	return atomic_read(&current->user->processes);
+}
+#define user_nr_processes get_user_processes()
+
 #ifdef CONFIG_MMU
 static inline int dup_mmap(struct mm_struct * mm, struct mm_struct * oldmm)
 {
@@ -869,6 +896,13 @@
 		goto fork_out;
 
 	retval = -ENOMEM;
+	
+	/*
+	 * Stop creation of new user process if limit is reached.
+	 */
+	if ( (current->user != &root_user) && (user_nr_processes >= max_user_threads) )
+		goto max_user_fork;
+	
 	p = dup_task_struct(current);
 	if (!p)
 		goto fork_out;
@@ -1109,6 +1143,9 @@
 		return ERR_PTR(retval);
 	return p;
 
+max_user_fork:
+	retval = -EAGAIN;
+	return ERR_PTR(retval);
 bad_fork_cleanup_namespace:
 	exit_namespace(p);
 bad_fork_cleanup_keys:
--- linux-2.6.12-rc1/kernel/sysctl.c	2005-03-29 00:53:38.000000000 +0200
+++ linux/kernel/sysctl.c	2005-03-29 00:54:19.000000000 +0200
@@ -56,6 +56,7 @@
 extern int sysctl_overcommit_memory;
 extern int sysctl_overcommit_ratio;
 extern int max_threads;
+extern int max_user_threads;
 extern int sysrq_enabled;
 extern int core_uses_pid;
 extern char core_pattern[];
@@ -642,6 +643,14 @@
 		.mode		= 0644,
 		.proc_handler	= &proc_dointvec,
 	},
+	{
+		.ctl_name	= KERN_MAX_USER_THREADS,
+		.procname	= "user_threads_max",
+		.data		= &max_user_threads,
+		.maxlen		= sizeof(int),
+		.mode		= 0644,
+		.proc_handler	= &proc_dointvec,
+	},
 
 	{ .ctl_name = 0 }
 };
--- linux-2.6.12-rc1/include/linux/sysctl.h	2005-03-29 00:54:06.000000000 +0200
+++ linux/include/linux/sysctl.h	2005-03-29 00:54:36.000000000 +0200
@@ -136,6 +136,7 @@
 	KERN_UNKNOWN_NMI_PANIC=66, /* int: unknown nmi panic flag */
 	KERN_BOOTLOADER_TYPE=67, /* int: boot loader type */
 	KERN_RANDOMIZE=68, /* int: randomize virtual address space */
+	KERN_MAX_USER_THREADS=69,	/* int: Maximum nr of threads per user in the system */
 };
 
 

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