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SubjectRe: forkbombing Linux distributions
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
--- 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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fn;quoted-printable:Jacek =C5=81uczak
n;quoted-printable:=C5=81uczak;Jacek
adr:;;Prof. Z. Szafrana 4a;Zielona Gora;;65-516;Poland
email;internet:difrost@pin.if.uz.zgora.pl
title:Linux Registered User # 337142
x-mozilla-html:FALSE
url:http://pin.if.uz.zgora.pl/~difrost
version:2.1
end:vcard

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