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Couple fork-bomb defense changes:

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- leave 5 processes for root-only use, the previous value of 1
  was unsufficient to execute additional commands once logged, and
  perhaps also not enough to actually login remotely with recent (open)sshd
- protect the log of "proc: table full" with ratecheck(), so that
  the message is only logged once per 10 seconds; though syslogd normally
  doesn't pass the repeated messages through, this avoids flooding
  syslogd and potentially also screen/logs
- If the process hits either system limit of number of processes in system,
  or user's limit of same, force the process to sleep for 0.5 seconds
  before returning failure. This turns 2000 rampaging fork monsters into
  2000 harmlessly snoozing fork monsters.
  The sleep is intentionally uninterruptible by signals.

These are not intended as ultimate protection agains fork-bombs.
Determined attacker can eat CPU differently than via repeating
fork() calls. But this is good enough to help protect against
programming mistakes or simple-minded tests.

Based on FreeBSD kern_fork.c change in revision 1.132 by
Mike Silbersack <silby at FreeBSD org>

Change also discussed on tech-kern@NetBSD.org, thread
'Fork bomb protection patch'.
This commit is contained in:
jdolecek 2002-12-05 16:24:46 +00:00
parent 72510c5d3b
commit 16e3e3cd55
1 changed files with 13 additions and 5 deletions
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18
sys/kern/kern_fork.c
View File

@ -1,4 +1,4 @@
/* $NetBSD: kern_fork.c,v 1.100 2002/11/30 11:20:51 manu Exp $ */
/* $NetBSD: kern_fork.c,v 1.101 2002/12/05 16:24:46 jdolecek Exp $ */
/*-
* Copyright (c) 1999, 2001 The NetBSD Foundation, Inc.
@ -78,7 +78,7 @@
*/
#include <sys/cdefs.h>
__KERNEL_RCSID(0, "$NetBSD: kern_fork.c,v 1.100 2002/11/30 11:20:51 manu Exp $");
__KERNEL_RCSID(0, "$NetBSD: kern_fork.c,v 1.101 2002/12/05 16:24:46 jdolecek Exp $");
#include "opt_ktrace.h"
#include "opt_systrace.h"
@ -189,6 +189,9 @@ sys___clone(struct proc *p, void *v, register_t *retval)
NULL, NULL, retval, NULL));
}
/* print the 'table full' message once per 10 seconds */
struct timeval fork_tfmrate = { 10, 0 };
int
fork1(struct proc *p1, int flags, int exitsig, void *stack, size_t stacksize,
void (*func)(void *), void *arg, register_t *retval,
@ -204,14 +207,18 @@ fork1(struct proc *p1, int flags, int exitsig, void *stack, size_t stacksize,
/*
* Although process entries are dynamically created, we still keep
* a global limit on the maximum number we will create. Don't allow
* a nonprivileged user to use the last process; don't let root
* a nonprivileged user to use the last few processes; don't let root
* exceed the limit. The variable nprocs is the current number of
* processes, maxproc is the limit.
*/
uid = p1->p_cred->p_ruid;
if (__predict_false((nprocs >= maxproc - 1 && uid != 0) ||
if (__predict_false((nprocs >= maxproc - 5 && uid != 0) ||
nprocs >= maxproc)) {
tablefull("proc", "increase kern.maxproc or NPROC");
static struct timeval lasttfm;
if (ratecheck(&lasttfm, &fork_tfmrate))
tablefull("proc", "increase kern.maxproc or NPROC");
(void)tsleep(&nprocs, PUSER, "forkmx", hz / 2);
return (EAGAIN);
}
nprocs++;
@ -225,6 +232,7 @@ fork1(struct proc *p1, int flags, int exitsig, void *stack, size_t stacksize,
p1->p_rlimit[RLIMIT_NPROC].rlim_cur)) {
(void)chgproccnt(uid, -1);
nprocs--;
(void)tsleep(&nprocs, PUSER, "forkulim", hz / 2);
return (EAGAIN);
}