From 9926e4c74300c4b31dee007298c6475d33369df0 Mon Sep 17 00:00:00 2001 From: Tom Alsberg Date: Tue, 8 May 2007 00:30:31 -0700 Subject: [PATCH] CPU time limit patch / setrlimit(RLIMIT_CPU, 0) cheat fix As discovered here today, the change in Kernel 2.6.17 intended to inhibit users from setting RLIMIT_CPU to 0 (as that is equivalent to unlimited) by "cheating" and setting it to 1 in such a case, does not make a difference, as the check is done in the wrong place (too late), and only applies to the profiling code. On all systems I checked running kernels above 2.6.17, no matter what the hard and soft CPU time limits were before, a user could escape them by issuing in the shell (sh/bash/zsh) "ulimit -t 0", and then the user's process was not ever killed. Attached is a trivial patch to fix that. Simply moving the check to a slightly earlier location (specifically, before the line that actually assigns the limit - *old_rlim = new_rlim), does the trick. Do note that at least the zsh (but not ash, dash, or bash) shell has the problem of "caching" the limits set by the ulimit command, so when running zsh the fix will not immediately be evident - after entering "ulimit -t 0", "ulimit -a" will show "-t: cpu time (seconds) 0", even though the actual limit as returned by getrlimit(...) will be 1. It can be verified by opening a subshell (which will not have the values of the parent shell in cache) and checking in it, or just by running a CPU intensive command like "echo '65536^1048576' | bc" and verifying that it dumps core after one second. Regardless of whether that is a misfeature in the shell, perhaps it would be better to return -EINVAL from setrlimit in such a case instead of cheating and setting to 1, as that does not really reflect the actual state of the process anymore. I do not however know what the ground for that decision was in the original 2.6.17 change, and whether there would be any "backward" compatibility issues, so I preferred not to touch that right now. Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- kernel/sys.c | 19 ++++++++++--------- 1 file changed, 10 insertions(+), 9 deletions(-) diff --git a/kernel/sys.c b/kernel/sys.c index fe1f3ab204..926bf9d7ac 100644 --- a/kernel/sys.c +++ b/kernel/sys.c @@ -1923,6 +1923,16 @@ asmlinkage long sys_setrlimit(unsigned int resource, struct rlimit __user *rlim) if (retval) return retval; + if (resource == RLIMIT_CPU && new_rlim.rlim_cur == 0) { + /* + * The caller is asking for an immediate RLIMIT_CPU + * expiry. But we use the zero value to mean "it was + * never set". So let's cheat and make it one second + * instead + */ + new_rlim.rlim_cur = 1; + } + task_lock(current->group_leader); *old_rlim = new_rlim; task_unlock(current->group_leader); @@ -1944,15 +1954,6 @@ asmlinkage long sys_setrlimit(unsigned int resource, struct rlimit __user *rlim) unsigned long rlim_cur = new_rlim.rlim_cur; cputime_t cputime; - if (rlim_cur == 0) { - /* - * The caller is asking for an immediate RLIMIT_CPU - * expiry. But we use the zero value to mean "it was - * never set". So let's cheat and make it one second - * instead - */ - rlim_cur = 1; - } cputime = secs_to_cputime(rlim_cur); read_lock(&tasklist_lock); spin_lock_irq(¤t->sighand->siglock); -- 2.39.5