path: root/kernel
diff options
authorPaul Jackson <pj@sgi.com>2006-09-29 02:01:48 -0700
committerLinus Torvalds <torvalds@g5.osdl.org>2006-09-29 09:18:25 -0700
commit181b64803661209cda64e5e874ad75f373a69de8 (patch)
tree0e0ee378b98e5ee72aeae428b1f155d031fe2597 /kernel
parent683e91cbd0582cb8e63daaf0429e0a62be9cc421 (diff)
[PATCH] cpuset: fix obscure attach_task vs exiting race
Fix obscure race condition in kernel/cpuset.c attach_task() code. There is basically zero chance of anyone accidentally being harmed by this race. It requires a special 'micro-stress' load and a special timing loop hacks in the kernel to hit in less than an hour, and even then you'd have to hit it hundreds or thousands of times, followed by some unusual and senseless cpuset configuration requests, including removing the top cpuset, to cause any visibly harm affects. One could, with perhaps a few days or weeks of such effort, get the reference count on the top cpuset below zero, and manage to crash the kernel by asking to remove the top cpuset. I found it by code inspection. The race was introduced when 'the_top_cpuset_hack' was introduced, and one piece of code was not updated. An old check for a possibly null task cpuset pointer needed to be changed to a check for a task marked PF_EXITING. The pointer can't be null anymore, thanks to the_top_cpuset_hack (documented in kernel/cpuset.c). But the task could have gone into PF_EXITING state after it was found in the task_list scan. If a task is PF_EXITING in this code, it is possible that its task->cpuset pointer is pointing to the top cpuset due to the_top_cpuset_hack, rather than because the top_cpuset was that tasks last valid cpuset. In that case, the wrong cpuset reference counter would be decremented. The fix is trivial. Instead of failing the system call if the tasks cpuset pointer is null here, fail it if the task is in PF_EXITING state. The code for 'the_top_cpuset_hack' that changes an exiting tasks cpuset to the top_cpuset is done without locking, so could happen at anytime. But it is done during the exit handling, after the PF_EXITING flag is set. So if we verify that a task is still not PF_EXITING after we copy out its cpuset pointer (into 'oldcs', below), we know that 'oldcs' is not one of these hack references to the top_cpuset. Signed-off-by: Paul Jackson <pj@sgi.com> Signed-off-by: Andrew Morton <akpm@osdl.org> Signed-off-by: Linus Torvalds <torvalds@osdl.org>
Diffstat (limited to 'kernel')
1 files changed, 6 insertions, 1 deletions
diff --git a/kernel/cpuset.c b/kernel/cpuset.c
index cc0395d7eba1..8c3c400cce91 100644
--- a/kernel/cpuset.c
+++ b/kernel/cpuset.c
@@ -1225,7 +1225,12 @@ static int attach_task(struct cpuset *cs, char *pidbuf, char **ppathbuf)
oldcs = tsk->cpuset;
- if (!oldcs) {
+ /*
+ * After getting 'oldcs' cpuset ptr, be sure still not exiting.
+ * If 'oldcs' might be the top_cpuset due to the_top_cpuset_hack
+ * then fail this attach_task(), to avoid breaking top_cpuset.count.
+ */
+ if (tsk->flags & PF_EXITING) {

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