Patch "pidns: disable pid allocation if pid_ns_prepare_proc() is failed in alloc_pid()" has been added to the 3.18-stable tree

This is a note to let you know that I've just added the patch titled

pidns: disable pid allocation if pid_ns_prepare_proc() is failed in alloc_pid()

to the 3.18-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
pidns-disable-pid-allocation-if-pid_ns_prepare_proc-is-failed-in-alloc_pid.patch
and it can be found in the queue-3.18 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Kirill Tkhai <[email protected]>
Date: Mon, 8 May 2017 15:56:34 -0700
Subject: pidns: disable pid allocation if pid_ns_prepare_proc() is failed in alloc_pid()

From: Kirill Tkhai <[email protected]>

[ Upstream commit 8896c23d2ef803f1883fea73117a435925c2b4c4 ]

alloc_pidmap() advances pid_namespace::last_pid. When first pid allocation fails, then next created process will have pid 2 and pid_ns_prepare_proc() won't be called. So, pid_namespace::proc_mnt will never be initialized (not to mention that there won't be a child reaper).

I saw crash stack of such case on kernel 3.10:

    BUG: unable to handle kernel NULL pointer dereference at (null)
    IP: proc_flush_task+0x8f/0x1b0
    Call Trace:
        release_task+0x3f/0x490
        wait_consider_task.part.10+0x7ff/0xb00
        do_wait+0x11f/0x280
        SyS_wait4+0x7d/0x110

We may fix this by restore of last_pid in 0 or by prohibiting of futher allocations. Since there was a similar issue in Oleg Nesterov's commit 314a8ad0f18a ("pidns: fix free_pid() to handle the first fork failure"). and it was fixed via prohibiting allocation, let's follow this way, and do the same.

---
 kernel/pid.c |    4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

@@ -316,8 +316,10 @@ struct pid *alloc_pid(struct pid_namespa
 	}
 
 	if (unlikely(is_child_reaper(pid))) {
-		if (pid_ns_prepare_proc(ns))
+		if (pid_ns_prepare_proc(ns)) {
+			disable_pid_allocation(ns);
 			goto out_free;
+		}
 	}
 
 	get_pid_ns(ns);

Patches currently in stable-queue which might be from [email protected] are

queue-3.18/pidns-disable-pid-allocation-if-pid_ns_prepare_proc-is-failed-in-alloc_pid.patch