[PATCH AUTOSEL for 4.9 022/281] pidns: disable pid allocation if pid_ns_prepare_proc() is failed in alloc_pid()

[Sasha Levin]

From: Kirill Tkhai <ktkhai@xxxxxxxxxxxxx>

[ 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.

Link: http://lkml.kernel.org/r/149201021004.4863.6762095011554287922.stgit@xxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Kirill Tkhai <ktkhai@xxxxxxxxxxxxx>
Acked-by: Cyrill Gorcunov <gorcunov@xxxxxxxxxx>
Cc: Andrei Vagin <avagin@xxxxxxxxxxxxx>
Cc: Andreas Gruenbacher <agruenba@xxxxxxxxxx>
Cc: Kees Cook <keescook@xxxxxxxxxxxx>
Cc: Michael Kerrisk <mtk.manpages@xxxxxxxxxxxxxx>
Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
Cc: Paul Moore <paul@xxxxxxxxxxxxxx>
Cc: Eric Biederman <ebiederm@xxxxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Serge Hallyn <serge@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Sasha Levin <alexander.levin@xxxxxxxxxxxxx>
---
 kernel/pid.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/kernel/pid.c b/kernel/pid.c
index 693a64385d59..fa704f88ff8e 100644
--- a/kernel/pid.c
+++ b/kernel/pid.c
@@ -322,8 +322,10 @@ struct pid *alloc_pid(struct pid_namespace *ns)
 	}
 
 	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);
-- 
2.14.1