Re: [PATCH 2/2] CRED: Fix __task_cred()'s lockdep check and banner comment

[David Howells <dhowells@redhat.com>]

Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp> wrote:

> I got below warning. Is this related to this patch?
> 
> [  140.173556] ===================================================
> [  140.215379] [ INFO: suspicious rcu_dereference_check() usage. ]
> [  140.216461] ---------------------------------------------------
> [  140.217530] kernel/signal.c:660 invoked rcu_dereference_check() without protection!

Yes.  The patch has uncovered a case of where we should be holding a lock, but
aren't.

Can you try the attached patch?

David
---
From: David Howells <dhowells@redhat.com>
Subject: [PATCH] CRED: Fix RCU warning due to previous patch fixing __task_cred()'s checks

A previous patch:

	commit 8f92054e7ca1d3a3ae50fb42d2253ac8730d9b2a
	Author: David Howells <dhowells@redhat.com>
	Date:   Thu Jul 29 12:45:55 2010 +0100
	Subject: CRED: Fix __task_cred()'s lockdep check and banner comment

fixed the lockdep checks on __task_cred().  This has shown up a place in the
signalling code where a lock should be held - namely that
check_kill_permission() requires its callers to hold the RCU lock.

It's may be that it would be better to add RCU read lock calls in
group_send_sig_info() only, around the call to check_kill_permission().  On the
other hand, some of the callers are either holding the RCU read lock already,
or have disabled interrupts, in which case, it's just extra overhead to do it
in g_s_s_i().

Without this patch, the following warning can occur:

[  140.173556] ===================================================
[  140.215379] [ INFO: suspicious rcu_dereference_check() usage. ]
[  140.216461] ---------------------------------------------------
[  140.217530] kernel/signal.c:660 invoked rcu_dereference_check() without protection!
[  140.218937] 
[  140.218938] other info that might help us debug this:
[  140.218939] 
[  140.220508] 
[  140.220509] rcu_scheduler_active = 1, debug_locks = 1
[  140.221991] 1 lock held by init/1:
[  140.222668]  #0:  (tasklist_lock){.+.+..}, at: [<c104a0ac>] kill_something_info+0x7c/0x160
[  140.224709] 
[  140.224711] stack backtrace:
[  140.225661] Pid: 1, comm: init Not tainted 2.6.35 #1
[  140.226576] Call Trace:
[  140.227111]  [<c103cca8>] ? printk+0x18/0x20
[  140.227908]  [<c1069884>] lockdep_rcu_dereference+0x94/0xb0
[  140.228931]  [<c104936a>] check_kill_permission+0x15a/0x170
[  140.229932]  [<c104a0ac>] ? kill_something_info+0x7c/0x160
[  140.230921]  [<c1049cca>] group_send_sig_info+0x1a/0x50
[  140.231866]  [<c1049d36>] __kill_pgrp_info+0x36/0x60
[  140.232780]  [<c104a0d0>] kill_something_info+0xa0/0x160
[  140.233740]  [<c10831c5>] ? __call_rcu+0xa5/0x110
[  140.234596]  [<c104b7ee>] sys_kill+0x5e/0x70
[  140.235387]  [<c10d1eee>] ? mntput_no_expire+0x1e/0xa0
[  140.236329]  [<c10bbd10>] ? __fput+0x170/0x220
[  140.257756]  [<c10bbdd9>] ? fput+0x19/0x20
[  140.258529]  [<c137ad94>] ? restore_all_notrace+0x0/0x18
[  140.259496]  [<c11bfb04>] ? trace_hardirqs_on_thunk+0xc/0x10
[  140.260531]  [<c137ad61>] syscall_call+0x7/0xb
[  144.627841] nfsd: last server has exited, flushing export cache
[  154.040420] Restarting system.
[  154.041061] machine restart

Reported-by: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Signed-off-by: David Howells <dhowells@redhat.com>
---

 kernel/exit.c   |    2 ++
 kernel/signal.c |    8 ++++++--
 2 files changed, 8 insertions(+), 2 deletions(-)


diff --git a/kernel/exit.c b/kernel/exit.c
index ceffc67..7858ebf 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -773,6 +773,7 @@ static void forget_original_parent(struct task_struct *father)
 
 	exit_ptrace(father);
 
+	rcu_read_lock();
 	write_lock_irq(&tasklist_lock);
 	reaper = find_new_reaper(father);
 
@@ -791,6 +792,7 @@ static void forget_original_parent(struct task_struct *father)
 		reparent_leader(father, p, &dead_children);
 	}
 	write_unlock_irq(&tasklist_lock);
+	rcu_read_unlock();
 
 	BUG_ON(!list_empty(&father->children));
 
diff --git a/kernel/signal.c b/kernel/signal.c
index 906ae5a..f771156 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -637,7 +637,7 @@ static inline bool si_fromuser(const struct siginfo *info)
 
 /*
  * Bad permissions for sending the signal
- * - the caller must hold at least the RCU read lock
+ * - the caller must hold the RCU read lock
  */
 static int check_kill_permission(int sig, struct siginfo *info,
 				 struct task_struct *t)
@@ -1127,7 +1127,7 @@ struct sighand_struct *lock_task_sighand(struct task_struct *tsk, unsigned long
 
 /*
  * send signal info to all the members of a group
- * - the caller must hold the RCU read lock at least
+ * - the caller must hold the RCU read lock
  */
 int group_send_sig_info(int sig, struct siginfo *info, struct task_struct *p)
 {
@@ -1151,11 +1151,13 @@ int __kill_pgrp_info(int sig, struct siginfo *info, struct pid *pgrp)
 
 	success = 0;
 	retval = -ESRCH;
+	rcu_read_lock();
 	do_each_pid_task(pgrp, PIDTYPE_PGID, p) {
 		int err = group_send_sig_info(sig, info, p);
 		success |= !err;
 		retval = err;
 	} while_each_pid_task(pgrp, PIDTYPE_PGID, p);
+	rcu_read_unlock();
 	return success ? 0 : retval;
 }
 
@@ -1261,6 +1263,7 @@ static int kill_something_info(int sig, struct siginfo *info, pid_t pid)
 		int retval = 0, count = 0;
 		struct task_struct * p;
 
+		rcu_read_lock();
 		for_each_process(p) {
 			if (task_pid_vnr(p) > 1 &&
 					!same_thread_group(p, current)) {
@@ -1270,6 +1273,7 @@ static int kill_something_info(int sig, struct siginfo *info, pid_t pid)
 					retval = err;
 			}
 		}
+		rcu_read_unlock();
 		ret = count ? retval : -ESRCH;
 	}
 	read_unlock(&tasklist_lock);