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From: David Howells on 4 Aug 2010 12:10
A previous patch:

commit 8f92054e7ca1d3a3ae50fb42d2253ac8730d9b2a
Author: David Howells <dhowells(a)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.

Fix group_send_sig_info() to get the RCU read lock around its call to
check_kill_permission().

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(a)i-love.sakura.ne.jp>
Signed-off-by: David Howells <dhowells(a)redhat.com>
Acked-by: Oleg Nesterov <oleg(a)redhat.com>
---

kernel/signal.c | 9 ++++++---
1 files changed, 6 insertions(+), 3 deletions(-)

diff --git a/kernel/signal.c b/kernel/signal.c
index 906ae5a..bded651 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)
@@ -1127,11 +1127,14 @@ 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
*/
int group_send_sig_info(int sig, struct siginfo *info, struct task_struct *p)
{
- int ret = check_kill_permission(sig, info, p);
+ int ret;
+
+ rcu_read_lock();
+ ret = check_kill_permission(sig, info, p);
+ rcu_read_unlock();

if (!ret && sig)
ret = do_send_sig_info(sig, info, p, true);

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