linux-kernel - [PATCH 1/2] locktorture: Fix deboosting nil ptr dereferencing

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Message-Id: <1457507286-653-2-git-send-email-dave@stgolabs.net>
Date:	Tue,  8 Mar 2016 23:08:05 -0800
From:	Davidlohr Bueso <dave@...olabs.net>
To:	paulmck@...ux.vnet.ibm.com
Cc:	mingo@...nel.org, wangkefeng.wang@...wei.com, peterz@...radead.org,
	josh@...htriplett.org, guohanjun@...wei.com, dave@...olabs.net,
	linux-kernel@...r.kernel.org, Davidlohr Bueso <dbueso@...e.de>
Subject: [PATCH 1/2] locktorture: Fix deboosting nil ptr dereferencing

For the case of rtmutex torturing we will randomly call into the
boost() handler, including upon module exiting when the tasks are
deboosted before stopping. In such cases the task may or may not have
already been boosted, and therefore the NULL being explicitly passed
can occur anywhere. Currently we only assume that the task will is
at a higher prio, and in consequence, dereference a nil pointer.

This patch fixes the case of a rmmod locktorture exploding while
pounding on the rtmutex lock (partial trace):

[83317.452251] task: ffff88081026cf80 ti: ffff880816120000 task.ti: ffff880816120000
[83317.452258] RIP: 0010:[<ffffffffa05c6185>]  [<ffffffffa05c6185>] torture_random+0x5/0x60 [torture]
[83317.452261] RSP: 0018:ffff880816123eb0  EFLAGS: 00010206
[83317.452264] RAX: ffff88081026cf80 RBX: ffff880816bfa630 RCX: 0000000000160d1b
[83317.452267] RDX: 0000000000000000 RSI: 0000000000000202 RDI: 0000000000000000
[83317.452269] RBP: ffff88081026cf80 R08: 000000000000001f R09: ffff88017c20ca80
[83317.452271] R10: 0000000000000000 R11: 000000000048c316 R12: ffffffffa05d1840
[83317.452273] R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
[83317.452275] FS:  0000000000000000(0000) GS:ffff88203f880000(0000) knlGS:0000000000000000
[83317.452277] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[83317.452279] CR2: 0000000000000008 CR3: 0000000001c0a000 CR4: 00000000000406e0
[83317.452281] Stack:
[83317.452288]  ffffffffa05d141d ffff880816bfa630 ffffffffa05d1922 ffff88081e70c2c0
[83317.452295]  ffff880816bfa630 ffffffff81095fed 0000000000000000 ffffffff8107bf60
[83317.452297]  ffff880816bfa630 ffffffff00000000 ffff880800000000 ffff880816123f08
[83317.452297] Call Trace:
[83317.452309]  [<ffffffffa05d141d>] torture_rtmutex_boost+0x1d/0x90 [locktorture]
[83317.452315]  [<ffffffffa05d1922>] lock_torture_writer+0xe2/0x170 [locktorture]
[83317.452321]  [<ffffffff81095fed>] kthread+0xbd/0xe0
[83317.452325]  [<ffffffff815cf40f>] ret_from_fork+0x3f/0x70

This patch ensures that if the random state pointer is not nil and current
is not boosted, then do nothing.

Signed-off-by: Davidlohr Bueso <dbueso@...e.de>
---
 kernel/locking/locktorture.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/kernel/locking/locktorture.c b/kernel/locking/locktorture.c
index 8ef1919d63b2..9e9c5f454f5c 100644
--- a/kernel/locking/locktorture.c
+++ b/kernel/locking/locktorture.c
@@ -394,12 +394,12 @@ static void torture_rtmutex_boost(struct torture_random_state *trsp)
 
 	if (!rt_task(current)) {
 		/*
-		 * (1) Boost priority once every ~50k operations. When the
+		 * Boost priority once every ~50k operations. When the
 		 * task tries to take the lock, the rtmutex it will account
 		 * for the new priority, and do any corresponding pi-dance.
 		 */
-		if (!(torture_random(trsp) %
-		      (cxt.nrealwriters_stress * factor))) {
+		if (trsp && !(torture_random(trsp) %
+			      (cxt.nrealwriters_stress * factor))) {
 			policy = SCHED_FIFO;
 			param.sched_priority = MAX_RT_PRIO - 1;
 		} else /* common case, do nothing */
-- 
2.1.4