Re: [PATCH 1/2] locktorture: Fix deboosting nil ptr dereferencing

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Tue, Mar 08, 2016 at 11:08:05PM -0800, Davidlohr Bueso wrote:
> 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@suse.de>

Queued for 4.7, thank you!!!

(If you need it earlier, please let me know.)

							Thanx, Paul

> ---
>  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
>