[PATCH 4.4 09/11] futex: Avoid freeing an active timer

From: Greg Kroah-Hartman
Date: Sun Aug 08 2021 - 03:24:16 EST


From: Thomas Gleixner <tglx@xxxxxxxxxxxxx>

[ Upstream commit 97181f9bd57405b879403763284537e27d46963d ]

Alexander reported a hrtimer debug_object splat:

ODEBUG: free active (active state 0) object type: hrtimer hint: hrtimer_wakeup (kernel/time/hrtimer.c:1423)

debug_object_free (lib/debugobjects.c:603)
destroy_hrtimer_on_stack (kernel/time/hrtimer.c:427)
futex_lock_pi (kernel/futex.c:2740)
do_futex (kernel/futex.c:3399)
SyS_futex (kernel/futex.c:3447 kernel/futex.c:3415)
do_syscall_64 (arch/x86/entry/common.c:284)
entry_SYSCALL64_slow_path (arch/x86/entry/entry_64.S:249)

Which was caused by commit:

cfafcd117da0 ("futex: Rework futex_lock_pi() to use rt_mutex_*_proxy_lock()")

... losing the hrtimer_cancel() in the shuffle. Where previously the
hrtimer_cancel() was done by rt_mutex_slowlock() we now need to do it
manually.

Reported-by: Alexander Levin <alexander.levin@xxxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Fixes: cfafcd117da0 ("futex: Rework futex_lock_pi() to use rt_mutex_*_proxy_lock()")
Link: http://lkml.kernel.org/r/alpine.DEB.2.20.1704101802370.2906@nanos
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
Signed-off-by: Zhen Lei <thunder.leizhen@xxxxxxxxxx>
Acked-by: Joe Korty <joe.korty@xxxxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
kernel/futex.c | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)

--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -2960,8 +2960,10 @@ out_unlock_put_key:
out_put_key:
put_futex_key(&q.key);
out:
- if (to)
+ if (to) {
+ hrtimer_cancel(&to->timer);
destroy_hrtimer_on_stack(&to->timer);
+ }
return ret != -EINTR ? ret : -ERESTARTNOINTR;

uaddr_faulted: