From: Gavin Guo <gavin.guo@xxxxxxxxxxxxx> The following message can be observed on the Ubuntu v3.13.0-65 with KASan backported: ================================================================== BUG: KASan: use after free in task_numa_find_cpu+0x64c/0x890 at addr ffff880dd393ecd8 Read of size 8 by task qemu-system-x86/3998900 ============================================================================= BUG kmalloc-128 (Tainted: G B ): kasan: bad access detected ----------------------------------------------------------------------------- INFO: Allocated in task_numa_fault+0xc1b/0xed0 age=41980 cpu=18 pid=3998890 __slab_alloc+0x4f8/0x560 __kmalloc+0x1eb/0x280 task_numa_fault+0xc1b/0xed0 do_numa_page+0x192/0x200 handle_mm_fault+0x808/0x1160 __do_page_fault+0x218/0x750 do_page_fault+0x1a/0x70 page_fault+0x28/0x30 SyS_poll+0x66/0x1a0 system_call_fastpath+0x1a/0x1f INFO: Freed in task_numa_free+0x1d2/0x200 age=62 cpu=18 pid=0 __slab_free+0x2ab/0x3f0 kfree+0x161/0x170 task_numa_free+0x1d2/0x200 finish_task_switch+0x1d2/0x210 __schedule+0x5d4/0xc60 schedule_preempt_disabled+0x40/0xc0 cpu_startup_entry+0x2da/0x340 start_secondary+0x28f/0x360 INFO: Slab 0xffffea00374e4f00 objects=37 used=17 fp=0xffff880dd393ecb0 flags=0x6ffff0000004080 INFO: Object 0xffff880dd393ecb0 @offset=11440 fp=0xffff880dd393f700 Bytes b4 ffff880dd393eca0: 0c 00 00 00 18 00 00 00 af 63 3a 04 01 00 00 00 .........c:..... Object ffff880dd393ecb0: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b kkkkkkkkkkkkkkkk Object ffff880dd393ecc0: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b kkkkkkkkkkkkkkkk Object ffff880dd393ecd0: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b kkkkkkkkkkkkkkkk Object ffff880dd393ece0: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b kkkkkkkkkkkkkkkk Object ffff880dd393ecf0: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b kkkkkkkkkkkkkkkk Object ffff880dd393ed00: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b kkkkkkkkkkkkkkkk Object ffff880dd393ed10: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b kkkkkkkkkkkkkkkk Object ffff880dd393ed20: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b a5 kkkkkkkkkkkkkkk. CPU: 61 PID: 3998900 Comm: qemu-system-x86 Tainted: G B 3.13.0-65-generic #105 Hardware name: Supermicro X8QB6/X8QB6, BIOS 2.0c 06/11/2 ffffea00374e4f00 ffff8816c572b420 ffffffff81a6ce35 ffff88045f00f500 ffff8816c572b450 ffffffff81244aed ffff88045f00f500 ffffea00374e4f00 ffff880dd393ecb0 0000000000000012 ffff8816c572b478 ffffffff8124ac36 Call Trace: [<ffffffff81a6ce35>] dump_stack+0x45/0x56 [<ffffffff81244aed>] print_trailer+0xfd/0x170 [<ffffffff8124ac36>] object_err+0x36/0x40 [<ffffffff8124cbf9>] kasan_report_error+0x1e9/0x3a0 [<ffffffff8124d260>] kasan_report+0x40/0x50 [<ffffffff810dda7c>] ? task_numa_find_cpu+0x64c/0x890 [<ffffffff8124bee9>] __asan_load8+0x69/0xa0 [<ffffffff814f5c38>] ? find_next_bit+0xd8/0x120 [<ffffffff810dda7c>] task_numa_find_cpu+0x64c/0x890 [<ffffffff810de16c>] task_numa_migrate+0x4ac/0x7b0 [<ffffffff810de523>] numa_migrate_preferred+0xb3/0xc0 [<ffffffff810e0b88>] task_numa_fault+0xb88/0xed0 [<ffffffff8120ef02>] do_numa_page+0x192/0x200 [<ffffffff81211038>] handle_mm_fault+0x808/0x1160 [<ffffffff810d7dbd>] ? sched_clock_cpu+0x10d/0x160 [<ffffffff81068c52>] ? native_load_tls+0x82/0xa0 [<ffffffff81a7bd68>] __do_page_fault+0x218/0x750 [<ffffffff810c2186>] ? hrtimer_try_to_cancel+0x76/0x160 [<ffffffff81a6f5e7>] ? schedule_hrtimeout_range_clock.part.24+0xf7/0x1c0 [<ffffffff81a7c2ba>] do_page_fault+0x1a/0x70 [<ffffffff81a772e8>] page_fault+0x28/0x30 [<ffffffff8128cbd4>] ? do_sys_poll+0x1c4/0x6d0 [<ffffffff810e64f6>] ? enqueue_task_fair+0x4b6/0xaa0 [<ffffffff810233c9>] ? sched_clock+0x9/0x10 [<ffffffff810cf70a>] ? resched_task+0x7a/0xc0 [<ffffffff810d0663>] ? check_preempt_curr+0xb3/0x130 [<ffffffff8128b5c0>] ? poll_select_copy_remaining+0x170/0x170 [<ffffffff810d3bc0>] ? wake_up_state+0x10/0x20 [<ffffffff8112a28f>] ? drop_futex_key_refs.isra.14+0x1f/0x90 [<ffffffff8112d40e>] ? futex_requeue+0x3de/0xba0 [<ffffffff8112e49e>] ? do_futex+0xbe/0x8f0 [<ffffffff81022c89>] ? read_tsc+0x9/0x20 [<ffffffff8111bd9d>] ? ktime_get_ts+0x12d/0x170 [<ffffffff8108f699>] ? timespec_add_safe+0x59/0xe0 [<ffffffff8128d1f6>] SyS_poll+0x66/0x1a0 [<ffffffff81a830dd>] system_call_fastpath+0x1a/0x1f Memory state around the buggy address: ffff880dd393eb80: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc ffff880dd393ec00: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc >ffff880dd393ec80: fc fc fc fc fc fc fb fb fb fb fb fb fb fb fb fb ^ ffff880dd393ed00: fb fb fb fb fb fb fc fc fc fc fc fc fc fc fc fc ffff880dd393ed80: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc ================================================================== As commit 1effd9f19324 ("sched/numa: Fix unsafe get_task_struct() in task_numa_assign()") points out, the rcu_read_lock() cannot protect the task_struct from being freed in the finish_task_switch(). And the bug happens in the process of calculation of imp which requires the access of p->numa_faults being freed in the following path: do_exit() current->flags |= PF_EXITING; release_task() ~~delayed_put_task_struct()~~ schedule() ... ... rq->curr = next; context_switch() finish_task_switch() put_task_struct() __put_task_struct() task_numa_free() The fix here to get_task_struct() early before end of dst_rq->lock to protect the calculation process and also put_task_struct() in the corresponding point if finally the dst_rq->curr somehow cannot be assigned. BugLink: https://bugs.launchpad.net/bugs/1527643 Signed-off-by: Gavin Guo <gavin.guo@xxxxxxxxxxxxx> Signed-off-by: Liang Chen <liangchen.linux@xxxxxxxxx> --- Currently, the bug still haven't been observed on the upstream kernel with KASan enabled. However, even in the Ubuntu v3.13.0-65, we took about 1 week or more to reproduce the bug. After comparing the source between v3.13.0-65 and latest mainline kernel, there seems not much difference in the logic of task_numa_compare. So, it has possibilities to happen in the tricky case. --- kernel/sched/fair.c | 29 ++++++++++++++++++++++------- 1 file changed, 22 insertions(+), 7 deletions(-) diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 1926606..6ef4033 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -1220,8 +1220,6 @@ static void task_numa_assign(struct task_numa_env *env, { if (env->best_task) put_task_struct(env->best_task); - if (p) - get_task_struct(p); env->best_task = p; env->best_imp = imp; @@ -1289,20 +1287,29 @@ static void task_numa_compare(struct task_numa_env *env, long imp = env->p->numa_group ? groupimp : taskimp; long moveimp = imp; int dist = env->dist; + bool assigned = false; rcu_read_lock(); raw_spin_lock_irq(&dst_rq->lock); cur = dst_rq->curr; /* - * No need to move the exiting task, and this ensures that ->curr - * wasn't reaped and thus get_task_struct() in task_numa_assign() - * is safe under RCU read lock. - * Note that rcu_read_lock() itself can't protect from the final - * put_task_struct() after the last schedule(). + * No need to move the exiting task or idle task. */ if ((cur->flags & PF_EXITING) || is_idle_task(cur)) cur = NULL; + else + /* + * The task_struct must be protected here to protect the + * p->numa_faults access in the task_weight since the + * numa_faults could already be freed in the following path: + * finish_task_switch() + * --> put_task_struct() + * --> __put_task_struct() + * --> task_numa_free() + */ + get_task_struct(cur); + raw_spin_unlock_irq(&dst_rq->lock); /* @@ -1386,6 +1393,7 @@ balance: */ if (!load_too_imbalanced(src_load, dst_load, env)) { imp = moveimp - 1; + put_task_struct(cur); cur = NULL; goto assign; } @@ -1411,9 +1419,16 @@ balance: env->dst_cpu = select_idle_sibling(env->p, env->dst_cpu); assign: + assigned = true; task_numa_assign(env, cur, imp); unlock: rcu_read_unlock(); + /* + * The dst_rq->curr isn't assigned. The protection for task_struct is + * finished. + */ + if (cur && !assigned) + put_task_struct(cur); } static void task_numa_find_cpu(struct task_numa_env *env, -- 2.0.0 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx"> email@xxxxxxxxx </a>