From: Vegard Nossum <vegard.nossum@xxxxxxxxxx>
commit 4d6501dce079c1eb6bf0b1d8f528a5e81770109e upstream.
If a kthread forks (e.g. usermodehelper since commit 1da5c46fa965) but
fails in copy_process() between calling dup_task_struct() and setting
p->set_child_tid, then the value of p->set_child_tid will be inherited
from the parent and get prematurely freed by free_kthread_struct().
kthread()
- worker_thread()
- process_one_work()
| - call_usermodehelper_exec_work()
| - kernel_thread()
| - _do_fork()
| - copy_process()
| - dup_task_struct()
| - arch_dup_task_struct()
| - tsk->set_child_tid = current->set_child_tid // implied
| - ...
| - goto bad_fork_*
| - ...
| - free_task(tsk)
| - free_kthread_struct(tsk)
| - kfree(tsk->set_child_tid)
- ...
- schedule()
- __schedule()
- wq_worker_sleeping()
- kthread_data(task)->flags // UAF
The problem started showing up with commit 1da5c46fa965 since it reused
->set_child_tid for the kthread worker data.
A better long-term solution might be to get rid of the ->set_child_tid
abuse. The comment in set_kthread_struct() also looks slightly wrong.
Debugged-by: Jamie Iles <jamie.iles@xxxxxxxxxx>
Fixes: 1da5c46fa965 ("kthread: Make struct kthread kmalloc'ed")
Signed-off-by: Vegard Nossum <vegard.nossum@xxxxxxxxxx>
Acked-by: Oleg Nesterov <oleg@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: Frederic Weisbecker <fweisbec@xxxxxxxxx>
Cc: Jamie Iles <jamie.iles@xxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Link: http://lkml.kernel.org/r/20170509073959.17858-1-vegard.nossum@xxxxxxxxxx
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Signed-off-by: Amit Pundir <amit.pundir@xxxxxxxxxx>
---
To be applied on 4.4.y and 3.18.y as well.
Build tested on v4.4.153 and v3.18.120.
kernel/fork.c | 17 ++++++++++++-----
1 file changed, 12 insertions(+), 5 deletions(-)
diff --git a/kernel/fork.c b/kernel/fork.c
index 2c98b987808d..46a6b0311ca3 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1530,6 +1530,18 @@ static __latent_entropy struct task_struct *copy_process(
if (!p)
goto fork_out;
+ /*
+ * This _must_ happen before we call free_task(), i.e. before we jump
+ * to any of the bad_fork_* labels. This is to avoid freeing
+ * p->set_child_tid which is (ab)used as a kthread's data pointer for
+ * kernel threads (PF_KTHREAD).
+ */
+ p->set_child_tid = (clone_flags & CLONE_CHILD_SETTID) ? child_tidptr : NULL;
+ /*
+ * Clear TID on mm_release()?
+ */
+ p->clear_child_tid = (clone_flags & CLONE_CHILD_CLEARTID) ? child_tidptr : NULL;
+
ftrace_graph_init_task(p);
rt_mutex_init_task(p);
@@ -1691,11 +1703,6 @@ static __latent_entropy struct task_struct *copy_process(
}
}
- p->set_child_tid = (clone_flags & CLONE_CHILD_SETTID) ? child_tidptr : NULL;
- /*
- * Clear TID on mm_release()?
- */
- p->clear_child_tid = (clone_flags & CLONE_CHILD_CLEARTID) ? child_tidptr : NULL;
#ifdef CONFIG_BLOCK
p->plug = NULL;
#endif
--
2.7.4
