[PATCH 3.12 06/86] ptrace: fix PTRACE_LISTEN race corrupting task->state

From: Jiri Slaby
Date: Thu May 04 2017 - 05:05:38 EST

From: "bsegall@xxxxxxxxxx" <bsegall@xxxxxxxxxx>

3.12-stable review patch. If anyone has any objections, please let me know.


commit 5402e97af667e35e54177af8f6575518bf251d51 upstream.

In PT_SEIZED + LISTEN mode STOP/CONT signals cause a wakeup against
__TASK_TRACED. If this races with the ptrace_unfreeze_traced at the end
of a PTRACE_LISTEN, this can wake the task /after/ the check against
__TASK_TRACED, but before the reset of state to TASK_TRACED. This
causes it to instead clobber TASK_WAKING, allowing a subsequent wakeup
against TRACED while the task is still on the rq wake_list, corrupting

Oleg said:
"The kernel can crash or this can lead to other hard-to-debug problems.
In short, "task->state = TASK_TRACED" in ptrace_unfreeze_traced()
assumes that nobody else can wake it up, but PTRACE_LISTEN breaks the
contract. Obviusly it is very wrong to manipulate task->state if this
task is already running, or WAKING, or it sleeps again"

[akpm@xxxxxxxxxxxxxxxxxxxx: coding-style fixes]
Fixes: 9899d11f ("ptrace: ensure arch_ptrace/ptrace_request can never race with SIGKILL")
Link: http://lkml.kernel.org/r/xm26y3vfhmkp.fsf_-_@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Ben Segall <bsegall@xxxxxxxxxx>
Acked-by: Oleg Nesterov <oleg@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Jiri Slaby <jslaby@xxxxxxx>
kernel/ptrace.c | 14 ++++++++++----
1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index 4524314ecbb4..3e3d7841179b 100644
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -150,11 +150,17 @@ static void ptrace_unfreeze_traced(struct task_struct *task)

WARN_ON(!task->ptrace || task->parent != current);

+ /*
+ * PTRACE_LISTEN can allow ptrace_trap_notify to wake us up remotely.
+ * Recheck state under the lock to close this race.
+ */
- if (__fatal_signal_pending(task))
- wake_up_state(task, __TASK_TRACED);
- else
- task->state = TASK_TRACED;
+ if (task->state == __TASK_TRACED) {
+ if (__fatal_signal_pending(task))
+ wake_up_state(task, __TASK_TRACED);
+ else
+ task->state = TASK_TRACED;
+ }