[PATCH 8/9] um: Fix possible race on task->mm

From: Anton Vorontsov
Date: Mon Apr 23 2012 - 03:10:50 EST

Checking for task->mm is dangerous as ->mm might disappear (exit_mm()
assigns NULL under task_lock(), so tasklist lock is not enough).

We can't use get_task_mm()/mmput() pair as mmput() might sleep,
so let's take the task lock while we care about its mm.

Note that we should also use find_lock_task_mm() to check all process'
threads for a valid mm, but for uml we'll do it in a separate patch.

Signed-off-by: Anton Vorontsov <anton.vorontsov@xxxxxxxxxx>
arch/um/kernel/reboot.c | 7 +++++--
1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/arch/um/kernel/reboot.c b/arch/um/kernel/reboot.c
index 66d754c..1411f4e 100644
--- a/arch/um/kernel/reboot.c
+++ b/arch/um/kernel/reboot.c
@@ -25,10 +25,13 @@ static void kill_off_processes(void)

for_each_process(p) {
- if (p->mm == NULL)
+ task_lock(p);
+ if (!p->mm) {
+ task_unlock(p);
+ }
pid = p->mm->context.id.u.pid;
+ task_unlock(p);
os_kill_ptraced_process(pid, 1);

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