um: properly check all process' threads for a live mm

kill_off_processes() might miss a valid process, this is because checking
for process->mm is not enough.  Process' main thread may exit or detach
its mm via use_mm(), but other threads may still have a valid mm.

To catch this we use find_lock_task_mm(), which walks up all threads and
returns an appropriate task (with task lock held).

Suggested-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Anton Vorontsov <anton.vorontsov@linaro.org>
Cc: Richard Weinberger <richard@nod.at>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 7 insertions, 6 deletions

diff --git a/arch/um/kernel/reboot.c b/arch/um/kernel/reboot.c
index 1411f4e79f9a..3d15243ce692 100644
--- a/arch/um/kernel/reboot.c
+++ b/arch/um/kernel/reboot.c
@@ -6,6 +6,7 @@
 #include "linux/sched.h"
 #include "linux/spinlock.h"
 #include "linux/slab.h"
+#include "linux/oom.h"
 #include "kern_util.h"
 #include "os.h"
 #include "skas.h"
@@ -25,13 +26,13 @@ static void kill_off_processes(void)
 
                 read_lock(&tasklist_lock);
                 for_each_process(p) {
-                        task_lock(p);
-                        if (!p->mm) {
-                                task_unlock(p);
+                        struct task_struct *t;
+
+                        t = find_lock_task_mm(p);
+                        if (!t)
                                 continue;
-                        }
-                        pid = p->mm->context.id.u.pid;
-                        task_unlock(p);
+                        pid = t->mm->context.id.u.pid;
+                        task_unlock(t);
                         os_kill_ptraced_process(pid, 1);
                 }
                 read_unlock(&tasklist_lock);