fork() clones all thread_info flags, including TIF_USER_RETURN_NOTIFY; if the
new task is first scheduled on a cpu which doesn't have user return notifiers
set, this causes user return notifiers to trigger without any way of clearing
itself.
This is easy to trigger with a forky workload on the host in parallel with
kvm, resulting in a cpu in an endless loop on the verge of returning to
userspace.
Fix by dropping the TIF_USER_RETURN_NOTIFY immediately after fork.
Signed-off-by: Avi Kivity <avi@xxxxxxxxxx>
---
include/linux/user-return-notifier.h | 7 +++++++
kernel/fork.c | 2 ++
2 files changed, 9 insertions(+), 0 deletions(-)
diff --git a/include/linux/user-return-notifier.h b/include/linux/user-return-notifier.h
index b6ac056..9c4a445 100644
--- a/include/linux/user-return-notifier.h
+++ b/include/linux/user-return-notifier.h
@@ -26,6 +26,11 @@ static inline void propagate_user_return_notify(struct task_struct *prev,
void fire_user_return_notifiers(void);
+static inline void clear_user_return_notifier(struct task_struct *p)
+{
+ clear_tsk_thread_flag(p, TIF_USER_RETURN_NOTIFY);
+}
+
#else
struct user_return_notifier {};
@@ -37,6 +42,8 @@ static inline void propagate_user_return_notify(struct task_struct *prev,
static inline void fire_user_return_notifiers(void) {}
+static inline void clear_user_return_notifier(struct task_struct *p) {}
+
#endif
#endif
diff --git a/kernel/fork.c b/kernel/fork.c
index 166b8c4..7d4a348 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -64,6 +64,7 @@
#include <linux/magic.h>
#include <linux/perf_event.h>
#include <linux/posix-timers.h>
+#include <linux/user-return-notifier.h>
#include <asm/pgtable.h>
#include <asm/pgalloc.h>
@@ -249,6 +250,7 @@ static struct task_struct *dup_task_struct(struct task_struct *orig)
goto out;
setup_thread_stack(tsk, orig);
+ clear_user_return_notifier(tsk);
stackend = end_of_stack(tsk);
*stackend = STACK_END_MAGIC; /* for overflow detection */
--
1.6.5.2
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