core: Fix user return notifier on fork() (8e7cac79) · Commits · 戴 / test

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@redhat.com> LKML-Reference: <1259505288-16559-1-git-send-email-avi@redhat.com> Signed-off-by:

Ingo Molnar <mingo@elte.hu>

include/linux/user-return-notifier.h

+7 −0

Original line number	Diff line number	Diff line
		@@ -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

kernel/fork.c

+2 −0

Original line number	Diff line number	Diff line
		@@ -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 */

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