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Commit 8e7cac79 authored by Avi Kivity's avatar Avi Kivity Committed by Ingo Molnar
Browse files

core: Fix user return notifier on fork() 



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: default avatarAvi Kivity <avi@redhat.com>
LKML-Reference: <1259505288-16559-1-git-send-email-avi@redhat.com>
Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
parent 3c912b6e

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 */