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core: Fix user return notifier on fork()

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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>
This commit is contained in:
Avi Kivity 2009-11-29 16:34:48 +02:00 committed by Ingo Molnar
parent 3c912b6eda
commit 8e7cac7980
2 changed files with 9 additions and 0 deletions
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7
include/linux/user-return-notifier.h
View file

@ -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

2
kernel/fork.c
View file

@ -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 */