[PATCH tip:x86/entry] core: fix user return notifier on fork()

[Avi Kivity]

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 <a...@redhat.com>
---
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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[tip-bot for Avi Kivity]

Commit-ID: 8e7cac79808b62f242069a6ac88d364d35621371
Gitweb: http://git.kernel.org/tip/8e7cac79808b62f242069a6ac88d364d35621371
Author: Avi Kivity <a...@redhat.com>
AuthorDate: Sun, 29 Nov 2009 16:34:48 +0200
Committer: Ingo Molnar <mi...@elte.hu>
CommitDate: Sun, 29 Nov 2009 22:03:04 +0100

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: Avi Kivity <a...@redhat.com>
LKML-Reference: <1259505288-16559-1...@redhat.com>
Signed-off-by: Ingo Molnar <mi...@elte.hu>

---
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 266c6af..1b7512d 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 */

--