[PATCH 4.4 00/12] Futex back-port from v4.9

[PATCH 4.4 00/12] Futex back-port from v4.9

[Lee Jones]

Ref: https://git.kernel.org/pub/scm/linux/kernel/git/stable/stable-queue.git/tree/pending/futex_issues.txt

This set required 1 additional patch dragged back from v4.14 to avoid build errors.

Arnd Bergmann (1):
  y2038: futex: Move compat implementation into futex.c

Thomas Gleixner (11):
  futex: Move futex exit handling into futex code
  futex: Replace PF_EXITPIDONE with a state
  exit/exec: Seperate mm_release()
  futex: Split futex_mm_release() for exit/exec
  futex: Set task::futex_state to DEAD right after handling futex exit
  futex: Mark the begin of futex exit explicitly
  futex: Sanitize exit state handling
  futex: Provide state handling for exec() as well
  futex: Add mutex around futex exit
  futex: Provide distinct return value when owner is exiting
  futex: Prevent exit livelock

 fs/exec.c              |   2 +-
 include/linux/compat.h |   2 -
 include/linux/futex.h  |  44 ++--
 include/linux/sched.h  |   9 +-
 kernel/Makefile        |   3 -
 kernel/exit.c          |  25 +--
 kernel/fork.c          |  40 ++--
 kernel/futex.c         | 446 ++++++++++++++++++++++++++++++++++++++---
 kernel/futex_compat.c  | 201 -------------------
 9 files changed, 466 insertions(+), 306 deletions(-)
 delete mode 100644 kernel/futex_compat.c

Cc: Stable Team <stable@vger.kernel.org>
-- 
2.25.1

[PATCH 01/12] y2038: futex: Move compat implementation into futex.c

[Lee Jones]

From: Arnd Bergmann <arnd@arndb.de>

commit 04e7712f4460585e5eed5b853fd8b82a9943958f upstream.

We are going to share the compat_sys_futex() handler between 64-bit
architectures and 32-bit architectures that need to deal with both 32-bit
and 64-bit time_t, and this is easier if both entry points are in the
same file.

In fact, most other system call handlers do the same thing these days, so
let's follow the trend here and merge all of futex_compat.c into futex.c.

In the process, a few minor changes have to be done to make sure everything
still makes sense: handle_futex_death() and futex_cmpxchg_enabled() become
local symbol, and the compat version of the fetch_robust_entry() function
gets renamed to compat_fetch_robust_entry() to avoid a symbol clash.

This is intended as a purely cosmetic patch, no behavior should
change.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
[Lee: Back-ported to satisfy a build dependency]
Signed-off-by: Lee Jones <lee.jones@linaro.org>
---
 include/linux/futex.h |   8 --
 kernel/Makefile       |   3 -
 kernel/futex.c        | 195 +++++++++++++++++++++++++++++++++++++++-
 kernel/futex_compat.c | 201 ------------------------------------------
 4 files changed, 192 insertions(+), 215 deletions(-)
 delete mode 100644 kernel/futex_compat.c

diff --git a/include/linux/futex.h b/include/linux/futex.h
index c015fa91e7cce..fb4e12cbe887e 100644
--- a/include/linux/futex.h
+++ b/include/linux/futex.h
@@ -11,9 +11,6 @@ union ktime;
 long do_futex(u32 __user *uaddr, int op, u32 val, union ktime *timeout,
 	      u32 __user *uaddr2, u32 val2, u32 val3);
 
-extern int
-handle_futex_death(u32 __user *uaddr, struct task_struct *curr, int pi);
-
 /*
  * Futexes are matched on equal values of this key.
  * The key type depends on whether it's a shared or private mapping.
@@ -58,11 +55,6 @@ union futex_key {
 #ifdef CONFIG_FUTEX
 extern void exit_robust_list(struct task_struct *curr);
 extern void exit_pi_state_list(struct task_struct *curr);
-#ifdef CONFIG_HAVE_FUTEX_CMPXCHG
-#define futex_cmpxchg_enabled 1
-#else
-extern int futex_cmpxchg_enabled;
-#endif
 #else
 static inline void exit_robust_list(struct task_struct *curr)
 {
diff --git a/kernel/Makefile b/kernel/Makefile
index 53abf008ecb39..a672bece1f499 100644
--- a/kernel/Makefile
+++ b/kernel/Makefile
@@ -36,9 +36,6 @@ obj-$(CONFIG_PROFILING) += profile.o
 obj-$(CONFIG_STACKTRACE) += stacktrace.o
 obj-y += time/
 obj-$(CONFIG_FUTEX) += futex.o
-ifeq ($(CONFIG_COMPAT),y)
-obj-$(CONFIG_FUTEX) += futex_compat.o
-endif
 obj-$(CONFIG_GENERIC_ISA_DMA) += dma.o
 obj-$(CONFIG_SMP) += smp.o
 ifneq ($(CONFIG_SMP),y)
diff --git a/kernel/futex.c b/kernel/futex.c
index e50b67674ba25..2815b1801ec5a 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -44,6 +44,7 @@
  *  along with this program; if not, write to the Free Software
  *  Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
  */
+#include <linux/compat.h>
 #include <linux/slab.h>
 #include <linux/poll.h>
 #include <linux/fs.h>
@@ -171,8 +172,10 @@
  * double_lock_hb() and double_unlock_hb(), respectively.
  */
 
-#ifndef CONFIG_HAVE_FUTEX_CMPXCHG
-int __read_mostly futex_cmpxchg_enabled;
+#ifdef CONFIG_HAVE_FUTEX_CMPXCHG
+#define futex_cmpxchg_enabled 1
+#else
+static int  __read_mostly futex_cmpxchg_enabled;
 #endif
 
 /*
@@ -3088,7 +3091,7 @@ err_unlock:
  * Process a futex-list entry, check whether it's owned by the
  * dying task, and do notification if so:
  */
-int handle_futex_death(u32 __user *uaddr, struct task_struct *curr, int pi)
+static int handle_futex_death(u32 __user *uaddr, struct task_struct *curr, int pi)
 {
 	u32 uval, uninitialized_var(nval), mval;
 
@@ -3318,6 +3321,192 @@ SYSCALL_DEFINE6(futex, u32 __user *, uaddr, int, op, u32, val,
 	return do_futex(uaddr, op, val, tp, uaddr2, val2, val3);
 }
 
+#ifdef CONFIG_COMPAT
+/*
+ * Fetch a robust-list pointer. Bit 0 signals PI futexes:
+ */
+static inline int
+compat_fetch_robust_entry(compat_uptr_t *uentry, struct robust_list __user **entry,
+		   compat_uptr_t __user *head, unsigned int *pi)
+{
+	if (get_user(*uentry, head))
+		return -EFAULT;
+
+	*entry = compat_ptr((*uentry) & ~1);
+	*pi = (unsigned int)(*uentry) & 1;
+
+	return 0;
+}
+
+static void __user *futex_uaddr(struct robust_list __user *entry,
+				compat_long_t futex_offset)
+{
+	compat_uptr_t base = ptr_to_compat(entry);
+	void __user *uaddr = compat_ptr(base + futex_offset);
+
+	return uaddr;
+}
+
+/*
+ * Walk curr->robust_list (very carefully, it's a userspace list!)
+ * and mark any locks found there dead, and notify any waiters.
+ *
+ * We silently return on any sign of list-walking problem.
+ */
+void compat_exit_robust_list(struct task_struct *curr)
+{
+	struct compat_robust_list_head __user *head = curr->compat_robust_list;
+	struct robust_list __user *entry, *next_entry, *pending;
+	unsigned int limit = ROBUST_LIST_LIMIT, pi, pip;
+	unsigned int uninitialized_var(next_pi);
+	compat_uptr_t uentry, next_uentry, upending;
+	compat_long_t futex_offset;
+	int rc;
+
+	if (!futex_cmpxchg_enabled)
+		return;
+
+	/*
+	 * Fetch the list head (which was registered earlier, via
+	 * sys_set_robust_list()):
+	 */
+	if (compat_fetch_robust_entry(&uentry, &entry, &head->list.next, &pi))
+		return;
+	/*
+	 * Fetch the relative futex offset:
+	 */
+	if (get_user(futex_offset, &head->futex_offset))
+		return;
+	/*
+	 * Fetch any possibly pending lock-add first, and handle it
+	 * if it exists:
+	 */
+	if (compat_fetch_robust_entry(&upending, &pending,
+			       &head->list_op_pending, &pip))
+		return;
+
+	next_entry = NULL;	/* avoid warning with gcc */
+	while (entry != (struct robust_list __user *) &head->list) {
+		/*
+		 * Fetch the next entry in the list before calling
+		 * handle_futex_death:
+		 */
+		rc = compat_fetch_robust_entry(&next_uentry, &next_entry,
+			(compat_uptr_t __user *)&entry->next, &next_pi);
+		/*
+		 * A pending lock might already be on the list, so
+		 * dont process it twice:
+		 */
+		if (entry != pending) {
+			void __user *uaddr = futex_uaddr(entry, futex_offset);
+
+			if (handle_futex_death(uaddr, curr, pi))
+				return;
+		}
+		if (rc)
+			return;
+		uentry = next_uentry;
+		entry = next_entry;
+		pi = next_pi;
+		/*
+		 * Avoid excessively long or circular lists:
+		 */
+		if (!--limit)
+			break;
+
+		cond_resched();
+	}
+	if (pending) {
+		void __user *uaddr = futex_uaddr(pending, futex_offset);
+
+		handle_futex_death(uaddr, curr, pip);
+	}
+}
+
+COMPAT_SYSCALL_DEFINE2(set_robust_list,
+		struct compat_robust_list_head __user *, head,
+		compat_size_t, len)
+{
+	if (!futex_cmpxchg_enabled)
+		return -ENOSYS;
+
+	if (unlikely(len != sizeof(*head)))
+		return -EINVAL;
+
+	current->compat_robust_list = head;
+
+	return 0;
+}
+
+COMPAT_SYSCALL_DEFINE3(get_robust_list, int, pid,
+			compat_uptr_t __user *, head_ptr,
+			compat_size_t __user *, len_ptr)
+{
+	struct compat_robust_list_head __user *head;
+	unsigned long ret;
+	struct task_struct *p;
+
+	if (!futex_cmpxchg_enabled)
+		return -ENOSYS;
+
+	rcu_read_lock();
+
+	ret = -ESRCH;
+	if (!pid)
+		p = current;
+	else {
+		p = find_task_by_vpid(pid);
+		if (!p)
+			goto err_unlock;
+	}
+
+	ret = -EPERM;
+	if (!ptrace_may_access(p, PTRACE_MODE_READ_REALCREDS))
+		goto err_unlock;
+
+	head = p->compat_robust_list;
+	rcu_read_unlock();
+
+	if (put_user(sizeof(*head), len_ptr))
+		return -EFAULT;
+	return put_user(ptr_to_compat(head), head_ptr);
+
+err_unlock:
+	rcu_read_unlock();
+
+	return ret;
+}
+
+COMPAT_SYSCALL_DEFINE6(futex, u32 __user *, uaddr, int, op, u32, val,
+		struct compat_timespec __user *, utime, u32 __user *, uaddr2,
+		u32, val3)
+{
+	struct timespec ts;
+	ktime_t t, *tp = NULL;
+	int val2 = 0;
+	int cmd = op & FUTEX_CMD_MASK;
+
+	if (utime && (cmd == FUTEX_WAIT || cmd == FUTEX_LOCK_PI ||
+		      cmd == FUTEX_WAIT_BITSET ||
+		      cmd == FUTEX_WAIT_REQUEUE_PI)) {
+		if (compat_get_timespec(&ts, utime))
+			return -EFAULT;
+		if (!timespec_valid(&ts))
+			return -EINVAL;
+
+		t = timespec_to_ktime(ts);
+		if (cmd == FUTEX_WAIT)
+			t = ktime_add_safe(ktime_get(), t);
+		tp = &t;
+	}
+	if (cmd == FUTEX_REQUEUE || cmd == FUTEX_CMP_REQUEUE ||
+	    cmd == FUTEX_CMP_REQUEUE_PI || cmd == FUTEX_WAKE_OP)
+		val2 = (int) (unsigned long) utime;
+
+	return do_futex(uaddr, op, val, tp, uaddr2, val2, val3);
+}
+#endif /* CONFIG_COMPAT */
+
 static void __init futex_detect_cmpxchg(void)
 {
 #ifndef CONFIG_HAVE_FUTEX_CMPXCHG
diff --git a/kernel/futex_compat.c b/kernel/futex_compat.c
deleted file mode 100644
index 4ae3232e7a28a..0000000000000
--- a/kernel/futex_compat.c
+++ /dev/null
@@ -1,201 +0,0 @@
-/*
- * linux/kernel/futex_compat.c
- *
- * Futex compatibililty routines.
- *
- * Copyright 2006, Red Hat, Inc., Ingo Molnar
- */
-
-#include <linux/linkage.h>
-#include <linux/compat.h>
-#include <linux/nsproxy.h>
-#include <linux/futex.h>
-#include <linux/ptrace.h>
-#include <linux/syscalls.h>
-
-#include <asm/uaccess.h>
-
-
-/*
- * Fetch a robust-list pointer. Bit 0 signals PI futexes:
- */
-static inline int
-fetch_robust_entry(compat_uptr_t *uentry, struct robust_list __user **entry,
-		   compat_uptr_t __user *head, unsigned int *pi)
-{
-	if (get_user(*uentry, head))
-		return -EFAULT;
-
-	*entry = compat_ptr((*uentry) & ~1);
-	*pi = (unsigned int)(*uentry) & 1;
-
-	return 0;
-}
-
-static void __user *futex_uaddr(struct robust_list __user *entry,
-				compat_long_t futex_offset)
-{
-	compat_uptr_t base = ptr_to_compat(entry);
-	void __user *uaddr = compat_ptr(base + futex_offset);
-
-	return uaddr;
-}
-
-/*
- * Walk curr->robust_list (very carefully, it's a userspace list!)
- * and mark any locks found there dead, and notify any waiters.
- *
- * We silently return on any sign of list-walking problem.
- */
-void compat_exit_robust_list(struct task_struct *curr)
-{
-	struct compat_robust_list_head __user *head = curr->compat_robust_list;
-	struct robust_list __user *entry, *next_entry, *pending;
-	unsigned int limit = ROBUST_LIST_LIMIT, pi, pip;
-	unsigned int uninitialized_var(next_pi);
-	compat_uptr_t uentry, next_uentry, upending;
-	compat_long_t futex_offset;
-	int rc;
-
-	if (!futex_cmpxchg_enabled)
-		return;
-
-	/*
-	 * Fetch the list head (which was registered earlier, via
-	 * sys_set_robust_list()):
-	 */
-	if (fetch_robust_entry(&uentry, &entry, &head->list.next, &pi))
-		return;
-	/*
-	 * Fetch the relative futex offset:
-	 */
-	if (get_user(futex_offset, &head->futex_offset))
-		return;
-	/*
-	 * Fetch any possibly pending lock-add first, and handle it
-	 * if it exists:
-	 */
-	if (fetch_robust_entry(&upending, &pending,
-			       &head->list_op_pending, &pip))
-		return;
-
-	next_entry = NULL;	/* avoid warning with gcc */
-	while (entry != (struct robust_list __user *) &head->list) {
-		/*
-		 * Fetch the next entry in the list before calling
-		 * handle_futex_death:
-		 */
-		rc = fetch_robust_entry(&next_uentry, &next_entry,
-			(compat_uptr_t __user *)&entry->next, &next_pi);
-		/*
-		 * A pending lock might already be on the list, so
-		 * dont process it twice:
-		 */
-		if (entry != pending) {
-			void __user *uaddr = futex_uaddr(entry, futex_offset);
-
-			if (handle_futex_death(uaddr, curr, pi))
-				return;
-		}
-		if (rc)
-			return;
-		uentry = next_uentry;
-		entry = next_entry;
-		pi = next_pi;
-		/*
-		 * Avoid excessively long or circular lists:
-		 */
-		if (!--limit)
-			break;
-
-		cond_resched();
-	}
-	if (pending) {
-		void __user *uaddr = futex_uaddr(pending, futex_offset);
-
-		handle_futex_death(uaddr, curr, pip);
-	}
-}
-
-COMPAT_SYSCALL_DEFINE2(set_robust_list,
-		struct compat_robust_list_head __user *, head,
-		compat_size_t, len)
-{
-	if (!futex_cmpxchg_enabled)
-		return -ENOSYS;
-
-	if (unlikely(len != sizeof(*head)))
-		return -EINVAL;
-
-	current->compat_robust_list = head;
-
-	return 0;
-}
-
-COMPAT_SYSCALL_DEFINE3(get_robust_list, int, pid,
-			compat_uptr_t __user *, head_ptr,
-			compat_size_t __user *, len_ptr)
-{
-	struct compat_robust_list_head __user *head;
-	unsigned long ret;
-	struct task_struct *p;
-
-	if (!futex_cmpxchg_enabled)
-		return -ENOSYS;
-
-	rcu_read_lock();
-
-	ret = -ESRCH;
-	if (!pid)
-		p = current;
-	else {
-		p = find_task_by_vpid(pid);
-		if (!p)
-			goto err_unlock;
-	}
-
-	ret = -EPERM;
-	if (!ptrace_may_access(p, PTRACE_MODE_READ_REALCREDS))
-		goto err_unlock;
-
-	head = p->compat_robust_list;
-	rcu_read_unlock();
-
-	if (put_user(sizeof(*head), len_ptr))
-		return -EFAULT;
-	return put_user(ptr_to_compat(head), head_ptr);
-
-err_unlock:
-	rcu_read_unlock();
-
-	return ret;
-}
-
-COMPAT_SYSCALL_DEFINE6(futex, u32 __user *, uaddr, int, op, u32, val,
-		struct compat_timespec __user *, utime, u32 __user *, uaddr2,
-		u32, val3)
-{
-	struct timespec ts;
-	ktime_t t, *tp = NULL;
-	int val2 = 0;
-	int cmd = op & FUTEX_CMD_MASK;
-
-	if (utime && (cmd == FUTEX_WAIT || cmd == FUTEX_LOCK_PI ||
-		      cmd == FUTEX_WAIT_BITSET ||
-		      cmd == FUTEX_WAIT_REQUEUE_PI)) {
-		if (compat_get_timespec(&ts, utime))
-			return -EFAULT;
-		if (!timespec_valid(&ts))
-			return -EINVAL;
-
-		t = timespec_to_ktime(ts);
-		if (cmd == FUTEX_WAIT)
-			t = ktime_add_safe(ktime_get(), t);
-		tp = &t;
-	}
-	if (cmd == FUTEX_REQUEUE || cmd == FUTEX_CMP_REQUEUE ||
-	    cmd == FUTEX_CMP_REQUEUE_PI || cmd == FUTEX_WAKE_OP)
-		val2 = (int) (unsigned long) utime;
-
-	return do_futex(uaddr, op, val, tp, uaddr2, val2, val3);
-}
-- 
2.25.1

[PATCH 02/12] futex: Move futex exit handling into futex code

[Lee Jones]

From: Thomas Gleixner <tglx@linutronix.de>

commit ba31c1a48538992316cc71ce94fa9cd3e7b427c0 upstream.

The futex exit handling is #ifdeffed into mm_release() which is not pretty
to begin with. But upcoming changes to address futex exit races need to add
more functionality to this exit code.

Split it out into a function, move it into futex code and make the various
futex exit functions static.

Preparatory only and no functional change.

Folded build fix from Borislav.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20191106224556.049705556@linutronix.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
---
 include/linux/compat.h |  2 --
 include/linux/futex.h  | 24 +++++++++++++++++-------
 kernel/fork.c          | 25 +++----------------------
 kernel/futex.c         | 28 ++++++++++++++++++++++++++--
 4 files changed, 46 insertions(+), 33 deletions(-)

diff --git a/include/linux/compat.h b/include/linux/compat.h
index a76c9172b2eb0..24dd42910d7c2 100644
--- a/include/linux/compat.h
+++ b/include/linux/compat.h
@@ -306,8 +306,6 @@ struct compat_kexec_segment;
 struct compat_mq_attr;
 struct compat_msgbuf;
 
-extern void compat_exit_robust_list(struct task_struct *curr);
-
 asmlinkage long
 compat_sys_set_robust_list(struct compat_robust_list_head __user *head,
 			   compat_size_t len);
diff --git a/include/linux/futex.h b/include/linux/futex.h
index fb4e12cbe887e..63d353cedfcde 100644
--- a/include/linux/futex.h
+++ b/include/linux/futex.h
@@ -1,6 +1,8 @@
 #ifndef _LINUX_FUTEX_H
 #define _LINUX_FUTEX_H
 
+#include <linux/sched.h>
+
 #include <uapi/linux/futex.h>
 
 struct inode;
@@ -53,14 +55,22 @@ union futex_key {
 #define FUTEX_KEY_INIT (union futex_key) { .both = { .ptr = 0ULL } }
 
 #ifdef CONFIG_FUTEX
-extern void exit_robust_list(struct task_struct *curr);
-extern void exit_pi_state_list(struct task_struct *curr);
-#else
-static inline void exit_robust_list(struct task_struct *curr)
-{
-}
-static inline void exit_pi_state_list(struct task_struct *curr)
+static inline void futex_init_task(struct task_struct *tsk)
 {
+	tsk->robust_list = NULL;
+#ifdef CONFIG_COMPAT
+	tsk->compat_robust_list = NULL;
+#endif
+	INIT_LIST_HEAD(&tsk->pi_state_list);
+	tsk->pi_state_cache = NULL;
 }
+
+void futex_mm_release(struct task_struct *tsk);
+
+long do_futex(u32 __user *uaddr, int op, u32 val, ktime_t *timeout,
+	      u32 __user *uaddr2, u32 val2, u32 val3);
+#else
+static inline void futex_init_task(struct task_struct *tsk) { }
+static inline void futex_mm_release(struct task_struct *tsk) { }
 #endif
 #endif
diff --git a/kernel/fork.c b/kernel/fork.c
index 5d35be1e0913b..d0ab6aff5efdc 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -890,20 +890,7 @@ static int wait_for_vfork_done(struct task_struct *child,
 void mm_release(struct task_struct *tsk, struct mm_struct *mm)
 {
 	/* Get rid of any futexes when releasing the mm */
-#ifdef CONFIG_FUTEX
-	if (unlikely(tsk->robust_list)) {
-		exit_robust_list(tsk);
-		tsk->robust_list = NULL;
-	}
-#ifdef CONFIG_COMPAT
-	if (unlikely(tsk->compat_robust_list)) {
-		compat_exit_robust_list(tsk);
-		tsk->compat_robust_list = NULL;
-	}
-#endif
-	if (unlikely(!list_empty(&tsk->pi_state_list)))
-		exit_pi_state_list(tsk);
-#endif
+	futex_mm_release(tsk);
 
 	uprobe_free_utask(tsk);
 
@@ -1511,14 +1498,8 @@ static struct task_struct *copy_process(unsigned long clone_flags,
 #ifdef CONFIG_BLOCK
 	p->plug = NULL;
 #endif
-#ifdef CONFIG_FUTEX
-	p->robust_list = NULL;
-#ifdef CONFIG_COMPAT
-	p->compat_robust_list = NULL;
-#endif
-	INIT_LIST_HEAD(&p->pi_state_list);
-	p->pi_state_cache = NULL;
-#endif
+	futex_init_task(p);
+
 	/*
 	 * sigaltstack should be cleared when sharing the same VM
 	 */
diff --git a/kernel/futex.c b/kernel/futex.c
index 2815b1801ec5a..5282b9b8d1ec1 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -331,6 +331,12 @@ static inline bool should_fail_futex(bool fshared)
 }
 #endif /* CONFIG_FAIL_FUTEX */
 
+#ifdef CONFIG_COMPAT
+static void compat_exit_robust_list(struct task_struct *curr);
+#else
+static inline void compat_exit_robust_list(struct task_struct *curr) { }
+#endif
+
 static inline void futex_get_mm(union futex_key *key)
 {
 	atomic_inc(&key->private.mm->mm_count);
@@ -889,7 +895,7 @@ static struct task_struct * futex_find_get_task(pid_t pid)
  * Kernel cleans up PI-state, but userspace is likely hosed.
  * (Robust-futex cleanup is separate and might save the day for userspace.)
  */
-void exit_pi_state_list(struct task_struct *curr)
+static void exit_pi_state_list(struct task_struct *curr)
 {
 	struct list_head *next, *head = &curr->pi_state_list;
 	struct futex_pi_state *pi_state;
@@ -3166,7 +3172,7 @@ static inline int fetch_robust_entry(struct robust_list __user **entry,
  *
  * We silently return on any sign of list-walking problem.
  */
-void exit_robust_list(struct task_struct *curr)
+static void exit_robust_list(struct task_struct *curr)
 {
 	struct robust_list_head __user *head = curr->robust_list;
 	struct robust_list __user *entry, *next_entry, *pending;
@@ -3229,6 +3235,24 @@ void exit_robust_list(struct task_struct *curr)
 				   curr, pip);
 }
 
+void futex_mm_release(struct task_struct *tsk)
+{
+	if (unlikely(tsk->robust_list)) {
+		exit_robust_list(tsk);
+		tsk->robust_list = NULL;
+	}
+
+#ifdef CONFIG_COMPAT
+	if (unlikely(tsk->compat_robust_list)) {
+		compat_exit_robust_list(tsk);
+		tsk->compat_robust_list = NULL;
+	}
+#endif
+
+	if (unlikely(!list_empty(&tsk->pi_state_list)))
+		exit_pi_state_list(tsk);
+}
+
 long do_futex(u32 __user *uaddr, int op, u32 val, ktime_t *timeout,
 		u32 __user *uaddr2, u32 val2, u32 val3)
 {
-- 
2.25.1

[PATCH 03/12] futex: Replace PF_EXITPIDONE with a state

[Lee Jones]

From: Thomas Gleixner <tglx@linutronix.de>

commit 3d4775df0a89240f671861c6ab6e8d59af8e9e41 upstream.

The futex exit handling relies on PF_ flags. That's suboptimal as it
requires a smp_mb() and an ugly lock/unlock of the exiting tasks pi_lock in
the middle of do_exit() to enforce the observability of PF_EXITING in the
futex code.

Add a futex_state member to task_struct and convert the PF_EXITPIDONE logic
over to the new state. The PF_EXITING dependency will be cleaned up in a
later step.

This prepares for handling various futex exit issues later.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20191106224556.149449274@linutronix.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
---
 include/linux/futex.h | 34 ++++++++++++++++++++++++++++++++++
 include/linux/sched.h |  2 +-
 kernel/exit.c         | 18 ++----------------
 kernel/futex.c        | 17 ++++++++---------
 4 files changed, 45 insertions(+), 26 deletions(-)

diff --git a/include/linux/futex.h b/include/linux/futex.h
index 63d353cedfcde..a0de6fe28e00b 100644
--- a/include/linux/futex.h
+++ b/include/linux/futex.h
@@ -55,6 +55,11 @@ union futex_key {
 #define FUTEX_KEY_INIT (union futex_key) { .both = { .ptr = 0ULL } }
 
 #ifdef CONFIG_FUTEX
+enum {
+	FUTEX_STATE_OK,
+	FUTEX_STATE_DEAD,
+};
+
 static inline void futex_init_task(struct task_struct *tsk)
 {
 	tsk->robust_list = NULL;
@@ -63,6 +68,34 @@ static inline void futex_init_task(struct task_struct *tsk)
 #endif
 	INIT_LIST_HEAD(&tsk->pi_state_list);
 	tsk->pi_state_cache = NULL;
+	tsk->futex_state = FUTEX_STATE_OK;
+}
+
+/**
+ * futex_exit_done - Sets the tasks futex state to FUTEX_STATE_DEAD
+ * @tsk:	task to set the state on
+ *
+ * Set the futex exit state of the task lockless. The futex waiter code
+ * observes that state when a task is exiting and loops until the task has
+ * actually finished the futex cleanup. The worst case for this is that the
+ * waiter runs through the wait loop until the state becomes visible.
+ *
+ * This has two callers:
+ *
+ * - futex_mm_release() after the futex exit cleanup has been done
+ *
+ * - do_exit() from the recursive fault handling path.
+ *
+ * In case of a recursive fault this is best effort. Either the futex exit
+ * code has run already or not. If the OWNER_DIED bit has been set on the
+ * futex then the waiter can take it over. If not, the problem is pushed
+ * back to user space. If the futex exit code did not run yet, then an
+ * already queued waiter might block forever, but there is nothing which
+ * can be done about that.
+ */
+static inline void futex_exit_done(struct task_struct *tsk)
+{
+	tsk->futex_state = FUTEX_STATE_DEAD;
 }
 
 void futex_mm_release(struct task_struct *tsk);
@@ -72,5 +105,6 @@ long do_futex(u32 __user *uaddr, int op, u32 val, ktime_t *timeout,
 #else
 static inline void futex_init_task(struct task_struct *tsk) { }
 static inline void futex_mm_release(struct task_struct *tsk) { }
+static inline void futex_exit_done(struct task_struct *tsk) { }
 #endif
 #endif
diff --git a/include/linux/sched.h b/include/linux/sched.h
index df5f53ea2f86c..bdd41a0127d10 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1704,6 +1704,7 @@ struct task_struct {
 #endif
 	struct list_head pi_state_list;
 	struct futex_pi_state *pi_state_cache;
+	unsigned int futex_state;
 #endif
 #ifdef CONFIG_PERF_EVENTS
 	struct perf_event_context *perf_event_ctxp[perf_nr_task_contexts];
@@ -2099,7 +2100,6 @@ extern void thread_group_cputime_adjusted(struct task_struct *p, cputime_t *ut,
  * Per process flags
  */
 #define PF_EXITING	0x00000004	/* getting shut down */
-#define PF_EXITPIDONE	0x00000008	/* pi exit done on shut down */
 #define PF_VCPU		0x00000010	/* I'm a virtual CPU */
 #define PF_WQ_WORKER	0x00000020	/* I'm a workqueue worker */
 #define PF_FORKNOEXEC	0x00000040	/* forked but didn't exec */
diff --git a/kernel/exit.c b/kernel/exit.c
index 5c20a32c95392..274a3c3834a15 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -695,16 +695,7 @@ void do_exit(long code)
 	 */
 	if (unlikely(tsk->flags & PF_EXITING)) {
 		pr_alert("Fixing recursive fault but reboot is needed!\n");
-		/*
-		 * We can do this unlocked here. The futex code uses
-		 * this flag just to verify whether the pi state
-		 * cleanup has been done or not. In the worst case it
-		 * loops once more. We pretend that the cleanup was
-		 * done as there is no way to return. Either the
-		 * OWNER_DIED bit is set by now or we push the blocked
-		 * task into the wait for ever nirwana as well.
-		 */
-		tsk->flags |= PF_EXITPIDONE;
+		futex_exit_done(tsk);
 		set_current_state(TASK_UNINTERRUPTIBLE);
 		schedule();
 	}
@@ -793,12 +784,7 @@ void do_exit(long code)
 	 * Make sure we are holding no locks:
 	 */
 	debug_check_no_locks_held();
-	/*
-	 * We can do this unlocked here. The futex code uses this flag
-	 * just to verify whether the pi state cleanup has been done
-	 * or not. In the worst case it loops once more.
-	 */
-	tsk->flags |= PF_EXITPIDONE;
+	futex_exit_done(tsk);
 
 	if (tsk->io_context)
 		exit_io_context(tsk);
diff --git a/kernel/futex.c b/kernel/futex.c
index 5282b9b8d1ec1..e531789aa440a 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -1094,19 +1094,18 @@ static int attach_to_pi_owner(u32 uval, union futex_key *key,
 	}
 
 	/*
-	 * We need to look at the task state flags to figure out,
-	 * whether the task is exiting. To protect against the do_exit
-	 * change of the task flags, we do this protected by
-	 * p->pi_lock:
+	 * We need to look at the task state to figure out, whether the
+	 * task is exiting. To protect against the change of the task state
+	 * in futex_exit_release(), we do this protected by p->pi_lock:
 	 */
 	raw_spin_lock_irq(&p->pi_lock);
-	if (unlikely(p->flags & PF_EXITING)) {
+	if (unlikely(p->futex_state != FUTEX_STATE_OK)) {
 		/*
-		 * The task is on the way out. When PF_EXITPIDONE is
-		 * set, we know that the task has finished the
-		 * cleanup:
+		 * The task is on the way out. When the futex state is
+		 * FUTEX_STATE_DEAD, we know that the task has finished
+		 * the cleanup:
 		 */
-		int ret = (p->flags & PF_EXITPIDONE) ? -ESRCH : -EAGAIN;
+		int ret = (p->futex_state = FUTEX_STATE_DEAD) ? -ESRCH : -EAGAIN;
 
 		raw_spin_unlock_irq(&p->pi_lock);
 		put_task_struct(p);
-- 
2.25.1

[PATCH 04/12] exit/exec: Seperate mm_release()

[Lee Jones]

From: Thomas Gleixner <tglx@linutronix.de>

commit 4610ba7ad877fafc0a25a30c6c82015304120426 upstream.

mm_release() contains the futex exit handling. mm_release() is called from
do_exit()->exit_mm() and from exec()->exec_mm().

In the exit_mm() case PF_EXITING and the futex state is updated. In the
exec_mm() case these states are not touched.

As the futex exit code needs further protections against exit races, this
needs to be split into two functions.

Preparatory only, no functional change.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20191106224556.240518241@linutronix.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
---
 fs/exec.c             |  2 +-
 include/linux/sched.h |  6 ++++--
 kernel/exit.c         |  2 +-
 kernel/fork.c         | 12 +++++++++++-
 4 files changed, 17 insertions(+), 5 deletions(-)

diff --git a/fs/exec.c b/fs/exec.c
index 46cc0c072246d..ce111af5784be 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -875,7 +875,7 @@ static int exec_mmap(struct mm_struct *mm)
 	/* Notify parent that we're no longer interested in the old VM */
 	tsk = current;
 	old_mm = current->mm;
-	mm_release(tsk, old_mm);
+	exec_mm_release(tsk, old_mm);
 
 	if (old_mm) {
 		sync_mm_rss(old_mm);
diff --git a/include/linux/sched.h b/include/linux/sched.h
index bdd41a0127d10..aba34bba5e9e3 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -2647,8 +2647,10 @@ extern struct mm_struct *get_task_mm(struct task_struct *task);
  * succeeds.
  */
 extern struct mm_struct *mm_access(struct task_struct *task, unsigned int mode);
-/* Remove the current tasks stale references to the old mm_struct */
-extern void mm_release(struct task_struct *, struct mm_struct *);
+/* Remove the current tasks stale references to the old mm_struct on exit() */
+extern void exit_mm_release(struct task_struct *, struct mm_struct *);
+/* Remove the current tasks stale references to the old mm_struct on exec() */
+extern void exec_mm_release(struct task_struct *, struct mm_struct *);
 
 #ifdef CONFIG_HAVE_COPY_THREAD_TLS
 extern int copy_thread_tls(unsigned long, unsigned long, unsigned long,
diff --git a/kernel/exit.c b/kernel/exit.c
index 274a3c3834a15..a098d76a9877e 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -389,7 +389,7 @@ static void exit_mm(struct task_struct *tsk)
 	struct mm_struct *mm = tsk->mm;
 	struct core_state *core_state;
 
-	mm_release(tsk, mm);
+	exit_mm_release(tsk, mm);
 	if (!mm)
 		return;
 	sync_mm_rss(mm);
diff --git a/kernel/fork.c b/kernel/fork.c
index d0ab6aff5efdc..43a50072dd5b4 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -887,7 +887,7 @@ static int wait_for_vfork_done(struct task_struct *child,
  * restoring the old one. . .
  * Eric Biederman 10 January 1998
  */
-void mm_release(struct task_struct *tsk, struct mm_struct *mm)
+static void mm_release(struct task_struct *tsk, struct mm_struct *mm)
 {
 	/* Get rid of any futexes when releasing the mm */
 	futex_mm_release(tsk);
@@ -924,6 +924,16 @@ void mm_release(struct task_struct *tsk, struct mm_struct *mm)
 		complete_vfork_done(tsk);
 }
 
+void exit_mm_release(struct task_struct *tsk, struct mm_struct *mm)
+{
+	mm_release(tsk, mm);
+}
+
+void exec_mm_release(struct task_struct *tsk, struct mm_struct *mm)
+{
+	mm_release(tsk, mm);
+}
+
 /*
  * Allocate a new mm structure and copy contents from the
  * mm structure of the passed in task structure.
-- 
2.25.1

[PATCH 05/12] futex: Split futex_mm_release() for exit/exec

[Lee Jones]

From: Thomas Gleixner <tglx@linutronix.de>

commit 150d71584b12809144b8145b817e83b81158ae5f upstream.

To allow separate handling of the futex exit state in the futex exit code
for exit and exec, split futex_mm_release() into two functions and invoke
them from the corresponding exit/exec_mm_release() callsites.

Preparatory only, no functional change.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20191106224556.332094221@linutronix.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
---
 include/linux/futex.h | 6 ++++--
 kernel/fork.c         | 5 ++---
 kernel/futex.c        | 7 ++++++-
 3 files changed, 12 insertions(+), 6 deletions(-)

diff --git a/include/linux/futex.h b/include/linux/futex.h
index a0de6fe28e00b..063a5cd00d770 100644
--- a/include/linux/futex.h
+++ b/include/linux/futex.h
@@ -98,13 +98,15 @@ static inline void futex_exit_done(struct task_struct *tsk)
 	tsk->futex_state = FUTEX_STATE_DEAD;
 }
 
-void futex_mm_release(struct task_struct *tsk);
+void futex_exit_release(struct task_struct *tsk);
+void futex_exec_release(struct task_struct *tsk);
 
 long do_futex(u32 __user *uaddr, int op, u32 val, ktime_t *timeout,
 	      u32 __user *uaddr2, u32 val2, u32 val3);
 #else
 static inline void futex_init_task(struct task_struct *tsk) { }
-static inline void futex_mm_release(struct task_struct *tsk) { }
 static inline void futex_exit_done(struct task_struct *tsk) { }
+static inline void futex_exit_release(struct task_struct *tsk) { }
+static inline void futex_exec_release(struct task_struct *tsk) { }
 #endif
 #endif
diff --git a/kernel/fork.c b/kernel/fork.c
index 43a50072dd5b4..2bd4c38efa095 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -889,9 +889,6 @@ static int wait_for_vfork_done(struct task_struct *child,
  */
 static void mm_release(struct task_struct *tsk, struct mm_struct *mm)
 {
-	/* Get rid of any futexes when releasing the mm */
-	futex_mm_release(tsk);
-
 	uprobe_free_utask(tsk);
 
 	/* Get rid of any cached register state */
@@ -926,11 +923,13 @@ static void mm_release(struct task_struct *tsk, struct mm_struct *mm)
 
 void exit_mm_release(struct task_struct *tsk, struct mm_struct *mm)
 {
+	futex_exit_release(tsk);
 	mm_release(tsk, mm);
 }
 
 void exec_mm_release(struct task_struct *tsk, struct mm_struct *mm)
 {
+	futex_exec_release(tsk);
 	mm_release(tsk, mm);
 }
 
diff --git a/kernel/futex.c b/kernel/futex.c
index e531789aa440a..32a606b605cbb 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -3234,7 +3234,7 @@ static void exit_robust_list(struct task_struct *curr)
 				   curr, pip);
 }
 
-void futex_mm_release(struct task_struct *tsk)
+void futex_exec_release(struct task_struct *tsk)
 {
 	if (unlikely(tsk->robust_list)) {
 		exit_robust_list(tsk);
@@ -3252,6 +3252,11 @@ void futex_mm_release(struct task_struct *tsk)
 		exit_pi_state_list(tsk);
 }
 
+void futex_exit_release(struct task_struct *tsk)
+{
+	futex_exec_release(tsk);
+}
+
 long do_futex(u32 __user *uaddr, int op, u32 val, ktime_t *timeout,
 		u32 __user *uaddr2, u32 val2, u32 val3)
 {
-- 
2.25.1

[PATCH 06/12] futex: Set task::futex_state to DEAD right after handling futex exit

[Lee Jones]

From: Thomas Gleixner <tglx@linutronix.de>

commit f24f22435dcc11389acc87e5586239c1819d217c upstream.

Setting task::futex_state in do_exit() is rather arbitrarily placed for no
reason. Move it into the futex code.

Note, this is only done for the exit cleanup as the exec cleanup cannot set
the state to FUTEX_STATE_DEAD because the task struct is still in active
use.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20191106224556.439511191@linutronix.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
---
 kernel/exit.c  | 1 -
 kernel/futex.c | 1 +
 2 files changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/exit.c b/kernel/exit.c
index a098d76a9877e..b39f4b3c0f37c 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -784,7 +784,6 @@ void do_exit(long code)
 	 * Make sure we are holding no locks:
 	 */
 	debug_check_no_locks_held();
-	futex_exit_done(tsk);
 
 	if (tsk->io_context)
 		exit_io_context(tsk);
diff --git a/kernel/futex.c b/kernel/futex.c
index 32a606b605cbb..f85635ff2fce1 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -3255,6 +3255,7 @@ void futex_exec_release(struct task_struct *tsk)
 void futex_exit_release(struct task_struct *tsk)
 {
 	futex_exec_release(tsk);
+	futex_exit_done(tsk);
 }
 
 long do_futex(u32 __user *uaddr, int op, u32 val, ktime_t *timeout,
-- 
2.25.1

[PATCH 07/12] futex: Mark the begin of futex exit explicitly

[Lee Jones]

From: Thomas Gleixner <tglx@linutronix.de>

commit 18f694385c4fd77a09851fd301236746ca83f3cb upstream.

Instead of relying on PF_EXITING use an explicit state for the futex exit
and set it in the futex exit function. This moves the smp barrier and the
lock/unlock serialization into the futex code.

As with the DEAD state this is restricted to the exit path as exec
continues to use the same task struct.

This allows to simplify that logic in a next step.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20191106224556.539409004@linutronix.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
---
 include/linux/futex.h | 31 +++----------------------------
 kernel/exit.c         |  8 +-------
 kernel/futex.c        | 37 ++++++++++++++++++++++++++++++++++++-
 3 files changed, 40 insertions(+), 36 deletions(-)

diff --git a/include/linux/futex.h b/include/linux/futex.h
index 063a5cd00d770..805508373fcea 100644
--- a/include/linux/futex.h
+++ b/include/linux/futex.h
@@ -57,6 +57,7 @@ union futex_key {
 #ifdef CONFIG_FUTEX
 enum {
 	FUTEX_STATE_OK,
+	FUTEX_STATE_EXITING,
 	FUTEX_STATE_DEAD,
 };
 
@@ -71,33 +72,7 @@ static inline void futex_init_task(struct task_struct *tsk)
 	tsk->futex_state = FUTEX_STATE_OK;
 }
 
-/**
- * futex_exit_done - Sets the tasks futex state to FUTEX_STATE_DEAD
- * @tsk:	task to set the state on
- *
- * Set the futex exit state of the task lockless. The futex waiter code
- * observes that state when a task is exiting and loops until the task has
- * actually finished the futex cleanup. The worst case for this is that the
- * waiter runs through the wait loop until the state becomes visible.
- *
- * This has two callers:
- *
- * - futex_mm_release() after the futex exit cleanup has been done
- *
- * - do_exit() from the recursive fault handling path.
- *
- * In case of a recursive fault this is best effort. Either the futex exit
- * code has run already or not. If the OWNER_DIED bit has been set on the
- * futex then the waiter can take it over. If not, the problem is pushed
- * back to user space. If the futex exit code did not run yet, then an
- * already queued waiter might block forever, but there is nothing which
- * can be done about that.
- */
-static inline void futex_exit_done(struct task_struct *tsk)
-{
-	tsk->futex_state = FUTEX_STATE_DEAD;
-}
-
+void futex_exit_recursive(struct task_struct *tsk);
 void futex_exit_release(struct task_struct *tsk);
 void futex_exec_release(struct task_struct *tsk);
 
@@ -105,7 +80,7 @@ long do_futex(u32 __user *uaddr, int op, u32 val, ktime_t *timeout,
 	      u32 __user *uaddr2, u32 val2, u32 val3);
 #else
 static inline void futex_init_task(struct task_struct *tsk) { }
-static inline void futex_exit_done(struct task_struct *tsk) { }
+static inline void futex_exit_recursive(struct task_struct *tsk) { }
 static inline void futex_exit_release(struct task_struct *tsk) { }
 static inline void futex_exec_release(struct task_struct *tsk) { }
 #endif
diff --git a/kernel/exit.c b/kernel/exit.c
index b39f4b3c0f37c..8d3c268fb1b8d 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -695,18 +695,12 @@ void do_exit(long code)
 	 */
 	if (unlikely(tsk->flags & PF_EXITING)) {
 		pr_alert("Fixing recursive fault but reboot is needed!\n");
-		futex_exit_done(tsk);
+		futex_exit_recursive(tsk);
 		set_current_state(TASK_UNINTERRUPTIBLE);
 		schedule();
 	}
 
 	exit_signals(tsk);  /* sets PF_EXITING */
-	/*
-	 * tsk->flags are checked in the futex code to protect against
-	 * an exiting task cleaning up the robust pi futexes.
-	 */
-	smp_mb();
-	raw_spin_unlock_wait(&tsk->pi_lock);
 
 	if (unlikely(in_atomic())) {
 		pr_info("note: %s[%d] exited with preempt_count %d\n",
diff --git a/kernel/futex.c b/kernel/futex.c
index f85635ff2fce1..5bd3afee4e139 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -3252,10 +3252,45 @@ void futex_exec_release(struct task_struct *tsk)
 		exit_pi_state_list(tsk);
 }
 
+/**
+ * futex_exit_recursive - Set the tasks futex state to FUTEX_STATE_DEAD
+ * @tsk:	task to set the state on
+ *
+ * Set the futex exit state of the task lockless. The futex waiter code
+ * observes that state when a task is exiting and loops until the task has
+ * actually finished the futex cleanup. The worst case for this is that the
+ * waiter runs through the wait loop until the state becomes visible.
+ *
+ * This is called from the recursive fault handling path in do_exit().
+ *
+ * This is best effort. Either the futex exit code has run already or
+ * not. If the OWNER_DIED bit has been set on the futex then the waiter can
+ * take it over. If not, the problem is pushed back to user space. If the
+ * futex exit code did not run yet, then an already queued waiter might
+ * block forever, but there is nothing which can be done about that.
+ */
+void futex_exit_recursive(struct task_struct *tsk)
+{
+	tsk->futex_state = FUTEX_STATE_DEAD;
+}
+
 void futex_exit_release(struct task_struct *tsk)
 {
+	tsk->futex_state = FUTEX_STATE_EXITING;
+	/*
+	 * Ensure that all new tsk->pi_lock acquisitions must observe
+	 * FUTEX_STATE_EXITING. Serializes against attach_to_pi_owner().
+	 */
+	smp_mb();
+	/*
+	 * Ensure that we must observe the pi_state in exit_pi_state_list().
+	 */
+	raw_spin_lock_irq(&tsk->pi_lock);
+	raw_spin_unlock_irq(&tsk->pi_lock);
+
 	futex_exec_release(tsk);
-	futex_exit_done(tsk);
+
+	tsk->futex_state = FUTEX_STATE_DEAD;
 }
 
 long do_futex(u32 __user *uaddr, int op, u32 val, ktime_t *timeout,
-- 
2.25.1

[PATCH 08/12] futex: Sanitize exit state handling

[Lee Jones]

From: Thomas Gleixner <tglx@linutronix.de>

commit 4a8e991b91aca9e20705d434677ac013974e0e30 upstream.

Instead of having a smp_mb() and an empty lock/unlock of task::pi_lock move
the state setting into to the lock section.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20191106224556.645603214@linutronix.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
---
 kernel/futex.c | 17 ++++++++++-------
 1 file changed, 10 insertions(+), 7 deletions(-)

diff --git a/kernel/futex.c b/kernel/futex.c
index 5bd3afee4e139..0efcb55455c2a 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -3276,16 +3276,19 @@ void futex_exit_recursive(struct task_struct *tsk)
 
 void futex_exit_release(struct task_struct *tsk)
 {
-	tsk->futex_state = FUTEX_STATE_EXITING;
-	/*
-	 * Ensure that all new tsk->pi_lock acquisitions must observe
-	 * FUTEX_STATE_EXITING. Serializes against attach_to_pi_owner().
-	 */
-	smp_mb();
 	/*
-	 * Ensure that we must observe the pi_state in exit_pi_state_list().
+	 * Switch the state to FUTEX_STATE_EXITING under tsk->pi_lock.
+	 *
+	 * This ensures that all subsequent checks of tsk->futex_state in
+	 * attach_to_pi_owner() must observe FUTEX_STATE_EXITING with
+	 * tsk->pi_lock held.
+	 *
+	 * It guarantees also that a pi_state which was queued right before
+	 * the state change under tsk->pi_lock by a concurrent waiter must
+	 * be observed in exit_pi_state_list().
 	 */
 	raw_spin_lock_irq(&tsk->pi_lock);
+	tsk->futex_state = FUTEX_STATE_EXITING;
 	raw_spin_unlock_irq(&tsk->pi_lock);
 
 	futex_exec_release(tsk);
-- 
2.25.1

[PATCH 09/12] futex: Provide state handling for exec() as well

[Lee Jones]

From: Thomas Gleixner <tglx@linutronix.de>

commit af8cbda2cfcaa5515d61ec500498d46e9a8247e2 upstream.

exec() attempts to handle potentially held futexes gracefully by running
the futex exit handling code like exit() does.

The current implementation has no protection against concurrent incoming
waiters. The reason is that the futex state cannot be set to
FUTEX_STATE_DEAD after the cleanup because the task struct is still active
and just about to execute the new binary.

While its arguably buggy when a task holds a futex over exec(), for
consistency sake the state handling can at least cover the actual futex
exit cleanup section. This provides state consistency protection accross
the cleanup. As the futex state of the task becomes FUTEX_STATE_OK after the
cleanup has been finished, this cannot prevent subsequent attempts to
attach to the task in case that the cleanup was not successfull in mopping
up all leftovers.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20191106224556.753355618@linutronix.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
---
 kernel/futex.c | 38 ++++++++++++++++++++++++++++++++++----
 1 file changed, 34 insertions(+), 4 deletions(-)

diff --git a/kernel/futex.c b/kernel/futex.c
index 0efcb55455c2a..feef5ce071aa5 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -3234,7 +3234,7 @@ static void exit_robust_list(struct task_struct *curr)
 				   curr, pip);
 }
 
-void futex_exec_release(struct task_struct *tsk)
+static void futex_cleanup(struct task_struct *tsk)
 {
 	if (unlikely(tsk->robust_list)) {
 		exit_robust_list(tsk);
@@ -3274,7 +3274,7 @@ void futex_exit_recursive(struct task_struct *tsk)
 	tsk->futex_state = FUTEX_STATE_DEAD;
 }
 
-void futex_exit_release(struct task_struct *tsk)
+static void futex_cleanup_begin(struct task_struct *tsk)
 {
 	/*
 	 * Switch the state to FUTEX_STATE_EXITING under tsk->pi_lock.
@@ -3290,10 +3290,40 @@ void futex_exit_release(struct task_struct *tsk)
 	raw_spin_lock_irq(&tsk->pi_lock);
 	tsk->futex_state = FUTEX_STATE_EXITING;
 	raw_spin_unlock_irq(&tsk->pi_lock);
+}
 
-	futex_exec_release(tsk);
+static void futex_cleanup_end(struct task_struct *tsk, int state)
+{
+	/*
+	 * Lockless store. The only side effect is that an observer might
+	 * take another loop until it becomes visible.
+	 */
+	tsk->futex_state = state;
+}
 
-	tsk->futex_state = FUTEX_STATE_DEAD;
+void futex_exec_release(struct task_struct *tsk)
+{
+	/*
+	 * The state handling is done for consistency, but in the case of
+	 * exec() there is no way to prevent futher damage as the PID stays
+	 * the same. But for the unlikely and arguably buggy case that a
+	 * futex is held on exec(), this provides at least as much state
+	 * consistency protection which is possible.
+	 */
+	futex_cleanup_begin(tsk);
+	futex_cleanup(tsk);
+	/*
+	 * Reset the state to FUTEX_STATE_OK. The task is alive and about
+	 * exec a new binary.
+	 */
+	futex_cleanup_end(tsk, FUTEX_STATE_OK);
+}
+
+void futex_exit_release(struct task_struct *tsk)
+{
+	futex_cleanup_begin(tsk);
+	futex_cleanup(tsk);
+	futex_cleanup_end(tsk, FUTEX_STATE_DEAD);
 }
 
 long do_futex(u32 __user *uaddr, int op, u32 val, ktime_t *timeout,
-- 
2.25.1

[PATCH 10/12] futex: Add mutex around futex exit

[Lee Jones]

From: Thomas Gleixner <tglx@linutronix.de>

commit 3f186d974826847a07bc7964d79ec4eded475ad9 upstream.

The mutex will be used in subsequent changes to replace the busy looping of
a waiter when the futex owner is currently executing the exit cleanup to
prevent a potential live lock.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20191106224556.845798895@linutronix.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
---
 include/linux/futex.h |  1 +
 include/linux/sched.h |  1 +
 kernel/futex.c        | 16 ++++++++++++++++
 3 files changed, 18 insertions(+)

diff --git a/include/linux/futex.h b/include/linux/futex.h
index 805508373fcea..0f294ae63c78c 100644
--- a/include/linux/futex.h
+++ b/include/linux/futex.h
@@ -70,6 +70,7 @@ static inline void futex_init_task(struct task_struct *tsk)
 	INIT_LIST_HEAD(&tsk->pi_state_list);
 	tsk->pi_state_cache = NULL;
 	tsk->futex_state = FUTEX_STATE_OK;
+	mutex_init(&tsk->futex_exit_mutex);
 }
 
 void futex_exit_recursive(struct task_struct *tsk);
diff --git a/include/linux/sched.h b/include/linux/sched.h
index aba34bba5e9e3..8c10e97f94fea 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1704,6 +1704,7 @@ struct task_struct {
 #endif
 	struct list_head pi_state_list;
 	struct futex_pi_state *pi_state_cache;
+	struct mutex futex_exit_mutex;
 	unsigned int futex_state;
 #endif
 #ifdef CONFIG_PERF_EVENTS
diff --git a/kernel/futex.c b/kernel/futex.c
index feef5ce071aa5..d21b151216aa3 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -3271,11 +3271,22 @@ static void futex_cleanup(struct task_struct *tsk)
  */
 void futex_exit_recursive(struct task_struct *tsk)
 {
+	/* If the state is FUTEX_STATE_EXITING then futex_exit_mutex is held */
+	if (tsk->futex_state == FUTEX_STATE_EXITING)
+		mutex_unlock(&tsk->futex_exit_mutex);
 	tsk->futex_state = FUTEX_STATE_DEAD;
 }
 
 static void futex_cleanup_begin(struct task_struct *tsk)
 {
+	/*
+	 * Prevent various race issues against a concurrent incoming waiter
+	 * including live locks by forcing the waiter to block on
+	 * tsk->futex_exit_mutex when it observes FUTEX_STATE_EXITING in
+	 * attach_to_pi_owner().
+	 */
+	mutex_lock(&tsk->futex_exit_mutex);
+
 	/*
 	 * Switch the state to FUTEX_STATE_EXITING under tsk->pi_lock.
 	 *
@@ -3299,6 +3310,11 @@ static void futex_cleanup_end(struct task_struct *tsk, int state)
 	 * take another loop until it becomes visible.
 	 */
 	tsk->futex_state = state;
+	/*
+	 * Drop the exit protection. This unblocks waiters which observed
+	 * FUTEX_STATE_EXITING to reevaluate the state.
+	 */
+	mutex_unlock(&tsk->futex_exit_mutex);
 }
 
 void futex_exec_release(struct task_struct *tsk)
-- 
2.25.1

[PATCH 11/12] futex: Provide distinct return value when owner is exiting

[Lee Jones]

From: Thomas Gleixner <tglx@linutronix.de>

commit ac31c7ff8624409ba3c4901df9237a616c187a5d upstream.

attach_to_pi_owner() returns -EAGAIN for various cases:

 - Owner task is exiting
 - Futex value has changed

The caller drops the held locks (hash bucket, mmap_sem) and retries the
operation. In case of the owner task exiting this can result in a live
lock.

As a preparatory step for seperating those cases, provide a distinct return
value (EBUSY) for the owner exiting case.

No functional change.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20191106224556.935606117@linutronix.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
---
 kernel/futex.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/kernel/futex.c b/kernel/futex.c
index d21b151216aa3..32d799b9bd205 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -1909,12 +1909,13 @@ retry_private:
 			if (!ret)
 				goto retry;
 			goto out;
+		case -EBUSY:
 		case -EAGAIN:
 			/*
 			 * Two reasons for this:
-			 * - Owner is exiting and we just wait for the
+			 * - EBUSY: Owner is exiting and we just wait for the
 			 *   exit to complete.
-			 * - The user space value changed.
+			 * - EAGAIN: The user space value changed.
 			 */
 			free_pi_state(pi_state);
 			pi_state = NULL;
@@ -2580,12 +2581,13 @@ retry_private:
 			goto out_unlock_put_key;
 		case -EFAULT:
 			goto uaddr_faulted;
+		case -EBUSY:
 		case -EAGAIN:
 			/*
 			 * Two reasons for this:
-			 * - Task is exiting and we just wait for the
+			 * - EBUSY: Task is exiting and we just wait for the
 			 *   exit to complete.
-			 * - The user space value changed.
+			 * - EAGAIN: The user space value changed.
 			 */
 			queue_unlock(hb);
 			put_futex_key(&q.key);
-- 
2.25.1

[PATCH 12/12] futex: Prevent exit livelock

[Lee Jones]

From: Thomas Gleixner <tglx@linutronix.de>

commit 3ef240eaff36b8119ac9e2ea17cbf41179c930ba upstream.

Oleg provided the following test case:

int main(void)
{
	struct sched_param sp = {};

	sp.sched_priority = 2;
	assert(sched_setscheduler(0, SCHED_FIFO, &sp) == 0);

	int lock = vfork();
	if (!lock) {
		sp.sched_priority = 1;
		assert(sched_setscheduler(0, SCHED_FIFO, &sp) == 0);
		_exit(0);
	}

	syscall(__NR_futex, &lock, FUTEX_LOCK_PI, 0,0,0);
	return 0;
}

This creates an unkillable RT process spinning in futex_lock_pi() on a UP
machine or if the process is affine to a single CPU. The reason is:

 parent	    	    			child

  set FIFO prio 2

  vfork()			->	set FIFO prio 1
   implies wait_for_child()	 	sched_setscheduler(...)
 			   		exit()
					do_exit()
 					....
					mm_release()
					  tsk->futex_state = FUTEX_STATE_EXITING;
					  exit_futex(); (NOOP in this case)
					  complete() --> wakes parent
  sys_futex()
    loop infinite because
    tsk->futex_state == FUTEX_STATE_EXITING

The same problem can happen just by regular preemption as well:

  task holds futex
  ...
  do_exit()
    tsk->futex_state = FUTEX_STATE_EXITING;

  --> preemption (unrelated wakeup of some other higher prio task, e.g. timer)

  switch_to(other_task)

  return to user
  sys_futex()
	loop infinite as above

Just for the fun of it the futex exit cleanup could trigger the wakeup
itself before the task sets its futex state to DEAD.

To cure this, the handling of the exiting owner is changed so:

   - A refcount is held on the task

   - The task pointer is stored in a caller visible location

   - The caller drops all locks (hash bucket, mmap_sem) and blocks
     on task::futex_exit_mutex. When the mutex is acquired then
     the exiting task has completed the cleanup and the state
     is consistent and can be reevaluated.

This is not a pretty solution, but there is no choice other than returning
an error code to user space, which would break the state consistency
guarantee and open another can of problems including regressions.

For stable backports the preparatory commits ac31c7ff8624 .. ba31c1a48538
are required as well, but for anything older than 5.3.y the backports are
going to be provided when this hits mainline as the other dependencies for
those kernels are definitely not stable material.

Fixes: 778e9a9c3e71 ("pi-futex: fix exit races and locking problems")
Reported-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Stable Team <stable@vger.kernel.org>
Link: https://lkml.kernel.org/r/20191106224557.041676471@linutronix.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
---
 kernel/futex.c | 106 ++++++++++++++++++++++++++++++++++++++++++-------
 1 file changed, 91 insertions(+), 15 deletions(-)

diff --git a/kernel/futex.c b/kernel/futex.c
index 32d799b9bd205..f1990e2a51e5a 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -1067,12 +1067,43 @@ out_state:
 	return 0;
 }
 
+/**
+ * wait_for_owner_exiting - Block until the owner has exited
+ * @exiting:	Pointer to the exiting task
+ *
+ * Caller must hold a refcount on @exiting.
+ */
+static void wait_for_owner_exiting(int ret, struct task_struct *exiting)
+{
+	if (ret != -EBUSY) {
+		WARN_ON_ONCE(exiting);
+		return;
+	}
+
+	if (WARN_ON_ONCE(ret == -EBUSY && !exiting))
+		return;
+
+	mutex_lock(&exiting->futex_exit_mutex);
+	/*
+	 * No point in doing state checking here. If the waiter got here
+	 * while the task was in exec()->exec_futex_release() then it can
+	 * have any FUTEX_STATE_* value when the waiter has acquired the
+	 * mutex. OK, if running, EXITING or DEAD if it reached exit()
+	 * already. Highly unlikely and not a problem. Just one more round
+	 * through the futex maze.
+	 */
+	mutex_unlock(&exiting->futex_exit_mutex);
+
+	put_task_struct(exiting);
+}
+
 /*
  * Lookup the task for the TID provided from user space and attach to
  * it after doing proper sanity checks.
  */
 static int attach_to_pi_owner(u32 uval, union futex_key *key,
-			      struct futex_pi_state **ps)
+			      struct futex_pi_state **ps,
+			      struct task_struct **exiting)
 {
 	pid_t pid = uval & FUTEX_TID_MASK;
 	struct futex_pi_state *pi_state;
@@ -1108,7 +1139,19 @@ static int attach_to_pi_owner(u32 uval, union futex_key *key,
 		int ret = (p->futex_state = FUTEX_STATE_DEAD) ? -ESRCH : -EAGAIN;
 
 		raw_spin_unlock_irq(&p->pi_lock);
-		put_task_struct(p);
+		/*
+		 * If the owner task is between FUTEX_STATE_EXITING and
+		 * FUTEX_STATE_DEAD then store the task pointer and keep
+		 * the reference on the task struct. The calling code will
+		 * drop all locks, wait for the task to reach
+		 * FUTEX_STATE_DEAD and then drop the refcount. This is
+		 * required to prevent a live lock when the current task
+		 * preempted the exiting task between the two states.
+		 */
+		if (ret == -EBUSY)
+			*exiting = p;
+		else
+			put_task_struct(p);
 		return ret;
 	}
 
@@ -1139,7 +1182,8 @@ static int attach_to_pi_owner(u32 uval, union futex_key *key,
 }
 
 static int lookup_pi_state(u32 uval, struct futex_hash_bucket *hb,
-			   union futex_key *key, struct futex_pi_state **ps)
+			   union futex_key *key, struct futex_pi_state **ps,
+			   struct task_struct **exiting)
 {
 	struct futex_q *match = futex_top_waiter(hb, key);
 
@@ -1154,7 +1198,7 @@ static int lookup_pi_state(u32 uval, struct futex_hash_bucket *hb,
 	 * We are the first waiter - try to look up the owner based on
 	 * @uval and attach to it.
 	 */
-	return attach_to_pi_owner(uval, key, ps);
+	return attach_to_pi_owner(uval, key, ps, exiting);
 }
 
 static int lock_pi_update_atomic(u32 __user *uaddr, u32 uval, u32 newval)
@@ -1180,6 +1224,8 @@ static int lock_pi_update_atomic(u32 __user *uaddr, u32 uval, u32 newval)
  *			lookup
  * @task:		the task to perform the atomic lock work for.  This will
  *			be "current" except in the case of requeue pi.
+ * @exiting:		Pointer to store the task pointer of the owner task
+ *			which is in the middle of exiting
  * @set_waiters:	force setting the FUTEX_WAITERS bit (1) or not (0)
  *
  * Return:
@@ -1188,11 +1234,17 @@ static int lock_pi_update_atomic(u32 __user *uaddr, u32 uval, u32 newval)
  * <0 - error
  *
  * The hb->lock and futex_key refs shall be held by the caller.
+ *
+ * @exiting is only set when the return value is -EBUSY. If so, this holds
+ * a refcount on the exiting task on return and the caller needs to drop it
+ * after waiting for the exit to complete.
  */
 static int futex_lock_pi_atomic(u32 __user *uaddr, struct futex_hash_bucket *hb,
 				union futex_key *key,
 				struct futex_pi_state **ps,
-				struct task_struct *task, int set_waiters)
+				struct task_struct *task,
+				struct task_struct **exiting,
+				int set_waiters)
 {
 	u32 uval, newval, vpid = task_pid_vnr(task);
 	struct futex_q *match;
@@ -1262,7 +1314,7 @@ static int futex_lock_pi_atomic(u32 __user *uaddr, struct futex_hash_bucket *hb,
 	 * attach to the owner. If that fails, no harm done, we only
 	 * set the FUTEX_WAITERS bit in the user space variable.
 	 */
-	return attach_to_pi_owner(uval, key, ps);
+	return attach_to_pi_owner(uval, key, ps, exiting);
 }
 
 /**
@@ -1688,6 +1740,8 @@ void requeue_pi_wake_futex(struct futex_q *q, union futex_key *key,
  * @key1:		the from futex key
  * @key2:		the to futex key
  * @ps:			address to store the pi_state pointer
+ * @exiting:		Pointer to store the task pointer of the owner task
+ *			which is in the middle of exiting
  * @set_waiters:	force setting the FUTEX_WAITERS bit (1) or not (0)
  *
  * Try and get the lock on behalf of the top waiter if we can do it atomically.
@@ -1695,16 +1749,20 @@ void requeue_pi_wake_futex(struct futex_q *q, union futex_key *key,
  * then direct futex_lock_pi_atomic() to force setting the FUTEX_WAITERS bit.
  * hb1 and hb2 must be held by the caller.
  *
+ * @exiting is only set when the return value is -EBUSY. If so, this holds
+ * a refcount on the exiting task on return and the caller needs to drop it
+ * after waiting for the exit to complete.
+ *
  * Return:
  *  0 - failed to acquire the lock atomically;
  * >0 - acquired the lock, return value is vpid of the top_waiter
  * <0 - error
  */
-static int futex_proxy_trylock_atomic(u32 __user *pifutex,
-				 struct futex_hash_bucket *hb1,
-				 struct futex_hash_bucket *hb2,
-				 union futex_key *key1, union futex_key *key2,
-				 struct futex_pi_state **ps, int set_waiters)
+static int
+futex_proxy_trylock_atomic(u32 __user *pifutex, struct futex_hash_bucket *hb1,
+			   struct futex_hash_bucket *hb2, union futex_key *key1,
+			   union futex_key *key2, struct futex_pi_state **ps,
+			   struct task_struct **exiting, int set_waiters)
 {
 	struct futex_q *top_waiter = NULL;
 	u32 curval;
@@ -1741,7 +1799,7 @@ static int futex_proxy_trylock_atomic(u32 __user *pifutex,
 	 */
 	vpid = task_pid_vnr(top_waiter->task);
 	ret = futex_lock_pi_atomic(pifutex, hb2, key2, ps, top_waiter->task,
-				   set_waiters);
+				   exiting, set_waiters);
 	if (ret == 1) {
 		requeue_pi_wake_futex(top_waiter, key2, hb2);
 		return vpid;
@@ -1861,6 +1919,8 @@ retry_private:
 	}
 
 	if (requeue_pi && (task_count - nr_wake < nr_requeue)) {
+		struct task_struct *exiting = NULL;
+
 		/*
 		 * Attempt to acquire uaddr2 and wake the top waiter. If we
 		 * intend to requeue waiters, force setting the FUTEX_WAITERS
@@ -1868,7 +1928,8 @@ retry_private:
 		 * faults rather in the requeue loop below.
 		 */
 		ret = futex_proxy_trylock_atomic(uaddr2, hb1, hb2, &key1,
-						 &key2, &pi_state, nr_requeue);
+						 &key2, &pi_state,
+						 &exiting, nr_requeue);
 
 		/*
 		 * At this point the top_waiter has either taken uaddr2 or is
@@ -1892,7 +1953,8 @@ retry_private:
 			 * rereading and handing potential crap to
 			 * lookup_pi_state.
 			 */
-			ret = lookup_pi_state(ret, hb2, &key2, &pi_state);
+			ret = lookup_pi_state(ret, hb2, &key2,
+					      &pi_state, &exiting);
 		}
 
 		switch (ret) {
@@ -1923,6 +1985,12 @@ retry_private:
 			hb_waiters_dec(hb2);
 			put_futex_key(&key2);
 			put_futex_key(&key1);
+			/*
+			 * Handle the case where the owner is in the middle of
+			 * exiting. Wait for the exit to complete otherwise
+			 * this task might loop forever, aka. live lock.
+			 */
+			wait_for_owner_exiting(ret, exiting);
 			cond_resched();
 			goto retry;
 		default:
@@ -2545,6 +2613,7 @@ static int futex_lock_pi(u32 __user *uaddr, unsigned int flags,
 			 ktime_t *time, int trylock)
 {
 	struct hrtimer_sleeper timeout, *to = NULL;
+	struct task_struct *exiting = NULL;
 	struct futex_hash_bucket *hb;
 	struct futex_q q = futex_q_init;
 	int res, ret;
@@ -2568,7 +2637,8 @@ retry:
 retry_private:
 	hb = queue_lock(&q);
 
-	ret = futex_lock_pi_atomic(uaddr, hb, &q.key, &q.pi_state, current, 0);
+	ret = futex_lock_pi_atomic(uaddr, hb, &q.key, &q.pi_state, current,
+				   &exiting, 0);
 	if (unlikely(ret)) {
 		/*
 		 * Atomic work succeeded and we got the lock,
@@ -2591,6 +2661,12 @@ retry_private:
 			 */
 			queue_unlock(hb);
 			put_futex_key(&q.key);
+			/*
+			 * Handle the case where the owner is in the middle of
+			 * exiting. Wait for the exit to complete otherwise
+			 * this task might loop forever, aka. live lock.
+			 */
+			wait_for_owner_exiting(ret, exiting);
 			cond_resched();
 			goto retry;
 		default:
-- 
2.25.1

Re: [PATCH 4.4 00/12] Futex back-port from v4.9

[Greg KH]

On Mon, Feb 01, 2021 at 03:12:02PM +0000, Lee Jones wrote:
> Ref: https://git.kernel.org/pub/scm/linux/kernel/git/stable/stable-queue.git/tree/pending/futex_issues.txt
> 
> This set required 1 additional patch dragged back from v4.14 to avoid build errors.
> 

Many thanks for these, now all queued up!

greg k-h

Re: [PATCH 4.4 00/12] Futex back-port from v4.9

[Sebastian Gottschall]

if CONFIG_FUTEX is not set

kernel/fork.c: In function 'exit_mm_release':
kernel/fork.c:1121:2: error: implicit declaration of function 
'futex_exit_release'; did you mean 'mutex_release'? 
[-Werror=implicit-function-declaration]
 1121 |  futex_exit_release(tsk);
      |  ^~~~~~~~~~~~~~~~~~
      |  mutex_release
kernel/fork.c: In function 'exec_mm_release':
kernel/fork.c:1127:2: error: implicit declaration of function 
'futex_exec_release'; did you mean 'mutex_release'? 
[-Werror=implicit-function-declaration]
 1127 |  futex_exec_release(tsk);
      |  ^~~~~~~~~~~~~~~~~~
      |  mutex_release
kernel/fork.c: In function 'copy_process':
kernel/fork.c:1705:2: error: implicit declaration of function 
'futex_init_task'; did you mean 'rt_mutex_init_task'? 
[-Werror=implicit-function-declaration]
 1705 |  futex_init_task(p);
      |  ^~~~~~~~~~~~~~~
      |  rt_mutex_init_task
cc1: some warnings being treated as errors
make[2]: *** [scripts/Makefile.build:304: kernel/fork.o] Error 1
make[2]: *** Waiting for unfinished jobs....
  CC      mm/mempool.o
  CC      fs/open.o
kernel/exit.c: In function 'do_exit':
kernel/exit.c:788:3: error: implicit declaration of function 
'futex_exit_recursive' [-Werror=implicit-function-declaration]
  788 |   futex_exit_recursive(tsk);
      |   ^~~~~~~~~~~~~~~~~~~~
cc1: some warnings being treated as errors

Am 01.02.2021 um 16:24 schrieb Greg KH:
> On Mon, Feb 01, 2021 at 03:12:02PM +0000, Lee Jones wrote:
>> Ref: https://git.kernel.org/pub/scm/linux/kernel/git/stable/stable-queue.git/tree/pending/futex_issues.txt
>>
>> This set required 1 additional patch dragged back from v4.14 to avoid build errors.
>>
> Many thanks for these, now all queued up!
>
> greg k-h
>

Re: [PATCH 4.4 00/12] Futex back-port from v4.9

[Sebastian Gottschall]

forget the last email. my fault while merging

Am 01.02.2021 um 16:24 schrieb Greg KH:
> On Mon, Feb 01, 2021 at 03:12:02PM +0000, Lee Jones wrote:
>> Ref: https://git.kernel.org/pub/scm/linux/kernel/git/stable/stable-queue.git/tree/pending/futex_issues.txt
>>
>> This set required 1 additional patch dragged back from v4.14 to avoid build errors.
>>
> Many thanks for these, now all queued up!
>
> greg k-h
>