FAILED: patch "[PATCH] futex: Fix mm reuse handling for FUT_OFF_MMSHARED" failed to apply to 5.15-stable tree

FAILED: patch "[PATCH] futex: Fix mm reuse handling for FUT_OFF_MMSHARED" failed to apply to 5.15-stable tree

[gregkh]

The patch below does not apply to the 5.15-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@vger.kernel.org>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-5.15.y
git checkout FETCH_HEAD
git cherry-pick -x 3267512ad1c2cc0c7768c5b014dc2cceb398a801
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<stable@vger.kernel.org>' --in-reply-to '2026102125-action-scroll-7041@gregkh' --subject-prefix 'PATCH 5.15.y' 'HEAD^..'

Possible dependencies:



thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 3267512ad1c2cc0c7768c5b014dc2cceb398a801 Mon Sep 17 00:00:00 2001
From: Jann Horn <jannh@google.com>
Date: Tue, 29 Sep 2026 18:31:32 +0200
Subject: [PATCH] futex: Fix mm reuse handling for FUT_OFF_MMSHARED

A FUT_OFF_MMSHARED futex is a shared futex that refers to an MM.
It is possible for a process to wait on a shared futex with a different MM
because a FUT_OFF_INODE waiter can be requeued onto a FUT_OFF_MMSHARED
futex by another process.
This can cause FUT_OFF_MMSHARED waiters on a freed MM to consume
wakeups intended for a newly allocated MM at the same address.

Fix it by keying FUT_OFF_MMSHARED using a unique 64-bit per-MM ID.
Leave private futexes as before to avoid influencing the performance of the
hotpath.

(Multi-threaded processes typically implicitly use FUT_OFF_MMSHARED by
setting clear_child_tid such that it points into anonymous memory, which
causes mm_release() in a multi-threaded mm to perform FUTEX_WAKE.)

Fixes: 222993395ed3 ("futex: Remove pointless mmgrap() + mmdrop()")
Closes: https://lore.kernel.org/r/CAG48ez0dLBpc3QtbAhMMVNHwHL94iHh2G+h-=BVFR4dDuzZr1g@mail.gmail.com/
Signed-off-by: Jann Horn <jannh@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20260929-futex-mmshared-fix-v1-1-262b1ffeb3d0@google.com
Cc: stable@vger.kernel.org

diff --git a/include/linux/futex.h b/include/linux/futex.h
index 18ed18d5cbc1..a895d8c69d66 100644
--- a/include/linux/futex.h
+++ b/include/linux/futex.h
@@ -23,28 +23,32 @@ struct task_struct;
  *  01 : Shared futex (PTHREAD_PROCESS_SHARED)
  *	mapped on a file (reference on the underlying inode)
  *  10 : Shared futex (PTHREAD_PROCESS_SHARED)
- *       (but private mapping on an mm, and reference taken on it)
+ *       (but private mapping on an mm)
 */
 
-#define FUT_OFF_INODE    1 /* We set bit 0 if key has a reference on inode */
-#define FUT_OFF_MMSHARED 2 /* We set bit 1 if key has a reference on mm */
+#define FUT_OFF_INODE    1 /* We set bit 0 if shared key identifies an inode */
+#define FUT_OFF_MMSHARED 2 /* We set bit 1 if shared key identifies an mm */
 
 union futex_key {
+	/* For FUT_OFF_INODE */
 	struct {
 		u64 i_seq;
 		unsigned long pgoff;
 		unsigned int offset;
 		/* unsigned int node; */
 	} shared;
+
+	/* For FUT_OFF_MMSHARED or private */
 	struct {
 		union {
-			struct mm_struct *mm;
-			u64 __tmp;
+			struct mm_struct *mm; /* for private */
+			u64 mm_seq; /* for FUT_OFF_MMSHARED */
 		};
 		unsigned long address;
 		unsigned int offset;
 		/* unsigned int node; */
 	} private;
+
 	struct {
 		u64 ptr;
 		unsigned long word;
@@ -152,11 +156,10 @@ static inline void futex_set_vdso_cs_range(struct futex_mm_data *fd, unsigned in
 static inline void futex_fixup_robust_unlock(struct pt_regs *regs) { }
 #endif /* !CONFIG_FUTEX_ROBUST_UNLOCK */
 
-
-#if defined(CONFIG_FUTEX_PRIVATE_HASH) || defined(CONFIG_FUTEX_ROBUST_UNLOCK)
+#ifdef CONFIG_FUTEX
 void futex_mm_init(struct mm_struct *mm);
 #else
 static inline void futex_mm_init(struct mm_struct *mm) { }
-#endif
+#endif /* !CONFIG_FUTEX */
 
 #endif /* _LINUX_FUTEX_H */
diff --git a/include/linux/futex_types.h b/include/linux/futex_types.h
index d320c0571f0c..34b16f2828e7 100644
--- a/include/linux/futex_types.h
+++ b/include/linux/futex_types.h
@@ -85,10 +85,12 @@ struct futex_unlock_cs_ranges { };
  * struct futex_mm_data - Futex related per MM data
  * @phash:	Futex private hash related data
  * @unlock:	Futex unlock VDSO critical sections
+ * @unique_id:	Unique ID of the MM, for FUT_OFF_MMSHARED futexes
  */
 struct futex_mm_data {
 	struct futex_mm_phash		phash;
 	struct futex_unlock_cs_ranges	unlock;
+	atomic64_t			unique_id;
 };
 #else  /* CONFIG_FUTEX */
 struct futex_sched_data { };
diff --git a/kernel/futex/core.c b/kernel/futex/core.c
index 095f9fe440e3..a240638f69e5 100644
--- a/kernel/futex/core.c
+++ b/kernel/futex/core.c
@@ -439,12 +439,35 @@ struct hrtimer_sleeper *futex_setup_timer(ktime_t *time, struct hrtimer_sleeper
 }
 
 /*
- * Generate a machine wide unique identifier for this inode.
+ * Generate a machine wide unique identifier for this object (inode or mm).
  *
  * This relies on u64 not wrapping in the life-time of the machine; which with
  * 1ns resolution means almost 585 years.
- *
- * This further relies on the fact that a well formed program will not unmap
+ */
+static u64 get_object_id(atomic64_t *object_seq)
+{
+	static atomic64_t last_assigned_seq;
+	u64 old;
+
+	/* Does the object already have a sequence number? */
+	old = atomic64_read(object_seq);
+	if (likely(old))
+		return old;
+
+	for (;;) {
+		u64 new = atomic64_inc_return(&last_assigned_seq);
+		if (WARN_ON_ONCE(!new))
+			continue;
+
+		old = 0;
+		if (!atomic64_try_cmpxchg_relaxed(object_seq, &old, new))
+			return old;
+		return new;
+	}
+}
+
+/*
+ * This relies on the fact that a well formed program will not unmap
  * the file while it has a (shared) futex waiting on it. This mapping will have
  * a file reference which pins the mount and inode.
  *
@@ -458,24 +481,7 @@ struct hrtimer_sleeper *futex_setup_timer(ktime_t *time, struct hrtimer_sleeper
  */
 static u64 get_inode_sequence_number(struct inode *inode)
 {
-	static atomic64_t i_seq;
-	u64 old;
-
-	/* Does the inode already have a sequence number? */
-	old = atomic64_read(&inode->i_sequence);
-	if (likely(old))
-		return old;
-
-	for (;;) {
-		u64 new = atomic64_inc_return(&i_seq);
-		if (WARN_ON_ONCE(!new))
-			continue;
-
-		old = 0;
-		if (!atomic64_try_cmpxchg_relaxed(&inode->i_sequence, &old, new))
-			return old;
-		return new;
-	}
+	return get_object_id(&inode->i_sequence);
 }
 
 /**
@@ -683,8 +689,8 @@ int get_futex_key(u32 __user *uaddr, unsigned int flags, union futex_key *key,
 			goto out;
 		}
 
-		key->both.offset |= FUT_OFF_MMSHARED; /* ref taken on mm */
-		key->private.mm = mm;
+		key->both.offset |= FUT_OFF_MMSHARED;
+		key->private.mm_seq = get_object_id(&mm->futex.unique_id);
 		key->private.address = address;
 
 	} else {
@@ -2055,13 +2061,12 @@ static void futex_robust_unlock_init_mm(struct futex_mm_data *fd)
 static inline void futex_robust_unlock_init_mm(struct futex_mm_data *fd) { }
 #endif /* !CONFIG_FUTEX_ROBUST_UNLOCK */
 
-#if defined(CONFIG_FUTEX_PRIVATE_HASH) || defined(CONFIG_FUTEX_ROBUST_UNLOCK)
 void futex_mm_init(struct mm_struct *mm)
 {
+	atomic64_set(&mm->futex.unique_id, 0);
 	futex_hash_init_mm(&mm->futex);
 	futex_robust_unlock_init_mm(&mm->futex);
 }
-#endif
 
 int futex_hash_prctl(unsigned long arg2, unsigned long arg3, unsigned long arg4)
 {

[PATCH 5.15.y] futex: Fix mm reuse handling for FUT_OFF_MMSHARED

[Sasha Levin]

From: Jann Horn <jannh@google.com>

[ Upstream commit 3267512ad1c2cc0c7768c5b014dc2cceb398a801 ]

A FUT_OFF_MMSHARED futex is a shared futex that refers to an MM.
It is possible for a process to wait on a shared futex with a different MM
because a FUT_OFF_INODE waiter can be requeued onto a FUT_OFF_MMSHARED
futex by another process.
This can cause FUT_OFF_MMSHARED waiters on a freed MM to consume
wakeups intended for a newly allocated MM at the same address.

Fix it by keying FUT_OFF_MMSHARED using a unique 64-bit per-MM ID.
Leave private futexes as before to avoid influencing the performance of the
hotpath.

(Multi-threaded processes typically implicitly use FUT_OFF_MMSHARED by
setting clear_child_tid such that it points into anonymous memory, which
causes mm_release() in a multi-threaded mm to perform FUTEX_WAKE.)

Fixes: 222993395ed3 ("futex: Remove pointless mmgrap() + mmdrop()")
Closes: https://lore.kernel.org/r/CAG48ez0dLBpc3QtbAhMMVNHwHL94iHh2G+h-=BVFR4dDuzZr1g@mail.gmail.com/
Signed-off-by: Jann Horn <jannh@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20260929-futex-mmshared-fix-v1-1-262b1ffeb3d0@google.com
Cc: stable@vger.kernel.org
[ sashal: Adapted futex ID storage to a direct mm_struct field initialized in mm_init(). ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 include/linux/futex.h    | 14 ++++++++----
 include/linux/mm_types.h |  4 ++++
 kernel/fork.c            |  3 +++
 kernel/futex/core.c      | 49 +++++++++++++++++++++-------------------
 4 files changed, 42 insertions(+), 28 deletions(-)

diff --git a/include/linux/futex.h b/include/linux/futex.h
index b70df27d7e85c..09f757f14d01f 100644
--- a/include/linux/futex.h
+++ b/include/linux/futex.h
@@ -23,26 +23,30 @@ struct task_struct;
  *  01 : Shared futex (PTHREAD_PROCESS_SHARED)
  *	mapped on a file (reference on the underlying inode)
  *  10 : Shared futex (PTHREAD_PROCESS_SHARED)
- *       (but private mapping on an mm, and reference taken on it)
+ *       (but private mapping on an mm)
 */
 
-#define FUT_OFF_INODE    1 /* We set bit 0 if key has a reference on inode */
-#define FUT_OFF_MMSHARED 2 /* We set bit 1 if key has a reference on mm */
+#define FUT_OFF_INODE    1 /* We set bit 0 if shared key identifies an inode */
+#define FUT_OFF_MMSHARED 2 /* We set bit 1 if shared key identifies an mm */
 
 union futex_key {
+	/* For FUT_OFF_INODE */
 	struct {
 		u64 i_seq;
 		unsigned long pgoff;
 		unsigned int offset;
 	} shared;
+
+	/* For FUT_OFF_MMSHARED or private */
 	struct {
 		union {
-			struct mm_struct *mm;
-			u64 __tmp;
+			struct mm_struct *mm; /* for private */
+			u64 mm_seq; /* for FUT_OFF_MMSHARED */
 		};
 		unsigned long address;
 		unsigned int offset;
 	} private;
+
 	struct {
 		u64 ptr;
 		unsigned long word;
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 07285b44d831a..4a663b39585b0 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -580,6 +580,10 @@ struct mm_struct {
 #endif
 		struct work_struct async_put_work;
 
+#ifdef CONFIG_FUTEX
+		atomic64_t futex_unique_id;
+#endif
+
 #ifdef CONFIG_IOMMU_SUPPORT
 		u32 pasid;
 #endif
diff --git a/kernel/fork.c b/kernel/fork.c
index 81355f325758c..25fb13362d38c 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1082,6 +1082,9 @@ static struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p,
 #endif
 	mm_init_uprobes_state(mm);
 	hugetlb_count_init(mm);
+#ifdef CONFIG_FUTEX
+	atomic64_set(&mm->futex_unique_id, 0);
+#endif
 
 	if (current->mm) {
 		mm->flags = current->mm->flags & MMF_INIT_MASK;
diff --git a/kernel/futex/core.c b/kernel/futex/core.c
index 8a3a27967973a..0055c446b0c60 100644
--- a/kernel/futex/core.c
+++ b/kernel/futex/core.c
@@ -473,39 +473,27 @@ futex_setup_timer(ktime_t *time, struct hrtimer_sleeper *timeout,
 }
 
 /*
- * Generate a machine wide unique identifier for this inode.
+ * Generate a machine wide unique identifier for this object (inode or mm).
  *
  * This relies on u64 not wrapping in the life-time of the machine; which with
  * 1ns resolution means almost 585 years.
- *
- * This further relies on the fact that a well formed program will not unmap
- * the file while it has a (shared) futex waiting on it. This mapping will have
- * a file reference which pins the mount and inode.
- *
- * If for some reason an inode gets evicted and read back in again, it will get
- * a new sequence number and will _NOT_ match, even though it is the exact same
- * file.
- *
- * It is important that match_futex() will never have a false-positive, esp.
- * for PI futexes that can mess up the state. The above argues that false-negatives
- * are only possible for malformed programs.
  */
-static u64 get_inode_sequence_number(struct inode *inode)
+static u64 get_object_id(atomic64_t *object_seq)
 {
-	static atomic64_t i_seq;
+	static atomic64_t last_assigned_seq;
 	u64 old;
 
-	/* Does the inode already have a sequence number? */
-	old = atomic64_read(&inode->i_sequence);
+	/* Does the object already have a sequence number? */
+	old = atomic64_read(object_seq);
 	if (likely(old))
 		return old;
 
 	for (;;) {
-		u64 new = atomic64_add_return(1, &i_seq);
+		u64 new = atomic64_add_return(1, &last_assigned_seq);
 		if (WARN_ON_ONCE(!new))
 			continue;
 
-		old = atomic64_cmpxchg_relaxed(&inode->i_sequence, 0, new);
+		old = atomic64_cmpxchg_relaxed(object_seq, 0, new);
 		if (old)
 			return old;
 		return new;
@@ -528,7 +516,7 @@ static u64 get_inode_sequence_number(struct inode *inode)
  *
  *   ( inode->i_sequence, page->index, offset_within_page )
  *
- * [ also see get_inode_sequence_number() ]
+ * [ also see get_object_id() ]
  *
  * For private mappings (or when !@fshared), the key is:
  *
@@ -680,8 +668,8 @@ static int get_futex_key(u32 __user *uaddr, bool fshared, union futex_key *key,
 			goto out;
 		}
 
-		key->both.offset |= FUT_OFF_MMSHARED; /* ref taken on mm */
-		key->private.mm = mm;
+		key->both.offset |= FUT_OFF_MMSHARED;
+		key->private.mm_seq = get_object_id(&mm->futex_unique_id);
 		key->private.address = address;
 
 	} else {
@@ -716,7 +704,22 @@ static int get_futex_key(u32 __user *uaddr, bool fshared, union futex_key *key,
 		}
 
 		key->both.offset |= FUT_OFF_INODE; /* inode-based key */
-		key->shared.i_seq = get_inode_sequence_number(inode);
+		/*
+		 * This relies on the fact that a well formed program will not
+		 * unmap the file while it has a (shared) futex waiting on it.
+		 * This mapping will have a file reference which pins the mount
+		 * and inode.
+		 *
+		 * If for some reason an inode gets evicted and read back in
+		 * again, it will get a new sequence number and will _NOT_ match,
+		 * even though it is the exact same file.
+		 *
+		 * It is important that match_futex() will never have a
+		 * false-positive, esp. for PI futexes that can mess up the state.
+		 * The above argues that false-negatives are only possible for
+		 * malformed programs.
+		 */
+		key->shared.i_seq = get_object_id(&inode->i_sequence);
 		key->shared.pgoff = page_to_pgoff(tail);
 		rcu_read_unlock();
 	}
-- 
2.53.0