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[PATCH] futex: Move futex_hash_free() back to __mmput()

Sebastian Andrzej Siewior posted 1 patch 1 month, 1 week ago
There is a newer version of this series
kernel/fork.c       |  2 +-
kernel/futex/core.c | 15 +++++++++++----
2 files changed, 12 insertions(+), 5 deletions(-)
[PATCH] futex: Move futex_hash_free() back to __mmput()
Posted by Sebastian Andrzej Siewior 1 month, 1 week ago 
To avoid a memory leak via mm_alloc() + mmdrop() the futex cleanup code
has been moved to __mmdrop(). This resulted in a warnings if the futex
hash table has been allocated via vmalloc() the mmdrop() was invoked
from atomic context.
The free path must stay in __mmput() to ensure it is invoked from
preemptible context.

In order to avoid the memory leak, delay the allocation of
mm_struct::mm->futex_ref to futex_hash_allocate(). This works because
neither the per-CPU counter nor the private hash has been allocated and
therefore
- futex_private_hash() callers (such as exit_pi_state_list()) don't
  acquire reference if there is no private hash yet. There is also no
  reference put.

- Regular callers (futex_hash()) fallback to global hash. No reference
  counting here.

The futex_ref member can be allocated in futex_hash_allocate() before
the private hash itself is allocated. This happens either while the
first thread is created or on request. In both cases the process has
just a single thread so there can be either futex operation in progress
or the request to create a private hash.

Move futex_hash_free() back to __mmput();
Move the allocation of mm_struct::futex_ref to futex_hash_allocate().

Fixes:  e703b7e247503 ("futex: Move futex cleanup to __mmdrop()")
Reported-by: Jakub Kicinski <kuba@kernel.org>
Closes: https://lore.kernel.org/all/20250821102721.6deae493@kernel.org/
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 kernel/fork.c       |  2 +-
 kernel/futex/core.c | 15 +++++++++++----
 2 files changed, 12 insertions(+), 5 deletions(-)

diff --git a/kernel/fork.c b/kernel/fork.c
index af673856499dc..c4ada32598bd5 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -689,7 +689,6 @@ void __mmdrop(struct mm_struct *mm)
 	mm_pasid_drop(mm);
 	mm_destroy_cid(mm);
 	percpu_counter_destroy_many(mm->rss_stat, NR_MM_COUNTERS);
-	futex_hash_free(mm);
 
 	free_mm(mm);
 }
@@ -1138,6 +1137,7 @@ static inline void __mmput(struct mm_struct *mm)
 	if (mm->binfmt)
 		module_put(mm->binfmt->module);
 	lru_gen_del_mm(mm);
+	futex_hash_free(mm);
 	mmdrop(mm);
 }
 
diff --git a/kernel/futex/core.c b/kernel/futex/core.c
index d9bb5567af0c5..fb63c13aa66fc 100644
--- a/kernel/futex/core.c
+++ b/kernel/futex/core.c
@@ -1724,10 +1724,6 @@ int futex_mm_init(struct mm_struct *mm)
 	/* futex-ref */
 	atomic_long_set(&mm->futex_atomic, 0);
 	mm->futex_batches = get_state_synchronize_rcu();
-	mm->futex_ref = alloc_percpu(unsigned int);
-	if (!mm->futex_ref)
-		return -ENOMEM;
-	this_cpu_inc(*mm->futex_ref); /* 0 -> 1 */
 	return 0;
 }
 
@@ -1801,6 +1797,17 @@ static int futex_hash_allocate(unsigned int hash_slots, unsigned int flags)
 		}
 	}
 
+	if (!mm->futex_ref) {
+		/*
+		 * This will always be allocated by the first thread and
+		 * therefore requires no locking.
+		 */
+		mm->futex_ref = alloc_percpu(unsigned int);
+		if (!mm->futex_ref)
+			return -ENOMEM;
+		this_cpu_inc(*mm->futex_ref); /* 0 -> 1 */
+	}
+
 	fph = kvzalloc(struct_size(fph, queues, hash_slots),
 		       GFP_KERNEL_ACCOUNT | __GFP_NOWARN);
 	if (!fph)
-- 
2.50.1
[tip: locking/urgent] futex: Move futex_hash_free() back to __mmput()
Posted by tip-bot2 for Sebastian Andrzej Siewior 1 month ago 
The following commit has been merged into the locking/urgent branch of tip:

Commit-ID:     d9b05321e21e4b218de4ce8a590bf375f58b6346
Gitweb:        https://git.kernel.org/tip/d9b05321e21e4b218de4ce8a590bf375f58b6346
Author:        Sebastian Andrzej Siewior <bigeasy@linutronix.de>
AuthorDate:    Fri, 22 Aug 2025 16:12:38 +02:00
Committer:     Borislav Petkov (AMD) <bp@alien8.de>
CommitterDate: Sun, 31 Aug 2025 11:48:19 +02:00

futex: Move futex_hash_free() back to __mmput()

To avoid a memory leak via mm_alloc() + mmdrop() the futex cleanup code
has been moved to __mmdrop(). This resulted in a warnings if the futex
hash table has been allocated via vmalloc() the mmdrop() was invoked
from atomic context.
The free path must stay in __mmput() to ensure it is invoked from
preemptible context.

In order to avoid the memory leak, delay the allocation of
mm_struct::mm->futex_ref to futex_hash_allocate(). This works because
neither the per-CPU counter nor the private hash has been allocated and
therefore
- futex_private_hash() callers (such as exit_pi_state_list()) don't
  acquire reference if there is no private hash yet. There is also no
  reference put.

- Regular callers (futex_hash()) fallback to global hash. No reference
  counting here.

The futex_ref member can be allocated in futex_hash_allocate() before
the private hash itself is allocated. This happens either while the
first thread is created or on request. In both cases the process has
just a single thread so there can be either futex operation in progress
or the request to create a private hash.

Move futex_hash_free() back to __mmput();
Move the allocation of mm_struct::futex_ref to futex_hash_allocate().

  [ bp: Fold a follow-up fix to prevent a use-after-free:
    https://lore.kernel.org/r/20250830213806.sEKuuGSm@linutronix.de ]

Fixes:  e703b7e247503 ("futex: Move futex cleanup to __mmdrop()")
Closes: https://lore.kernel.org/all/20250821102721.6deae493@kernel.org/
Reported-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lkml.kernel.org/r/20250822141238.PfnkTjFb@linutronix.de
---
 kernel/fork.c       |  2 +-
 kernel/futex/core.c | 16 ++++++++++++----
 2 files changed, 13 insertions(+), 5 deletions(-)

diff --git a/kernel/fork.c b/kernel/fork.c
index af67385..c4ada32 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -689,7 +689,6 @@ void __mmdrop(struct mm_struct *mm)
 	mm_pasid_drop(mm);
 	mm_destroy_cid(mm);
 	percpu_counter_destroy_many(mm->rss_stat, NR_MM_COUNTERS);
-	futex_hash_free(mm);
 
 	free_mm(mm);
 }
@@ -1138,6 +1137,7 @@ static inline void __mmput(struct mm_struct *mm)
 	if (mm->binfmt)
 		module_put(mm->binfmt->module);
 	lru_gen_del_mm(mm);
+	futex_hash_free(mm);
 	mmdrop(mm);
 }
 
diff --git a/kernel/futex/core.c b/kernel/futex/core.c
index d9bb556..125804f 100644
--- a/kernel/futex/core.c
+++ b/kernel/futex/core.c
@@ -1722,12 +1722,9 @@ int futex_mm_init(struct mm_struct *mm)
 	RCU_INIT_POINTER(mm->futex_phash, NULL);
 	mm->futex_phash_new = NULL;
 	/* futex-ref */
+	mm->futex_ref = NULL;
 	atomic_long_set(&mm->futex_atomic, 0);
 	mm->futex_batches = get_state_synchronize_rcu();
-	mm->futex_ref = alloc_percpu(unsigned int);
-	if (!mm->futex_ref)
-		return -ENOMEM;
-	this_cpu_inc(*mm->futex_ref); /* 0 -> 1 */
 	return 0;
 }
 
@@ -1801,6 +1798,17 @@ static int futex_hash_allocate(unsigned int hash_slots, unsigned int flags)
 		}
 	}
 
+	if (!mm->futex_ref) {
+		/*
+		 * This will always be allocated by the first thread and
+		 * therefore requires no locking.
+		 */
+		mm->futex_ref = alloc_percpu(unsigned int);
+		if (!mm->futex_ref)
+			return -ENOMEM;
+		this_cpu_inc(*mm->futex_ref); /* 0 -> 1 */
+	}
+
 	fph = kvzalloc(struct_size(fph, queues, hash_slots),
 		       GFP_KERNEL_ACCOUNT | __GFP_NOWARN);
 	if (!fph)
[tip: locking/urgent] futex: Move futex_hash_free() back to __mmput()
Posted by tip-bot2 for Sebastian Andrzej Siewior 1 month, 1 week ago 
The following commit has been merged into the locking/urgent branch of tip:

Commit-ID:     1b708b38414d32838baa39c9dee59d40731ed202
Gitweb:        https://git.kernel.org/tip/1b708b38414d32838baa39c9dee59d40731ed202
Author:        Sebastian Andrzej Siewior <bigeasy@linutronix.de>
AuthorDate:    Fri, 22 Aug 2025 16:12:38 +02:00
Committer:     Peter Zijlstra <peterz@infradead.org>
CommitterDate: Wed, 27 Aug 2025 13:31:07 +02:00

futex: Move futex_hash_free() back to __mmput()

To avoid a memory leak via mm_alloc() + mmdrop() the futex cleanup code
has been moved to __mmdrop(). This resulted in a warnings if the futex
hash table has been allocated via vmalloc() the mmdrop() was invoked
from atomic context.
The free path must stay in __mmput() to ensure it is invoked from
preemptible context.

In order to avoid the memory leak, delay the allocation of
mm_struct::mm->futex_ref to futex_hash_allocate(). This works because
neither the per-CPU counter nor the private hash has been allocated and
therefore
- futex_private_hash() callers (such as exit_pi_state_list()) don't
  acquire reference if there is no private hash yet. There is also no
  reference put.

- Regular callers (futex_hash()) fallback to global hash. No reference
  counting here.

The futex_ref member can be allocated in futex_hash_allocate() before
the private hash itself is allocated. This happens either while the
first thread is created or on request. In both cases the process has
just a single thread so there can be either futex operation in progress
or the request to create a private hash.

Move futex_hash_free() back to __mmput();
Move the allocation of mm_struct::futex_ref to futex_hash_allocate().

Fixes:  e703b7e247503 ("futex: Move futex cleanup to __mmdrop()")
Closes: https://lore.kernel.org/all/20250821102721.6deae493@kernel.org/
Reported-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20250822141238.PfnkTjFb@linutronix.de
---
 kernel/fork.c       |  2 +-
 kernel/futex/core.c | 15 +++++++++++----
 2 files changed, 12 insertions(+), 5 deletions(-)

diff --git a/kernel/fork.c b/kernel/fork.c
index af67385..c4ada32 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -689,7 +689,6 @@ void __mmdrop(struct mm_struct *mm)
 	mm_pasid_drop(mm);
 	mm_destroy_cid(mm);
 	percpu_counter_destroy_many(mm->rss_stat, NR_MM_COUNTERS);
-	futex_hash_free(mm);
 
 	free_mm(mm);
 }
@@ -1138,6 +1137,7 @@ static inline void __mmput(struct mm_struct *mm)
 	if (mm->binfmt)
 		module_put(mm->binfmt->module);
 	lru_gen_del_mm(mm);
+	futex_hash_free(mm);
 	mmdrop(mm);
 }
 
diff --git a/kernel/futex/core.c b/kernel/futex/core.c
index d9bb556..fb63c13 100644
--- a/kernel/futex/core.c
+++ b/kernel/futex/core.c
@@ -1724,10 +1724,6 @@ int futex_mm_init(struct mm_struct *mm)
 	/* futex-ref */
 	atomic_long_set(&mm->futex_atomic, 0);
 	mm->futex_batches = get_state_synchronize_rcu();
-	mm->futex_ref = alloc_percpu(unsigned int);
-	if (!mm->futex_ref)
-		return -ENOMEM;
-	this_cpu_inc(*mm->futex_ref); /* 0 -> 1 */
 	return 0;
 }
 
@@ -1801,6 +1797,17 @@ static int futex_hash_allocate(unsigned int hash_slots, unsigned int flags)
 		}
 	}
 
+	if (!mm->futex_ref) {
+		/*
+		 * This will always be allocated by the first thread and
+		 * therefore requires no locking.
+		 */
+		mm->futex_ref = alloc_percpu(unsigned int);
+		if (!mm->futex_ref)
+			return -ENOMEM;
+		this_cpu_inc(*mm->futex_ref); /* 0 -> 1 */
+	}
+
 	fph = kvzalloc(struct_size(fph, queues, hash_slots),
 		       GFP_KERNEL_ACCOUNT | __GFP_NOWARN);
 	if (!fph)

Patchew 2.1-devel-b67e4c2

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