From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.remote-ip=100.103.45.18 ARC-Seal: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; cv=none; t=1793210193; b=Xl1RhlDTSMl5SMH63K9PKzHOF4crzHU7+aGXDWWHjWtFLEtiPzi1VsOQAFvEC3+b6iwJ JCWvaYkc4mt1mZHF24o4ZOHwaBsOx6jlp7Nh6keBIy3jzSiyNmIiiNOylcB3Pv07J7ied Ull6c28hQRGwYQbwI2kYzPscbu998PfhB647yLyvxCSGgefICViQucl/lG/+Ij8InLcwO XlfLDHaXs0RBKk+NrVXH+gJh44CeyWRzFFpOu8DKdeCfFV53ptf3KZWH8Fo5Yk0lXdBtI RF4sEc5KpENImbc1T1PW3gypMQ+zIiLVS6hi9Bmw7EBNFqMizaIluN9/t+E8dWLG15g== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1793210193; h=Received:DKIM-Signature:From:To:Cc:Subject:Date:Message-ID:X-Mailer: In-Reply-To:References:User-Agent:X-stable:X-Patchwork-Hint: MIME-Version:Content-Transfer-Encoding; bh=bQnABk5FwES44iQO9xA0JdQ7jHnbC0jd3jIOs7RdT+I=; b=3uTTL8ADkKupgDr6sk/xwXQ5gHjHNLizsoegOLS/UOcvrNlP1DX4PajyJCYKPJmZAHfd 1P7Ex2RheZu8zxwau1cQ2nxE41NXShp/1tvKP7h0ywmWD3IoII4ZxZVvafI5db1Yiv8kh tZ8JznzP6j+LQY/EOMIbUqV/e+sOtTXDN1KiqxwwiI7UMg4stSl7qHxkruF8R0e0g2yod PcTdAP2zrDWxxC5B0FpAxaENxK5nevq/O6dqxDdhP99oJkKFqvONopmVjdAh6cKNHRJZf 9CyGmUMdzusvnlFUrJaU6qLj6b3pYwzYsH9jD0LXpJ6zW5d76paEK/rRoQLXrAjybgQ== ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dkim=pass header.d=linuxfoundation.org header.i=@linuxfoundation.org header.a=rsa-sha256 header.s=korg header.b=dVj7LMs7; arc=none smtp.remote-ip=100.103.45.18 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 060AE606A32E; Wed, 28 Oct 2026 17:56:33 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key, unprotected) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.a=rsa-sha256 header.s=korg header.b=dVj7LMs7 Received: by smtp.kernel.org (Postfix) with ESMTPSA id 578CA1F01828; Wed, 28 Oct 2026 17:56:32 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linuxfoundation.org; s=korg; t=1793210192; bh=bQnABk5FwES44iQO9xA0JdQ7jHnbC0jd3jIOs7RdT+I=; h=From:To:Cc:Subject:Date:In-Reply-To:References; b=dVj7LMs7eydnWuMb68SMaENReYWpdHgM0HzN+Der/a80E4jgIyxPHdCrYN5/t9xSy YzT7mDVyIZJA4ceAarVZP8Ia41O3jsfTTYrabaYh7Z4Spj6I6pqhK0+hMxdXvTKd6G GB0x6YrAuwaraNPFMdBAfzbx+aH/qeVBXYQseFjA= From: Greg Kroah-Hartman To: stable@vger.kernel.org Cc: Greg Kroah-Hartman , patches@lists.linux.dev, Jann Horn , "Peter Zijlstra (Intel)" , Sasha Levin Subject: [PATCH 6.6 110/168] futex: Fix mm reuse handling for FUT_OFF_MMSHARED Date: Wed, 28 Oct 2026 18:00:46 +0100 Message-ID: <20261028165945.690792353@linuxfoundation.org> X-Mailer: git-send-email 2.56.0 In-Reply-To: <20261028165943.089486480@linuxfoundation.org> References: <20261028165943.089486480@linuxfoundation.org> User-Agent: quilt/0.69 X-stable: review X-Patchwork-Hint: ignore X-Mailing-List: patches@lists.linux.dev List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 6.6-stable review patch. If anyone has any objections, please let me know. ------------------ From: Jann Horn [ 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 Signed-off-by: Peter Zijlstra (Intel) Link: https://patch.msgid.link/20260929-futex-mmshared-fix-v1-1-262b1ffeb3d0@google.com Cc: stable@vger.kernel.org [ sashal: adapted futex ID handling to direct mm_struct storage initialized by mm_init() because the dedicated per-MM futex infrastructure is absent. ] Signed-off-by: Sasha Levin --- include/linux/futex.h | 14 ++++++++---- include/linux/mm_types.h | 3 +++ kernel/fork.c | 3 +++ kernel/futex/core.c | 49 +++++++++++++++++++++------------------- 4 files changed, 41 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 ce3370823f9c7..91c10d8b00c02 100644 --- a/include/linux/mm_types.h +++ b/include/linux/mm_types.h @@ -849,6 +849,9 @@ struct mm_struct { */ int mm_lock_seq; #endif +#ifdef CONFIG_FUTEX + atomic64_t futex_unique_id; +#endif unsigned long hiwater_rss; /* High-watermark of RSS usage */ diff --git a/kernel/fork.c b/kernel/fork.c index 73e8a6b430a05..396a1d4b05e59 100644 --- a/kernel/fork.c +++ b/kernel/fork.c @@ -1299,6 +1299,9 @@ 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 = mmf_init_flags(current->mm->flags); diff --git a/kernel/futex/core.c b/kernel/futex/core.c index 660d7061740b8..8fb54800ce0ad 100644 --- a/kernel/futex/core.c +++ b/kernel/futex/core.c @@ -151,39 +151,27 @@ 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 futex_match() 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; @@ -206,7 +194,7 @@ 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: * @@ -358,8 +346,8 @@ 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 { @@ -394,7 +382,22 @@ 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 futex_match() 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