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=1792673892; b=H668EzQma0IthN2HePacKDe02KlmRBPZ51SA37PRF+BUsDRRus7i3N+1BzqVHVAvXHgT QXbu9yo0vlQaZlnL6MaTFKVLm71kvzx/gejACTNH/vvQpImgYSq7qbW5lWg9Gna4MG/Tq S7ZFxOX8opSXu91CD1t8JnUjMnvKNWyqabhA8sv+WDy+fA9kwdUU64y3CaovBKbswhZwl CWb+VHJnIwuX/c+LNVb0vmSDkFFWRs46dlsI6Kj8vh+muqLt1OynWFrMgb6FXuAFnyaKl gPg+weN88v77v6jKzoCaPOBZP7tcwm/AblwSD+XnCNvGG9lQQYMobgsusnNhZrvxHvg== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1792673892; h=Received:DKIM-Signature:From:To:Cc:Subject:Date:Message-ID:X-Mailer: In-Reply-To:References:MIME-Version:Content-Transfer-Encoding; bh=QRgAYzrydbf/BE76yEzQ2A++wHiNjgfJ9bvNKbkRwQE=; b=UURgi1Iqjh6Ip7LfMVtk5GkvxYIk2XwTTxUre4Hc8pdrU9gxAF1uvaL8OH60YkGOuVng sYmVKBTPdI94bzXzXrP5gTh62Wls5OziRNhIpuYJ35yfPZ2uUGnOJMi3PtBhv5fau293H ZzPFulVJ2CRYyH+Y1hqB9jqmdclyczum2WLTjqnKnbztzz5LzBu8ir1JX7yTwQylu60b7 GdpVQcW7RtRo54EoRE2SBtoI9sP33uoha6adWmcxWSFYFR/Vnf3A8+MMoULVdrR1y6Do/ KBFbMuVH7NmvJ+AsFzgWauK3r8+ijV/tZd18Lltmmi4J0Fhmif9LCcF16JBPbjnptPA== ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dkim=pass header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=LibqHHni; 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 C728C606A31F for ; Thu, 22 Oct 2026 12:58:11 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key, unprotected) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=LibqHHni Received: by smtp.kernel.org (Postfix) with ESMTPSA id D83591F000C6; Thu, 22 Oct 2026 12:58:10 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1792673891; bh=QRgAYzrydbf/BE76yEzQ2A++wHiNjgfJ9bvNKbkRwQE=; h=From:To:Cc:Subject:Date:In-Reply-To:References; b=LibqHHniYjKSrIZIckmYhkx35na+eyLvfN1R+A9XZyh5zPLs6qSbKgSMvVEDHETpK OIQpGxLCRgjgpJQ1YuaVJ2jV5OZF74hBCmNCtvvWG/Cz0eIxygOihNY/qEylBItBG7 2DK2FtB2YaB/2nikaM9XA2qqDAAxz+zRyLDGRJnZdueIL4hvbvcQFGXr+SUvTfBwAq wDCg9VrEXRvUybfn/wpepOMp2ICN0LEhQ/YLF1HWy3rrPXwL4T3Tlv8zvxYgsQlDmS gjRi2VBwlw49zAdBeHTHRYrPEJYp/1mbfrAQhsDlFxBA9LD2rJpePOhajUWG0a2ksA VSFjQ7tSoifsw== From: Sasha Levin To: stable@vger.kernel.org Cc: Jann Horn , "Peter Zijlstra (Intel)" , Sasha Levin Subject: [PATCH 5.15.y] futex: Fix mm reuse handling for FUT_OFF_MMSHARED Date: Thu, 22 Oct 2026 08:58:09 -0400 Message-ID: <20261022125420.1192801-1-sashal@kernel.org> X-Mailer: git-send-email 2.53.0 In-Reply-To: <2026102125-action-scroll-7041@gregkh> References: <2026102125-action-scroll-7041@gregkh> X-Mailing-List: stable@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 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 storage to a direct mm_struct field initialized in mm_init(). ] Signed-off-by: Sasha Levin --- 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