From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 1FCEE52E053 for ; Wed, 16 Sep 2026 15:28:28 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789572511; cv=none; b=kjTlEZvCMfaFTRMqKWnshduHvBGEQn3WGd1RUZ5KKlpkymgdjSFk1QQ/wB5Y1paWH+hSF5DIuijjSeoHcgjE+0qjkZEb/MxczRUGbXLylCwGDkTkkpwdPciRWe96hyeYT+EFjCGUEs18HYruPu0ocKBhOjRuQ95iwNOLAkNnSI0= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789572511; c=relaxed/simple; bh=HVg5XnrKHJ6j2S/S+q+YVG+SvZggZgbzhU/Bfvq1UXk=; h=Subject:To:Cc:From:Date:Message-ID:MIME-Version:Content-Type; b=WSAWdSP8O1rBViItsF9g5C/NVcEaJDWvmf6q9H0OrZq1oZSF8VtDIAAGoanwgk9SXzgQTYgroAh0u9Imkl+5nKU0VhtdRmDVgf+KGYXiJShlWr9uiI4hgXr6INeWkUaUK1CkJi6LSLHuaPL3ECNorVUxDlOLdyrF5cT5UkA1BAc= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b=JKzaPMAl; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b="JKzaPMAl" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 221161F0089A; Wed, 16 Sep 2026 15:28:24 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linuxfoundation.org; s=korg; t=1789572505; bh=l5EfZpLBagMxePdyN9UtB6usxfqB1GvJXpOBxVwfQ6g=; h=Subject:To:Cc:From:Date; b=JKzaPMAl4k7z528R5Hj6OenAJGZTZSoNcY5Z3HV/kxDODS06X/Ez/aaYQBYEQMjhY lqos+76p7tYnnB1fhar/NsfVbDI75I0FpqG2hPIMn7vyhjmwck2AYSpagNXwYljaJD 8dtCAyM0tnttAPOrQ4Bv+VTszDxSsW3zOSAdCr4w= Subject: FAILED: patch "[PATCH] exit: hold a reference to thread_pid across proc_flush_pid" failed to apply to 6.18-stable tree To: 4ncienth@gmail.com,brads@mainlining.org,brauner@kernel.org,oleg@redhat.com Cc: From: Date: Wed, 16 Sep 2026 16:16:58 +0100 Message-ID: <2026091657-curry-myself-be92@gregkh> Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=ANSI_X3.4-1968 Content-Transfer-Encoding: 8bit The patch below does not apply to the 6.18-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 . 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-6.18.y git checkout FETCH_HEAD git cherry-pick -x cdd812d0683dee14ead02c9eded568685e61b23f # git commit -s git send-email --to '' --in-reply-to '2026091657-curry-myself-be92@gregkh' --subject-prefix 'PATCH 6.18.y' 'HEAD^..' Possible dependencies: thanks, greg k-h ------------------ original commit in Linus's tree ------------------ >From cdd812d0683dee14ead02c9eded568685e61b23f Mon Sep 17 00:00:00 2001 From: Daehyeon Ko <4ncienth@gmail.com> Date: Mon, 31 Aug 2026 09:12:21 +0900 Subject: [PATCH] exit: hold a reference to thread_pid across proc_flush_pid Commit 0a36bad01731 ("release_task: kill the no longer needed get/put_pid(thread_pid)") removed the reference around proc_flush_pid(). It assumed that free_pids(post.pids) at the end of release_task() would keep thread_pid alive until then. That assumption is wrong. __change_pid() only records a detached PID in post.pids when pid_has_task() is false for every PIDTYPE. If another task still uses the exiting task's PID as its process group or session ID, __unhash_process() removes the exiting task's PIDTYPE_PID link but leaves the PID out of post.pids. release_task() therefore holds no reference to it after dropping tasklist_lock. The other task can then remove the remaining PIDTYPE links. Its free_pids() call schedules delayed_put_pid(), and the RCU callback can free the PID before the first release_task() reaches proc_flush_pid(). An unprivileged reproducer races wait4(-1) against setsid() to trigger this ordering. Three of three fresh v7.2 KASAN boots reported: BUG: KASAN: slab-use-after-free in proc_invalidate_siblings_dcache+0x3e2/0x3f0 Read of size 8 by task h7_pid_reaper/1921 Call Trace: proc_invalidate_siblings_dcache release_task wait_consider_task __do_wait do_wait kernel_wait4 Freed by task 0: kmem_cache_free put_pid delayed_put_pid rcu_core Last potentially related work creation: __call_rcu_common free_pids ksys_setsid KASAN identified a 144-byte object from the pid cache and located the bad read 80 bytes into the freed object, matching pid->inodes. With an explicit reference, three of three fresh boots completed without a KASAN report. The concurrent RCU callback dropped its reference while proc_flush_pid() was protected, and the balancing put_pid() performed the final free afterward. Take a reference before __unhash_process() clears p->thread_pid and release it after proc_flush_pid() completes. A tested source reproducer is available privately on request. No controlled read or write, information leak, or privilege escalation is claimed. The mainline patch applies directly to v6.19.y and newer; v6.16.y through v6.18.y need a context-adjusted backport. Fixes: 0a36bad01731 ("release_task: kill the no longer needed get/put_pid(thread_pid)") Reported-by: syzbot+0aee5e8066eddbbe7397@syzkaller.appspotmail.com Closes: https://syzkaller.appspot.com/bug?extid=0aee5e8066eddbbe7397 Reported-by: syzbot+e8b3520b53e78e90034e@syzkaller.appspotmail.com Link: https://syzkaller.appspot.com/bug?extid=e8b3520b53e78e90034e Cc: stable@vger.kernel.org # see patch description, needs adjustments for 6.16.y-6.18.y Signed-off-by: Daehyeon Ko <4ncienth@gmail.com> Link: https://patch.msgid.link/20260831001221.3755948-1-4ncienth@gmail.com Acked-by: Oleg Nesterov Reviewed-by: Bradley Morgan Signed-off-by: Christian Brauner (Amutable) diff --git a/kernel/exit.c b/kernel/exit.c index 182c06671c78..e12072f1507c 100644 --- a/kernel/exit.c +++ b/kernel/exit.c @@ -261,8 +261,11 @@ void release_task(struct task_struct *p) pidfs_exit(p); cgroup_task_release(p); - /* Retrieve @thread_pid before __unhash_process() may set it to NULL. */ - thread_pid = task_pid(p); + /* + * Pin @thread_pid before __unhash_process() clears it. The last + * PIDTYPE detach can otherwise free it before proc_flush_pid(). + */ + thread_pid = get_pid(task_pid(p)); write_lock_irq(&tasklist_lock); ptrace_release_task(p); @@ -291,8 +294,8 @@ void release_task(struct task_struct *p) } write_unlock_irq(&tasklist_lock); - /* @thread_pid can't go away until free_pids() below */ proc_flush_pid(thread_pid); + put_pid(thread_pid); exit_cred_namespaces(p); add_device_randomness(&p->se.sum_exec_runtime, sizeof(p->se.sum_exec_runtime));