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 7479846E011 for ; Wed, 16 Sep 2026 20:37:03 +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=1789591041; cv=none; b=oBzqASIPsNbXI5pxDdhhzMDH493Ah9qOf+uGTFvWIHMTdKnwTOdlCmZIPS3dPtk+zLHuGgl1Eyi0NY9eAZ03TFk/AA8NMl8MCvzlrHJ7Z5+L6S3Gnr4hAFtuuga7/3rlsIfsRShlRdMRgkkn7WJ22uf0nBl0LGzjQhtcS/ObNVk= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789591041; c=relaxed/simple; bh=vGUq/mq6pjETsh6rrtpLcBUOl3E68jLaOnnA0gs0QQ8=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=VyJ1y2FNLxtq8qIrpo7TYG+pQ4ZlZg9aQJQyIsO7Lp4NNHcNppm5bNmncwEGFoieZ2WhAzHfSNVqG18AS+HkSkABigxBKnEbSMIB2NLcfkkX+ro4voLDR4yYt2YPLyrjVReLoJQQtMB1Owx1VVLqHhAIZUkVDq2pvbplGnMQlQs= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=PCkOLaSC; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="PCkOLaSC" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 9C4E21F00898; Wed, 16 Sep 2026 20:36:59 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1789591020; bh=D9NqxqYkWaoLVyJo16XIoac/mxONFnR1Sy3SgThOHw4=; h=From:To:Cc:Subject:Date:In-Reply-To:References; b=PCkOLaSC6gb7gz6y03bf7jjJL1H3w+Jy8wQ6xMgKbTwgXZBVB4JhCxYZwGrX+ZkxW wtFIhldIWpfHozmfEhzzkG0YsrDI/aBzWulyKlyLclxmT91t7BZsUZyEUYGNoho7ss 5Q4WKcoPNf9ciViGerH8p/eLZm734X4rd+g+UeDrSV4HWtpbZbbEGbXRuRJydhEMTC /FMwerwdvrzLDkIempGt7zzsWgrOpA6dulFwgtmn7iD5FLSmEJNzk9fvk7dZZxqPxe BEZIOXqr/8i3DZVIJCmWhN6jnDgEqW3posFmPsO7UgN/f+bGbhye8jmtYRQxTayWHg GFt9Txv6tL6vQ== From: Sasha Levin To: stable@vger.kernel.org Cc: Daehyeon Ko <4ncienth@gmail.com>, syzbot+0aee5e8066eddbbe7397@syzkaller.appspotmail.com, syzbot+e8b3520b53e78e90034e@syzkaller.appspotmail.com, Oleg Nesterov , Bradley Morgan , "Christian Brauner (Amutable)" , Sasha Levin Subject: [PATCH 6.18.y 3/3] exit: hold a reference to thread_pid across proc_flush_pid Date: Wed, 16 Sep 2026 16:36:56 -0400 Message-ID: <20260916203656.2689289-3-sashal@kernel.org> X-Mailer: git-send-email 2.53.0 In-Reply-To: <20260916203656.2689289-1-sashal@kernel.org> References: <2026091657-curry-myself-be92@gregkh> <20260916203656.2689289-1-sashal@kernel.org> Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit From: Daehyeon Ko <4ncienth@gmail.com> [ Upstream commit cdd812d0683dee14ead02c9eded568685e61b23f ] 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) [ Adjusted context for cgroup_release() and the missing exit_cred_namespaces() call. ] Signed-off-by: Sasha Levin --- kernel/exit.c | 9 ++++++--- 1 file changed, 6 insertions(+), 3 deletions(-) diff --git a/kernel/exit.c b/kernel/exit.c index b22ede84c0ec9..d60c1241923a9 100644 --- a/kernel/exit.c +++ b/kernel/exit.c @@ -264,8 +264,11 @@ void release_task(struct task_struct *p) pidfs_exit(p); cgroup_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); @@ -294,8 +297,8 @@ 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); add_device_randomness(&p->se.sum_exec_runtime, sizeof(p->se.sum_exec_runtime)); free_pids(post.pids); -- 2.53.0