From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from pdx-out-008.esa.us-west-2.outbound.mail-perimeter.amazon.com (pdx-out-008.esa.us-west-2.outbound.mail-perimeter.amazon.com [52.42.203.116]) (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 851CC407569 for ; Fri, 10 Jul 2026 22:49:13 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=52.42.203.116 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1783723754; cv=none; b=Cc4PE6zR/IY7kzSTFrNLa7e7jSfI292F89t7Fu/G9ByK7OiPCPpbNtesBQp6XJ2d4oYYAyS/YPnemX/hkjQLvjgwAoYgBKcr6u0COIEbS/8MZoIHOlxeJgoLaCHgOWVqOGpJP0afPJxbhcimUbHltmgPwZyYJQKwW8Qxgh/5HXM= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1783723754; c=relaxed/simple; bh=RoTrQNAhjijjxjz8QLC9hr+IRo+isuz0jRyjUBA43DQ=; h=From:To:CC:Subject:Date:Message-ID:MIME-Version:Content-Type; b=tcHlmO4gZJYxun+9VfcFzAFt+LqSbaUd7DaCLRuQZhtMfZfi9g6ov5otO+Q+lOln5WhBk+TOFSrzWee0YbxdkRUyfD4+nMcI0d3ffc94SkLf6o3Nxy3kcryvNW9Ngm2AZ5q/+mNDZqjA9HuVa8/3Xw/RW257DzGEYi1XH0ncUio= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=amazon.com; spf=pass smtp.mailfrom=amazon.com; dkim=pass (2048-bit key) header.d=amazon.com header.i=@amazon.com header.b=W6bM1rRL; arc=none smtp.client-ip=52.42.203.116 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=amazon.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=amazon.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=amazon.com header.i=@amazon.com header.b="W6bM1rRL" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=amazon.com; i=@amazon.com; q=dns/txt; s=amazoncorp2; t=1783723753; x=1815259753; h=from:to:cc:subject:date:message-id:mime-version: content-transfer-encoding; bh=IwI4uz+WfRpjV0aFIUyhlHeYWbfOGsS9Lftxs9VMhxU=; b=W6bM1rRLkRBqFNFSWSrVXGOZrkjlGs5XxKTYzbcrq3AVVn8nLJ2Oo8EJ w7hihnOx80gVax6bi25dQjPXhy6UQiUiF9MlTONqgpwlD3puvnctaajnD 89MyFWMGPDVaKgzJp+thFWRz8slOXCKzu0bN638SaHJ7MDBzjU3SZxcAR jzTW/NPhkSYjdUqBN8GUEfIP7+/SbVtYnuAtNbRXQk9juX0FXgQ/ys14v S4ht6WovCt0xEX+RX1XftRBe7kaFWaN+yssCN/NdgNeisVj0luQ1EYuX0 GisLpmgEXTvbn0DEwFH+79woSHDBR5igvN3cctJ3nWugJ/NyuGbU3jfcL w==; X-CSE-ConnectionGUID: r5GXHeQrTCicy6j1+c1kBg== X-CSE-MsgGUID: vJVZru1gRCiLz5YjMCqyZw== X-IronPort-AV: E=Sophos;i="6.25,154,1779148800"; d="scan'208";a="23486973" Received: from ip-10-5-6-203.us-west-2.compute.internal (HELO smtpout.naws.us-west-2.prod.farcaster.email.amazon.dev) ([10.5.6.203]) by internal-pdx-out-008.esa.us-west-2.outbound.mail-perimeter.amazon.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 10 Jul 2026 22:49:13 +0000 Received: from EX19MTAUWA002.ant.amazon.com [205.251.233.234:2616] by smtpin.naws.us-west-2.prod.farcaster.email.amazon.dev [10.0.26.212:2525] with esmtp (Farcaster) id 91575453-a2e4-4f77-8070-3c7c87c9bfb3; Fri, 10 Jul 2026 22:49:12 +0000 (UTC) X-Farcaster-Flow-ID: 91575453-a2e4-4f77-8070-3c7c87c9bfb3 Received: from EX19D001UWA001.ant.amazon.com (10.13.138.214) by EX19MTAUWA002.ant.amazon.com (10.250.64.202) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA) id 15.2.2562.43; Fri, 10 Jul 2026 22:49:12 +0000 Received: from dev-dsk-mkbund-2b-ce767ba1.us-west-2.amazon.com (10.169.40.81) by EX19D001UWA001.ant.amazon.com (10.13.138.214) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA) id 15.2.2562.43; Fri, 10 Jul 2026 22:49:12 +0000 From: Mark Bundschuh To: CC: Edward Adam Davis , , Chen Ridong , Tejun Heo Subject: [PATCH 6.12.y] sched/psi: fix race between file release and pressure write Date: Fri, 10 Jul 2026 22:48:45 +0000 Message-ID: <20260710224845.3892042-1-mkbund@amazon.com> X-Mailer: git-send-email 2.50.1 Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Content-Type: text/plain X-ClientProxiedBy: EX19D032UWA003.ant.amazon.com (10.13.139.37) To EX19D001UWA001.ant.amazon.com (10.13.138.214) From: Edward Adam Davis [ Upstream commit a5b98009f16d8a5fb4a8ff9a193f5735515c38fa ] A potential race condition exists between pressure write and cgroup file release regarding the priv member of struct kernfs_open_file, which triggers the uaf reported in [1]. Consider the following scenario involving execution on two separate CPUs: CPU0 CPU1 ==== ==== vfs_rmdir() kernfs_iop_rmdir() cgroup_rmdir() cgroup_kn_lock_live() cgroup_destroy_locked() cgroup_addrm_files() cgroup_rm_file() kernfs_remove_by_name() kernfs_remove_by_name_ns() vfs_write() __kernfs_remove() new_sync_write() kernfs_drain() kernfs_fop_write_iter() kernfs_drain_open_files() cgroup_file_write() kernfs_release_file() pressure_write() cgroup_file_release() ctx = of->priv; kfree(ctx); of->priv = NULL; cgroup_kn_unlock() cgroup_kn_lock_live() cgroup_get(cgrp) cgroup_kn_unlock() if (ctx->psi.trigger) // here, trigger uaf for ctx, that is of->priv The cgroup_rmdir() is protected by the cgroup_mutex, it also safeguards the memory deallocation of of->priv performed within cgroup_file_release(). However, the operations involving of->priv executed within pressure_write() are not entirely covered by the protection of cgroup_mutex. Consequently, if the code in pressure_write(), specifically the section handling the ctx variable executes after cgroup_file_release() has completed, a uaf vulnerability involving of->priv is triggered. Therefore, the issue can be resolved by extending the scope of the cgroup_mutex lock within pressure_write() to encompass all code paths involving of->priv, thereby properly synchronizing the race condition occurring between cgroup_file_release() and pressure_write(). And, if an live kn lock can be successfully acquired while executing the pressure write operation, it indicates that the cgroup deletion process has not yet reached its final stage; consequently, the priv pointer within open_file cannot be NULL. Therefore, the operation to retrieve the ctx value must be moved to a point *after* the live kn lock has been successfully acquired. In another situation, specifically after entering cgroup_kn_lock_live() but before acquiring cgroup_mutex, there exists a different class of race condition: CPU0: write memory.pressure CPU1: write cgroup.pressure=0 =========================== ============================= kernfs_fop_write_iter() kernfs_get_active_of(of) pressure_write() cgroup_kn_lock_live(memory.pressure) cgroup_tryget(cgrp) kernfs_break_active_protection(kn) ... blocks on cgroup_mutex cgroup_pressure_write() cgroup_kn_lock_live(cgroup.pressure) cgroup_file_show(memory.pressure, false) kernfs_show(false) kernfs_drain_open_files() cgroup_file_release(of) kfree(ctx) of->priv = NULL cgroup_kn_unlock() ... acquires cgroup_mutex ctx = of->priv; // may now be NULL if (ctx->psi.trigger) // NULL dereference Consequently, there is a possibility that of->priv is NULL, the pressure write needs to check for this. Now that the scope of the cgroup_mutex has been expanded, the original explicit cgroup_get/put operations are no longer necessary, this is because acquiring/releasing the live kn lock inherently executes a cgroup get/put operation. [1] BUG: KASAN: slab-use-after-free in pressure_write+0xa4/0x210 kernel/cgroup/cgroup.c:4011 Call Trace: pressure_write+0xa4/0x210 kernel/cgroup/cgroup.c:4011 cgroup_file_write+0x36f/0x790 kernel/cgroup/cgroup.c:4311 kernfs_fop_write_iter+0x3b0/0x540 fs/kernfs/file.c:352 Allocated by task 9352: cgroup_file_open+0x90/0x3a0 kernel/cgroup/cgroup.c:4256 kernfs_fop_open+0x9eb/0xcb0 fs/kernfs/file.c:724 do_dentry_open+0x83d/0x13e0 fs/open.c:949 Freed by task 9353: cgroup_file_release+0xd6/0x100 kernel/cgroup/cgroup.c:4283 kernfs_release_file fs/kernfs/file.c:764 [inline] kernfs_drain_open_files+0x392/0x720 fs/kernfs/file.c:834 kernfs_drain+0x470/0x600 fs/kernfs/dir.c:525 Fixes: 0e94682b73bf ("psi: introduce psi monitor") Reported-by: syzbot+33e571025d88efd1312c@syzkaller.appspotmail.com Closes: https://syzkaller.appspot.com/bug?extid=33e571025d88efd1312c Tested-by: syzbot+33e571025d88efd1312c@syzkaller.appspotmail.com Signed-off-by: Edward Adam Davis Reviewed-by: Chen Ridong Signed-off-by: Tejun Heo Signed-off-by: Mark Bundschuh diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c --- a/kernel/cgroup/cgroup.c +++ b/kernel/cgroup/cgroup.c @@ -3876,33 +3876,41 @@ static int cgroup_cpu_pressure_show(struct seq_file *seq, void *v) static ssize_t pressure_write(struct kernfs_open_file *of, char *buf, size_t nbytes, enum psi_res res) { - struct cgroup_file_ctx *ctx = of->priv; + struct cgroup_file_ctx *ctx; struct psi_trigger *new; struct cgroup *cgrp; struct psi_group *psi; + ssize_t ret = 0; cgrp = cgroup_kn_lock_live(of->kn, false); if (!cgrp) return -ENODEV; - cgroup_get(cgrp); - cgroup_kn_unlock(of->kn); + ctx = of->priv; + if (!ctx) { + ret = -ENODEV; + goto out_unlock; + } /* Allow only one trigger per file descriptor */ if (ctx->psi.trigger) { - cgroup_put(cgrp); - return -EBUSY; + ret = -EBUSY; + goto out_unlock; } psi = cgroup_psi(cgrp); new = psi_trigger_create(psi, buf, res, of->file, of); if (IS_ERR(new)) { - cgroup_put(cgrp); - return PTR_ERR(new); + ret = PTR_ERR(new); + goto out_unlock; } smp_store_release(&ctx->psi.trigger, new); - cgroup_put(cgrp); + +out_unlock: + cgroup_kn_unlock(of->kn); + if (ret) + return ret; return nbytes; }