From patchwork Thu Dec 19 12:02:03 2019 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Will Deacon X-Patchwork-Id: 11303367 Return-Path: Received: from mail.kernel.org (pdx-korg-mail-1.web.codeaurora.org [172.30.200.123]) by pdx-korg-patchwork-2.web.codeaurora.org (Postfix) with ESMTP id C0604138D for ; Thu, 19 Dec 2019 12:02:22 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 9E1A424650 for ; Thu, 19 Dec 2019 12:02:22 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1576756942; bh=4tSN8Ty0g9jbGMWwle8pw+lMUclGXrafZ3F/UVZPngM=; h=From:To:Cc:Subject:Date:List-ID:From; b=UmKgy5KbApuze0eJIAFwltRgzOdsekchKBkD8PSn/+foBVDB63gZ4qaUSQwMoKgmr T3XBbn64YcKTKOwMjaCZz08VUfjp9sFRBrFPJ4GXxmx2jQ05Zhy29uT+GOIoWdiJ0c gXtiOHVVaGGkB1Ou/uOVR6CjZHLJdR9bqEH/bS/8= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726712AbfLSMCS (ORCPT ); Thu, 19 Dec 2019 07:02:18 -0500 Received: from mail.kernel.org ([198.145.29.99]:34856 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726668AbfLSMCS (ORCPT ); Thu, 19 Dec 2019 07:02:18 -0500 Received: from localhost.localdomain (236.31.169.217.in-addr.arpa [217.169.31.236]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 82457222C2; Thu, 19 Dec 2019 12:02:15 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1576756937; bh=4tSN8Ty0g9jbGMWwle8pw+lMUclGXrafZ3F/UVZPngM=; h=From:To:Cc:Subject:Date:From; b=vFw2XExoEj0Ip/k2VkBkAdc26WWv6SqBqelcFAXwcnHAkDmFafozwzuP/k46Bl1yS bDfIb2uoEW8VTWXutcdBjXnrqOl7jo1NrMZkskPGjFQ0BOufjeBvmtyN3ILermmoFP y94WAjaVc9H1NkGRKGtat1m67EaWSNS9yNVrV6ws= From: Will Deacon To: linux-kernel@vger.kernel.org Cc: linux-fsdevel@vger.kernel.org, kernel-team@android.com, Will Deacon , Greg KH , Hillf Danton , Andrew Morton , Al Viro , syzbot+82defefbbd8527e1c2cb@syzkaller.appspotmail.com Subject: [PATCH] chardev: Avoid potential use-after-free in 'chrdev_open()' Date: Thu, 19 Dec 2019 12:02:03 +0000 Message-Id: <20191219120203.32691-1-will@kernel.org> X-Mailer: git-send-email 2.20.1 MIME-Version: 1.0 Sender: linux-fsdevel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org 'chrdev_open()' calls 'cdev_get()' to obtain a reference to the 'struct cdev *' stashed in the 'i_cdev' field of the target inode structure. If the pointer is NULL, then it is initialised lazily by looking up the kobject in the 'cdev_map' and so the whole procedure is protected by the 'cdev_lock' spinlock to serialise initialisation of the shared pointer. Unfortunately, it is possible for the initialising thread to fail *after* installing the new pointer, for example if the subsequent '->open()' call on the file fails. In this case, 'cdev_put()' is called, the reference count on the kobject is dropped and, if nobody else has taken a reference, the release function is called which finally clears 'inode->i_cdev' from 'cdev_purge()' before potentially freeing the object. The problem here is that a racing thread can happily take the 'cdev_lock' and see the non-NULL pointer in the inode, which can result in a refcount increment from zero and a warning: | ------------[ cut here ]------------ | refcount_t: addition on 0; use-after-free. | WARNING: CPU: 2 PID: 6385 at lib/refcount.c:25 refcount_warn_saturate+0x6d/0xf0 | Modules linked in: | CPU: 2 PID: 6385 Comm: repro Not tainted 5.5.0-rc2+ #22 | Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.12.0-1 04/01/2014 | RIP: 0010:refcount_warn_saturate+0x6d/0xf0 | Code: 05 55 9a 15 01 01 e8 9d aa c8 ff 0f 0b c3 80 3d 45 9a 15 01 00 75 ce 48 c7 c7 00 9c 62 b3 c6 08 | RSP: 0018:ffffb524c1b9bc70 EFLAGS: 00010282 | RAX: 0000000000000000 RBX: ffff9e9da1f71390 RCX: 0000000000000000 | RDX: ffff9e9dbbd27618 RSI: ffff9e9dbbd18798 RDI: ffff9e9dbbd18798 | RBP: 0000000000000000 R08: 000000000000095f R09: 0000000000000039 | R10: 0000000000000000 R11: ffffb524c1b9bb20 R12: ffff9e9da1e8c700 | R13: ffffffffb25ee8b0 R14: 0000000000000000 R15: ffff9e9da1e8c700 | FS: 00007f3b87d26700(0000) GS:ffff9e9dbbd00000(0000) knlGS:0000000000000000 | CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 | CR2: 00007fc16909c000 CR3: 000000012df9c000 CR4: 00000000000006e0 | DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000 | DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400 | Call Trace: | kobject_get+0x5c/0x60 | cdev_get+0x2b/0x60 | chrdev_open+0x55/0x220 | ? cdev_put.part.3+0x20/0x20 | do_dentry_open+0x13a/0x390 | path_openat+0x2c8/0x1470 | do_filp_open+0x93/0x100 | ? selinux_file_ioctl+0x17f/0x220 | do_sys_open+0x186/0x220 | do_syscall_64+0x48/0x150 | entry_SYSCALL_64_after_hwframe+0x44/0xa9 | RIP: 0033:0x7f3b87efcd0e | Code: 89 54 24 08 e8 a3 f4 ff ff 8b 74 24 0c 48 8b 3c 24 41 89 c0 44 8b 54 24 08 b8 01 01 00 00 89 f4 | RSP: 002b:00007f3b87d259f0 EFLAGS: 00000293 ORIG_RAX: 0000000000000101 | RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007f3b87efcd0e | RDX: 0000000000000000 RSI: 00007f3b87d25a80 RDI: 00000000ffffff9c | RBP: 00007f3b87d25e90 R08: 0000000000000000 R09: 0000000000000000 | R10: 0000000000000000 R11: 0000000000000293 R12: 00007ffe188f504e | R13: 00007ffe188f504f R14: 00007f3b87d26700 R15: 0000000000000000 | ---[ end trace 24f53ca58db8180a ]--- Since 'cdev_get()' can already fail to obtain a reference, simply move it over to use 'kobject_get_unless_zero()' instead of 'kobject_get()', which will cause the racing thread to return -ENXIO if the initialising thread fails unexpectedly. Cc: Greg KH Cc: Hillf Danton Cc: Andrew Morton Cc: Al Viro Reported-by: syzbot+82defefbbd8527e1c2cb@syzkaller.appspotmail.com Signed-off-by: Will Deacon --- fs/char_dev.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/fs/char_dev.c b/fs/char_dev.c index 00dfe17871ac..c5e6eff5a381 100644 --- a/fs/char_dev.c +++ b/fs/char_dev.c @@ -352,7 +352,7 @@ static struct kobject *cdev_get(struct cdev *p) if (owner && !try_module_get(owner)) return NULL; - kobj = kobject_get(&p->kobj); + kobj = kobject_get_unless_zero(&p->kobj); if (!kobj) module_put(owner); return kobj;