Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755264Ab1C1RwZ (ORCPT ); Mon, 28 Mar 2011 13:52:25 -0400 Received: from mail4.comsite.net ([205.238.176.238]:35519 "EHLO mail4.comsite.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752443Ab1C1RwX (ORCPT ); Mon, 28 Mar 2011 13:52:23 -0400 X-Default-Received-SPF: pass (skip=forwardok (res=PASS)) x-ip-name=71.22.127.106; To: Amit Shah From: Milton Miller Cc: Rusty Russell , linux-kernel@vger.kernel.org, benh@kernel.crashing.org, greg@kroah.com, linuxppc-dev@ozlabs.org Subject: Re: hvc_console: Don't access hvc_task if not initialised Message-ID: References:

<20110325084714.GA28356@amit-x200.redhat.com> In-Reply-To: <20110325084714.GA28356@amit-x200.redhat.com> Date: Mon, 28 Mar 2011 11:52:05 -0600 X-Originating-IP: 71.22.127.106 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 5406 Lines: 122 On Fri, 25 Mar 2011 about 14:17:14 +0530, Amit Shah wrote: > On (Thu) 24 Mar 2011 [08:58:04], Milton Miller wrote: > > On Thu, 24 Mar 2011 07:29:58 -0000, Amit Shah wrote: > > > hvc_open() can be called without having any backing device. This > > > results in a call to hvc_kick() which calls wake_up_process on a NULL > > > pointer. > > > > How is hvc_open called without a hvc_driver registered to the tty layer? > > This gets reproduced in a couple of scenarios, I'm trying to get more > information. > > > > Ensure hvc is initialised by checking for a non-NULL hvc_task > > > before waking up the hvc thread. > > > > No if the task is missing the subsystem is really stuck. Put a check > > in open and refuse to open. Just wanted to add emphasis. Making hvc_kick a do nothing is not acceptable. > > > > > > > > This was found by an autotest run for virtio_console without having a > > > console backend. Are you sure? the virtio_console module was loaded. > > > > > > > stack trace please > > Yes, should have included that: > > BUG: unable to handle kernel NULL pointer dereference at 0000000000000008 > IP: [] task_rq_lock+0x42/0xa0 > PGD 1d66a067 PUD 1d598067 PMD 0 > Oops: 0000 [#1] SMP > last sysfs file: /sys/devices/pci0000:00/0000:00:04.0/virtio1/virtio-ports/vport0p0/name > CPU 0 > Modules linked in: virtio_console virtio_net i2c_piix4 i2c_core ext4 > mbcache jbd2 sr_mod cdrom virtio_blk pata_acpi ata_generic ata_piix > virtio_pci virtio_ring virtio dm_mod [last unloaded: scsi_wait_scan] > > Modules linked in: virtio_console virtio_net i2c_piix4 i2c_core ext4 > mbcache jbd2 sr_mod cdrom virtio_blk pata_acpi ata_generic ata_piix > virtio_pci virtio_ring virtio dm_mod [last unloaded: scsi_wait_scan] > Pid: 672, comm: console_check Not tainted 2.6.32-125.el6.x86_64 #1 KVM So 2.6.32-125.el6.x86_64, not mainline. Please check for any patches affecting hvc_task and/or hvc_driver. > RIP: 0010:[] [] task_rq_lock+0x42/0xa0 > RSP: 0018:ffff880017cbbb98 EFLAGS: 00010082 > RAX: 0000000000000282 RBX: 0000000000015f40 RCX: 0000000000000002 > RDX: 0000000000000282 RSI: ffff880017cbbbf0 RDI: 0000000000000000 > RBP: ffff880017cbbbb8 R08: ffff88001bf39208 R09: ffff88001d67a000 > R10: 0000000000000001 R11: 0000000000000000 R12: 0000000000000000 > R13: ffff880017cbbbf0 R14: 0000000000000000 R15: 000000000000000f > FS: 00007f3592683700(0000) GS:ffff880002000000(0000) knlGS:0000000000000000 > CS: 0010 DS: 0000 ES: 0000 CR0: 000000008005003b > CR2: 0000000000000008 CR3: 000000001d595000 CR4: 00000000000006f0 > DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000 > DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400 so seems to be x86_64 > Process console_check (pid: 672, threadinfo ffff880017cba000, task ffff88001f5fe100) > Stack: > 0000000000000000 ffff88001bf39000 0000000000000000 0000000000000000 > <0> ffff880017cbbc28 ffffffff8105c74c ffff88001d5966a0 0000000000008800 > <0> ffff880017cbbbe8 ffffffff813081c1 ffff880017cbbc28 0000000000000282 > Call Trace: > [] try_to_wake_up+0x3c/0x400 > [] ? tty_ldisc_enable+0x31/0x40 > [] wake_up_process+0x15/0x20 > [] hvc_kick+0x1f/0x30 > [] hvc_open+0xca/0x120 > [] tty_open+0x211/0x510 > [] chrdev_open+0x125/0x230 > [] ? chrdev_open+0x0/0x230 > [] __dentry_open+0x10a/0x360 > [] ? selinux_inode_permission+0x72/0xb0 > [] ? security_inode_permission+0x1f/0x30 > [] nameidata_to_filp+0x54/0x70 > [] do_filp_open+0x700/0xd90 > [] ? audit_alloc_name+0x62/0x100 > [] ? mntput_no_expire+0x30/0x110 > [] ? alloc_fd+0x92/0x160 > [] do_sys_open+0x69/0x140 > [] sys_open+0x20/0x30 > [] system_call_fastpath+0x16/0x1b > Code: 89 74 24 18 0f 1f 44 00 00 48 c7 c3 40 5f 01 00 49 89 fc 49 89 > f5 9c 58 0f 1f 44 00 00 48 89 c2 fa 66 0f 1f 44 00 00 49 89 55 00 <49> > 8b 44 24 08 49 89 de 8b 40 18 4c 03 34 c5 40 46 b9 81 4c 89 > RIP [] task_rq_lock+0x42/0xa0 > RSP > CR2: 0000000000000008 > ---[ end trace a6f40a29150796da ]--- > > Amit Looking at 2.6.38+ without taking the time to audit my assumptions about the tty code, it would appear that we wait to register the driver until hvc_task is checked for IS_ERR(), and we only set hvc_driver after setting hvc_task. If it was some loosely ordered architecture I would worry about barriers. I also did a quick scan of patches from 2.6.32 to 38. I see several potential problems such as hvc_task is left IS_ERR, and we kill hvc_task before unregistering the tty, and we could end up with no kick with hvc_console backend registered (that should be ok, as there is still no tty driver to be called), but I don't see hvc_console in your module list and the only code to kill hvc_task and zero once its set is in the module exit. milton -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/