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Date:   Fri, 18 Dec 2020 11:32:46 +0100
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Hillf Danton <hdanton@...a.com>
Cc:     syzbot <syzbot+9e04e2df4a32fb661daf@...kaller.appspotmail.com>,
        andreyknvl@...gle.com, gustavoars@...nel.org,
        ingrassia@...genesys.com, lee.jones@...aro.org,
        linux-kernel@...r.kernel.org, linux-usb@...r.kernel.org,
        syzkaller-bugs@...glegroups.com
Subject: Re: KASAN: use-after-free Read in service_outstanding_interrupt

On Fri, Dec 18, 2020 at 06:01:34PM +0800, Hillf Danton wrote:
> On Fri, 18 Dec 2020 09:28:16 +0100 Greg KH wrote:
> >On Fri, Dec 18, 2020 at 04:21:13PM +0800, Hillf Danton wrote:
> >> On Thu, 17 Dec 2020 19:21:10 -0800
> >> > syzbot has found a reproducer for the following issue on:
> >> > 
> >> > HEAD commit:    5e60366d Merge tag 'fallthrough-fixes-clang-5.11-rc1' of g..
> >> > git tree:       https://git.kernel.org/pub/scm/linux/kernel/git/gregkh/usb.git usb-testing
> >> > console output: https://syzkaller.appspot.com/x/log.txt?x=12c5b623500000
> >> > kernel config:  https://syzkaller.appspot.com/x/.config?x=5cea7506b7139727
> >> > dashboard link: https://syzkaller.appspot.com/bug?extid=9e04e2df4a32fb661daf
> >> > compiler:       gcc (GCC) 10.1.0-syz 20200507
> >> > syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=175adf07500000
> >> > C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=1672680f500000
> >> > 
> >> > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> >> > Reported-by: syzbot+9e04e2df4a32fb661daf@...kaller.appspotmail.com
> >> > 
> >> > ==================================================================
> >> > BUG: KASAN: use-after-free in usb_submit_urb+0x1210/0x1560 drivers/usb/core/urb.c:383
> >> > Read of size 4 at addr ffff888101d21018 by task syz-executor166/4405
> >> > 
> >> > CPU: 0 PID: 4405 Comm: syz-executor166 Not tainted 5.10.0-syzkaller #0
> >> > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
> >> > Call Trace:
> >> >  __dump_stack lib/dump_stack.c:79 [inline]
> >> >  dump_stack+0x107/0x163 lib/dump_stack.c:120
> >> >  print_address_description.constprop.0.cold+0xae/0x4c8 mm/kasan/report.c:385
> >> >  __kasan_report mm/kasan/report.c:545 [inline]
> >> >  kasan_report.cold+0x1f/0x37 mm/kasan/report.c:562
> >> >  usb_submit_urb+0x1210/0x1560 drivers/usb/core/urb.c:383
> >> >  service_outstanding_interrupt.part.0+0x5f/0xa0 drivers/usb/class/cdc-wdm.c:470
> >> >  service_outstanding_interrupt drivers/usb/class/cdc-wdm.c:465 [inline]
> >> >  wdm_read+0x9a0/0xbd0 drivers/usb/class/cdc-wdm.c:583
> >> >  vfs_read+0x1b5/0x570 fs/read_write.c:494
> >> >  ksys_read+0x12d/0x250 fs/read_write.c:634
> >> >  do_syscall_64+0x2d/0x40 arch/x86/entry/common.c:46
> >> >  entry_SYSCALL_64_after_hwframe+0x44/0xa9
> >> > RIP: 0033:0x44b529
> >> > Code: e8 bc b4 02 00 48 83 c4 18 c3 0f 1f 80 00 00 00 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 0f 83 8b cb fb ff c3 66 2e 0f 1f 84 00 00 00 00
> >> > RSP: 002b:00007f2dfcb6ed98 EFLAGS: 00000246 ORIG_RAX: 0000000000000000
> >> > RAX: ffffffffffffffda RBX: 00000000006dcc38 RCX: 000000000044b529
> >> > RDX: 0000000000001000 RSI: 0000000020001000 RDI: 0000000000000004
> >> > RBP: 00000000006dcc30 R08: 0000000000000000 R09: 0000000000000000
> >> > R10: 0000000000000000 R11: 0000000000000246 R12: 00000000006dcc3c
> >> > R13: 0142006002090100 R14: 04010040a4157d25 R15: 4000000200000112
> >> > 
> >> > Allocated by task 2632:
> >> >  kasan_save_stack+0x1b/0x40 mm/kasan/common.c:48
> >> >  kasan_set_track mm/kasan/common.c:56 [inline]
> >> >  __kasan_kmalloc.constprop.0+0xc2/0xd0 mm/kasan/common.c:461
> >> >  kmalloc include/linux/slab.h:552 [inline]
> >> >  kzalloc include/linux/slab.h:682 [inline]
> >> >  usb_alloc_dev+0x51/0xef0 drivers/usb/core/usb.c:582
> >> >  hub_port_connect drivers/usb/core/hub.c:5129 [inline]
> >> >  hub_port_connect_change drivers/usb/core/hub.c:5363 [inline]
> >> >  port_event drivers/usb/core/hub.c:5509 [inline]
> >> >  hub_event+0x1def/0x42d0 drivers/usb/core/hub.c:5591
> >> >  process_one_work+0x98d/0x15c0 kernel/workqueue.c:2275
> >> >  worker_thread+0x64c/0x1120 kernel/workqueue.c:2421
> >> >  kthread+0x38c/0x460 kernel/kthread.c:292
> >> >  ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:296
> >> > 
> >> > Freed by task 2181:
> >> >  kasan_save_stack+0x1b/0x40 mm/kasan/common.c:48
> >> >  kasan_set_track+0x1c/0x30 mm/kasan/common.c:56
> >> >  kasan_set_free_info+0x1b/0x30 mm/kasan/generic.c:352
> >> >  __kasan_slab_free+0x102/0x140 mm/kasan/common.c:422
> >> >  slab_free_hook mm/slub.c:1544 [inline]
> >> >  slab_free_freelist_hook+0x5d/0x150 mm/slub.c:1577
> >> >  slab_free mm/slub.c:3140 [inline]
> >> >  kfree+0xdb/0x3a0 mm/slub.c:4122
> >> >  device_release+0x9f/0x240 drivers/base/core.c:1962
> >> >  kobject_cleanup lib/kobject.c:705 [inline]
> >> >  kobject_release lib/kobject.c:736 [inline]
> >> >  kref_put include/linux/kref.h:65 [inline]
> >> >  kobject_put+0x1c8/0x540 lib/kobject.c:753
> >> >  put_device+0x1b/0x30 drivers/base/core.c:3190
> >> >  hub_port_connect drivers/usb/core/hub.c:5074 [inline]
> >> >  hub_port_connect_change drivers/usb/core/hub.c:5363 [inline]
> >> >  port_event drivers/usb/core/hub.c:5509 [inline]
> >> >  hub_event+0x1c8a/0x42d0 drivers/usb/core/hub.c:5591
> >> >  process_one_work+0x98d/0x15c0 kernel/workqueue.c:2275
> >> >  worker_thread+0x64c/0x1120 kernel/workqueue.c:2421
> >> >  kthread+0x38c/0x460 kernel/kthread.c:292
> >> >  ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:296
> >> > 
> >> > The buggy address belongs to the object at ffff888101d21000
> >> >  which belongs to the cache kmalloc-2k of size 2048
> >> > The buggy address is located 24 bytes inside of
> >> >  2048-byte region [ffff888101d21000, ffff888101d21800)
> >> > The buggy address belongs to the page:
> >> > page:0000000019761127 refcount:1 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x101d20
> >> > head:0000000019761127 order:3 compound_mapcount:0 compound_pincount:0
> >> > flags: 0x200000000010200(slab|head)
> >> > raw: 0200000000010200 dead000000000100 dead000000000122 ffff888100042000
> >> > raw: 0000000000000000 0000000080080008 00000001ffffffff 0000000000000000
> >> > page dumped because: kasan: bad access detected
> >> > 
> >> > Memory state around the buggy address:
> >> >  ffff888101d20f00: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
> >> >  ffff888101d20f80: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
> >> > >ffff888101d21000: fa fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> >> >                             ^
> >> >  ffff888101d21080: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> >> >  ffff888101d21100: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> >> > ==================================================================
> >> > 
> >> Bail out without urb submitted if wdm device is disconnecting.
> >> 
> >> --- a/drivers/usb/class/cdc-wdm.c
> >> +++ b/drivers/usb/class/cdc-wdm.c
> >> @@ -474,18 +474,25 @@ out:
> >>  	return rv;
> >>  }
> >>  
> >> -static ssize_t wdm_read
> >> -(struct file *file, char __user *buffer, size_t count, loff_t *ppos)
> >> +static ssize_t wdm_read(struct file *file, char __user *buffer,
> >> +			size_t count, loff_t *ppos)
> >>  {
> >>  	int rv, cntr;
> >>  	int i = 0;
> >>  	struct wdm_device *desc = file->private_data;
> >>  
> >> +	if (test_bit(WDM_DISCONNECTING, &desc->flags))
> >> +	 	return -ENODEV;
> >>  
> >>  	rv = mutex_lock_interruptible(&desc->rlock); /*concurrent reads */
> >>  	if (rv < 0)
> >>  		return -ERESTARTSYS;
> >>  
> >> +	if (test_bit(WDM_DISCONNECTING, &desc->flags)) {
> >> +		rv = -ENODEV;
> >> +		goto err;
> >> +	}
> >
> >Why test before the lock if you test right after the lock?
> 
> Any test is supposed to make sense.

I do not understand.

I am trying to say, why do you even have the test outside of the lock as
that makes no sense?

> >and what happens if the bit is set right _after_ you test this?
> 
> Your question explains the above double test.

No it doesn't :)

> If we get desc->rlock and fail to test WDM_DISCONNECTING in the
> read path then we are safe because the same lock is acquired also
> in the disconnect path.

Then only check the bit _when_ the lock is held.  Otherwise it is a
pointless check.

thanks,

greg k-h

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