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Date:   Wed, 23 Feb 2022 09:38:20 -0500
From:   "stern@rowland.harvard.edu" <stern@rowland.harvard.edu>
To:     "gregkh@linuxfoundation.org" <gregkh@linuxfoundation.org>
Cc:     "Zhang, Qiang1" <qiang1.zhang@intel.com>,
        syzbot <syzbot+348b571beb5eeb70a582@syzkaller.appspotmail.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "rafael@kernel.org" <rafael@kernel.org>,
        "syzkaller-bugs@googlegroups.com" <syzkaller-bugs@googlegroups.com>,
        "balbi@kernel.org" <balbi@kernel.org>
Subject: Re: [syzbot] KASAN: use-after-free Read in dev_uevent
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On Wed, Feb 23, 2022 at 12:29:03PM +0100, gregkh@linuxfoundation.org wrote:
> On Wed, Feb 23, 2022 at 11:17:07AM +0000, Zhang, Qiang1 wrote:
> > 
> > syzbot has found a reproducer for the following issue on:
> > 
> > HEAD commit:    4f12b742eb2b Merge tag 'nfs-for-5.17-3' of git://git.linux..
> > git tree:       upstream
> > console output: https://syzkaller.appspot.com/x/log.txt?x=110a6df2700000
> > kernel config:  https://syzkaller.appspot.com/x/.config?x=f6a069ed94a1ed1d
> > dashboard link: https://syzkaller.appspot.com/bug?extid=348b571beb5eeb70a582
> > compiler:       gcc (Debian 10.2.1-6) 10.2.1 20210110, GNU ld (GNU Binutils for Debian) 2.35.2
> > syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=12377296700000
> > 
> > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > Reported-by: syzbot+348b571beb5eeb70a582@syzkaller.appspotmail.com
> > 
> > ==================================================================
> > BUG: KASAN: use-after-free in dev_uevent+0x712/0x780 drivers/base/core.c:2320 Read of size 8 at addr ffff88802b934098 by task udevd/3689
> > 
> > CPU: 2 PID: 3689 Comm: udevd Not tainted 5.17.0-rc4-syzkaller-00229-g4f12b742eb2b #0 Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.14.0-2 04/01/2014 Call Trace:
> >  <TASK>
> >  __dump_stack lib/dump_stack.c:88 [inline]
> >  dump_stack_lvl+0xcd/0x134 lib/dump_stack.c:106
> >  print_address_description.constprop.0.cold+0x8d/0x303 mm/kasan/report.c:255  __kasan_report mm/kasan/report.c:442 [inline]  kasan_report.cold+0x83/0xdf mm/kasan/report.c:459
> >  dev_uevent+0x712/0x780 drivers/base/core.c:2320
> >  uevent_show+0x1b8/0x380 drivers/base/core.c:2391
> >  dev_attr_show+0x4b/0x90 drivers/base/core.c:2094
> >  sysfs_kf_seq_show+0x219/0x3d0 fs/sysfs/file.c:59
> >  seq_read_iter+0x4f5/0x1280 fs/seq_file.c:230
> >  kernfs_fop_read_iter+0x514/0x6f0 fs/kernfs/file.c:241  call_read_iter include/linux/fs.h:2068 [inline]
> >  new_sync_read+0x429/0x6e0 fs/read_write.c:400
> >  vfs_read+0x35c/0x600 fs/read_write.c:481
> >  ksys_read+0x12d/0x250 fs/read_write.c:619
> >  do_syscall_x64 arch/x86/entry/common.c:50 [inline]
> >  do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80  entry_SYSCALL_64_after_hwframe+0x44/0xae
> > RIP: 0033:0x7f964cc558fe
> > Code: c0 e9 e6 fe ff ff 50 48 8d 3d 0e c7 09 00 e8 c9 cf 01 00 66 0f 1f 84 00 00 00 00 00 64 8b 04 25 18 00 00 00 85 c0 75 14 0f 05 <48> 3d 00 f0 ff ff 77 5a c3 66 0f 1f 84 00 00 00 00 00 48 83 ec 28
> > RSP: 002b:00007ffc0133d258 EFLAGS: 00000246 ORIG_RAX: 0000000000000000
> > RAX: ffffffffffffffda RBX: 000056497b21a140 RCX: 00007f964cc558fe
> > RDX: 0000000000001000 RSI: 000056497b218650 RDI: 0000000000000008
> > RBP: 00007f964cd22380 R08: 0000000000000008 R09: 00007f964cd25a60
> > R10: 0000000000000008 R11: 0000000000000246 R12: 000056497b21a140
> > R13: 0000000000000d68 R14: 00007f964cd21780 R15: 0000000000000d68  </TASK>
> > 
> > Cc: Alan Stern 
> >        Felipe Balbi
> > 
> > Hello syzbot, Please try it:
> > 
> > From 574d45ff924e2d2f9b9f5cc3e846f8004498a811 Mon Sep 17 00:00:00 2001
> > From: Zqiang <qiang1.zhang@intel.com>
> > Date: Wed, 23 Feb 2022 18:18:22 +0800
> > Subject: [PATCH] driver core: Fix use-after-free in dev_uevent()
> > 
> > In dev_uevent(), if the "dev->driver" is valid, the "dev->driver->name"
> > be accessed, there may be a window period between these two operations.
> 
> There should not be any such window period.  The bus locks should
> prevent this, unless some driver is doing odd things with the pointers
> that it should not be doing.

Which bus locks are you referring to?  I'm not aware of any locks that 
synchronize dev_uevent() with anything (in particular, with driver 
unbinding).

And as far as I know, usb_gadget_remove_driver() doesn't play any odd 
tricks with pointers.

> > in this window period if the "dev->driver" is set to null
> > (in usb_gadget_unregister_driver function), when the "dev->driver->name"
> > is accessed again, invalid address will be accessed. fix it by checking
> > "dev->driver" again.
> > 
> > Signed-off-by: Zqiang <qiang1.zhang@intel.com>
> > ---
> >  drivers/base/core.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/drivers/base/core.c b/drivers/base/core.c
> > index 3d6430eb0c6a..a45b927ee76e 100644
> > --- a/drivers/base/core.c
> > +++ b/drivers/base/core.c
> > @@ -2317,7 +2317,7 @@ static int dev_uevent(struct kobject *kobj, struct kobj_uevent_env *env)
> >                 add_uevent_var(env, "DEVTYPE=%s", dev->type->name);
> > 
> >         if (dev->driver)
> > -               add_uevent_var(env, "DRIVER=%s", dev->driver->name);
> > +               add_uevent_var(env, "DRIVER=%s", dev_driver_string(dev));
> 
> What's to prevent the "window" from happening in the middle of the
> dev_driver_string() call?

Nothing prevents it.  But if you read the code for dev_driver_string(), 
you will see that it doesn't matter if dev->driver gets set to NULL 
while the routine is running.

(Of course, there is still the possibility that the driver structure 
itself might get deallocated while dev_driver_string() is running.  
This whole area could perhaps use a little careful thought.  Driver 
unbinding might be a good application for SRCU.)

Alan Stern