Re: [PATCH] chardev: Avoid potential use-after-free in 'chrdev_open()'

[Will Deacon]

On Thu, Dec 19, 2019 at 01:15:07PM +0100, Greg KH wrote:
> On Thu, Dec 19, 2019 at 12:02:03PM +0000, Will Deacon wrote:
> > '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 <gregkh@xxxxxxxxxxxxxxxxxxx>
> > Cc: Hillf Danton <hdanton@xxxxxxxx>
> > Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> > Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
> > Reported-by: syzbot+82defefbbd8527e1c2cb@xxxxxxxxxxxxxxxxxxxxxxxxx
> > Signed-off-by: Will Deacon <will@xxxxxxxxxx>
> > ---
> >  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;
> > -- 
> > 2.24.1.735.g03f4e72817-goog
> > 
> 
> Ugh.
> 
> Due to the location of the lock, this looks like the only viable
> solution.
> 
> Al, any objection for me taking this into my tree now to send to Linus?
> Will, this did fix the syzbot reproducer, right?

Yes, looks like it. It ran overnight without failing (even with my mdelay()
hacks applied to make the race more likely).

Will