Re: WARNING: refcount bug in cdev_get

[Will Deacon]

Hi all,

On Tue, Dec 10, 2019 at 11:44:45AM +0000, Will Deacon wrote:
> On Wed, Dec 04, 2019 at 01:31:48PM +0100, Greg KH wrote:
> > But I thought we had a lock in play here, so why would changing this
> > actually fix anything?
> 
> I don't think the lock is always used. For example, look at chrdev_open(),
> which appears in the backtrace; the locked code is:
> 
> 	spin_lock(&cdev_lock);
> 	p = inode->i_cdev;
> 	if (!p) {
> 		struct kobject *kobj;
> 		int idx;
> 		spin_unlock(&cdev_lock);
> 		kobj = kobj_lookup(cdev_map, inode->i_rdev, &idx);
> 		if (!kobj)
> 			return -ENXIO;
> 		new = container_of(kobj, struct cdev, kobj);
> 		spin_lock(&cdev_lock);
> 		/* Check i_cdev again in case somebody beat us to it while
> 		   we dropped the lock. */
> 		p = inode->i_cdev;
> 		if (!p) {
> 			inode->i_cdev = p = new;
> 			list_add(&inode->i_devices, &p->list);
> 			new = NULL;
> 		} else if (!cdev_get(p))
> 			ret = -ENXIO;
> 	} else if (!cdev_get(p))
> 		ret = -ENXIO;
> 	spin_unlock(&cdev_lock);
> 	cdev_put(new);
> 
> So the idea is that multiple threads serialise on the 'cdev_lock' and then
> check 'inode->i_cdev' to figure out if the device has already been probed,
> taking a reference to it if it's available or probing it via kobj_lookup()
> otherwise. I think that's backwards with respect to things like cdev_put(),
> where the refcount is dropped *before* 'inode->i_cdev' is cleared to NULL.
> In which case, if a concurrent call to cdev_put() can drop the refcount
> to zero without 'cdev_lock' held, then you could get a use-after-free on
> this path thanks to a dangling pointer in 'inode->i_cdev'..
> 
> Looking slightly ahead in this same function, there are error paths which
> appear to do exactly that:
> 
> 	fops = fops_get(p->ops);
> 	if (!fops)
> 		goto out_cdev_put;
> 
> 	replace_fops(filp, fops);
> 	if (filp->f_op->open) {
> 		ret = filp->f_op->open(inode, filp);
> 		if (ret)
> 			goto out_cdev_put;
> 	}
> 
> 	return 0;
> 
>  out_cdev_put:
> 	cdev_put(p);
> 	return ret;
> 
> In which case the thread which installed 'inode->i_cdev' earlier on can
> now drop its refcount to zero without the lock held if, for example, the
> filp->f_op->open() call fails.
> 
> But note, this is purely based on code inspection -- the C reproducer from
> syzkaller doesn't work for me, so I've not been able to test any fixes either.
> It's also worth noting that cdev_put() is called from __fput(), but I think the
> reference counting on the file means we're ok there.
> 
> > This code hasn't changed in 15+ years, what suddenly changed that causes
> > problems here?
> 
> I suppose one thing to consider is that the refcount code is relatively new,
> so it could be that the actual use-after-free is extremely rare, but we're
> now seeing that it's at least potentially an issue.
> 
> Thoughts?

FWIW, I added some mdelay()s to make this race more likely, and I can now
trigger it reasonably reliably. See below.

Will

--->8

[   89.512353] ------------[ cut here ]------------
[   89.513350] refcount_t: addition on 0; use-after-free.
[   89.513977] WARNING: CPU: 2 PID: 6385 at lib/refcount.c:25 refcount_warn_saturate+0x6d/0xf0
[   89.514943] Modules linked in:
[   89.515307] CPU: 2 PID: 6385 Comm: repro Not tainted 5.5.0-rc2+ #22
[   89.516039] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.12.0-1 04/01/2014
[   89.517047] RIP: 0010:refcount_warn_saturate+0x6d/0xf0
[   89.517647] 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
[   89.519749] RSP: 0018:ffffb524c1b9bc70 EFLAGS: 00010282
[   89.520353] RAX: 0000000000000000 RBX: ffff9e9da1f71390 RCX: 0000000000000000
[   89.521184] RDX: ffff9e9dbbd27618 RSI: ffff9e9dbbd18798 RDI: ffff9e9dbbd18798
[   89.522020] RBP: 0000000000000000 R08: 000000000000095f R09: 0000000000000039
[   89.522854] R10: 0000000000000000 R11: ffffb524c1b9bb20 R12: ffff9e9da1e8c700
[   89.523689] R13: ffffffffb25ee8b0 R14: 0000000000000000 R15: ffff9e9da1e8c700
[   89.524512] FS:  00007f3b87d26700(0000) GS:ffff9e9dbbd00000(0000) knlGS:0000000000000000
[   89.525439] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   89.526105] CR2: 00007fc16909c000 CR3: 000000012df9c000 CR4: 00000000000006e0
[   89.526937] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[   89.527759] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[   89.528587] Call Trace:
[   89.528889]  kobject_get+0x5c/0x60
[   89.529290]  cdev_get+0x2b/0x60
[   89.529656]  chrdev_open+0x55/0x220
[   89.530060]  ? cdev_put.part.3+0x20/0x20
[   89.530515]  do_dentry_open+0x13a/0x390
[   89.530961]  path_openat+0x2c8/0x1470
[   89.531383]  do_filp_open+0x93/0x100
[   89.531797]  ? selinux_file_ioctl+0x17f/0x220
[   89.532297]  do_sys_open+0x186/0x220
[   89.532708]  do_syscall_64+0x48/0x150
[   89.533129]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
[   89.533704] RIP: 0033:0x7f3b87efcd0e
[   89.534115] 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
[   89.536227] RSP: 002b:00007f3b87d259f0 EFLAGS: 00000293 ORIG_RAX: 0000000000000101
[   89.537085] RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007f3b87efcd0e
[   89.537891] RDX: 0000000000000000 RSI: 00007f3b87d25a80 RDI: 00000000ffffff9c
[   89.538693] RBP: 00007f3b87d25e90 R08: 0000000000000000 R09: 0000000000000000
[   89.539493] R10: 0000000000000000 R11: 0000000000000293 R12: 00007ffe188f504e
[   89.540291] R13: 00007ffe188f504f R14: 00007f3b87d26700 R15: 0000000000000000
[   89.541090] ---[ end trace 24f53ca58db8180a ]---