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SubjectRe: WARNING: refcount bug in cdev_get
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 ]---

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