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SubjectRe: WARNING: refcount bug in cdev_get
On Wed, Dec 18, 2019 at 05:08:55PM +0000, Will Deacon wrote:
> 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 ]---

No hint as to _where_ you put the mdelay()? :)


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