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SubjectRe: INFO: rcu detected stall in hub_event
On Mon, 25 Nov 2019, Alan Stern wrote:

> Jiri:
>
> On Sat, 23 Nov 2019, Andrey Konovalov wrote:
>
> > I'm not sure, but the stack trace reminds me of this issue, so this
> > report might be related:
> >
> > https://groups.google.com/d/msg/syzkaller-bugs/X0zVbh8aFEM/NsPcshjxBgAJ
>
> No, the issue is quite different, although it is also a bug in the HID
> parser. The big problem is that the parser assumes all usages will
> belong to a collection.
>
> There's also a second, smaller bug: hid_apply_multipler() assumes every
> Resolution Multiplier control is associated with a Logical Collection
> (i.e., there's no way the routine can ever set multiplier_collection to
> NULL) even though there's a big quotation from the HID Usage Table
> manual at the start of the function saying that they don't have to be.
> This bug can be fixed easily, though.
>
> The first bug is more troublesome. hid_add_usage() explicitly sets the
> parser->local.collection_index[] entry to 0 if the current collection
> stack is empty. But there's no way to distinguish this 0 from a
> genuine index value that happens to point to the first collection!
>
> So what should happen when a usage appears outside of all collections?
> Is it a bug in the report descriptor (the current code suggests that it
> is not)?
>
> Or should we use a different sentinel value for the collection_index[]
> entry, one that cannot be confused with a genuine value, such as
> UINT_MAX?

On Tue, 26 Nov 2019, Jiri Kosina wrote:

> Alan, did you get a test result from syzbot on this patch? My mailbox
> doesn't seem to have it.

On Tue, 26 Nov 2019, syzbot wrote:

> Hello,
>
> syzbot has tested the proposed patch and the reproducer did not trigger
> crash:
>
> Reported-and-tested-by:
> syzbot+ec5f884c4a135aa0dbb9@syzkaller.appspotmail.com

Jiri:

Now we've got the answer. The current question is: What should I do
with the patch? It seems rather ad-hoc, not a proper solution to the
problem.

To refresh your memory, here is the patch that syzbot tested:

drivers/hid/hid-core.c | 5 +++++
1 file changed, 5 insertions(+)

Index: usb-devel/drivers/hid/hid-core.c
===================================================================
--- usb-devel.orig/drivers/hid/hid-core.c
+++ usb-devel/drivers/hid/hid-core.c
@@ -1057,6 +1057,8 @@ static void hid_apply_multiplier(struct
while (multiplier_collection->parent_idx != -1 &&
multiplier_collection->type != HID_COLLECTION_LOGICAL)
multiplier_collection = &hid->collection[multiplier_collection->parent_idx];
+ if (multiplier_collection->type != HID_COLLECTION_LOGICAL)
+ multiplier_collection = NULL;

effective_multiplier = hid_calculate_multiplier(hid, multiplier);

@@ -1191,6 +1193,9 @@ int hid_open_report(struct hid_device *d
}
device->collection_size = HID_DEFAULT_NUM_COLLECTIONS;

+ /* Needed for usages before the first collection */
+ device->collection[0].parent_idx = -1;
+
ret = -EINVAL;
while ((start = fetch_item(start, end, &item)) != NULL) {

Alan Stern
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