I made some mistakes in the previous e-mail. Please ignore that. There are a lot of things going on...Sorry for that. On Mon, Jul 20, 2020 at 03:12:57PM +0300, Dan Carpenter wrote: > So another option would be to just add HIDIOCGUSAGE and HIDIOCSUSAGE to > the earlier check. That risks breaking userspace. Another option is to > just add a check like you did earlier to the HIDIOCGUSAGE case. > Probably just do option #2 and resend. Sure, I will just add the same check to the HIDIOCGUSAGE case for the time being. Thank you for the detailed explanation. Here's what I found after digging a bit further though: hid_open_report() calls different functions in order to process different type of items: drivers/hid/hid-core.c:1193: static int (*dispatch_type[])(struct hid_parser *parser, struct hid_item *item) = { hid_parser_main, hid_parser_global, hid_parser_local, hid_parser_reserved }; In this case, hid_parser_main() calls hid_add_field(), which in turn calls hid_register_field(), which allocates the `field` object as you mentioned: drivers/hid/hid-core.c:102: field = kzalloc((sizeof(struct hid_field) + usages * sizeof(struct hid_usage) + values * sizeof(unsigned)), GFP_KERNEL); Here, `values` equals to `global.report_count`. See how it is being called: drivers/hid/hid-core.c:303: field = hid_register_field(report, usages, parser->global.report_count); In hid_open_report(), `global.report_count` can be set by calling hid_parser_global(). However, the syzkaller reproducer made hid_open_report() to call hid_parser_main() __before__ `global.report_count` is properly set. It's zero. So hid_register_field() allocated `field` with `values` equals to zero - No room for value[] at all. I believe this caused the bug. Apparently hid_open_report() doesn't care about which item (main, local, global and reserved) gets processed first. I am new to this code and I don't know whether this is by design, but this arbitrarity is apparently causing some issues. As another example, in hid_add_field(): drivers/hid/hid-core.c:289: report->size += parser->global.report_size * parser->global.report_count; If `global.report_count` is zero, `report->size` gets increased by zero. Is this working as intended? It seems weird to me. Thank you, Peilin Ye