[Linux-kernel-mentees] [PATCH v1] usbhid: Fix slab-out-of-bounds write in hiddev_ioctl_usage()

[Peilin Ye]

`uref->usage_index` is not always being properly checked, causing
hiddev_ioctl_usage() to go out of bounds under some cases. Fix it.

This patch fixes the following syzbot bug:

    
https://syzkaller.appspot.com/bug?id=f2aebe90b8c56806b050a20b36f51ed6acabe802

Reported-by: syzbot+34ee1b45d88571c2f...@syzkaller.appspotmail.com
Signed-off-by: Peilin Ye <yepeilin...@gmail.com>
---
This patch fixes the bug, but in an ugly way. Checks on `uref` are already
being done in this code:

                        if (cmd == HIDIOCGCOLLECTIONINDEX) {
                                if (uref->usage_index >= field->maxusage)
                                        goto inval;
                                uref->usage_index =
                                        array_index_nospec(uref->usage_index,
                                                           field->maxusage);
                        } else if (uref->usage_index >= field->report_count)
                                goto inval;

However it did not catch this bug since it's in an `else` bracket. Should
we move the above code out of the bracket? Would like to hear your opinion.
Thank you!

 drivers/hid/usbhid/hiddev.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/hid/usbhid/hiddev.c b/drivers/hid/usbhid/hiddev.c
index 4140dea693e9..c63d07caacef 100644
--- a/drivers/hid/usbhid/hiddev.c
+++ b/drivers/hid/usbhid/hiddev.c
@@ -525,6 +525,8 @@ static noinline int hiddev_ioctl_usage(struct hiddev 
*hiddev, unsigned int cmd,
                        goto goodreturn;
 
                case HIDIOCSUSAGE:
+                       if (uref->usage_index >= field->report_count)
+                               goto inval;
                        field->value[uref->usage_index] = uref->value;
                        goto goodreturn;
 
-- 
2.25.1