[PATCH] HID: core: detect and skip invalid inputs to snto32()

From: Randy Dunlap
Date: Wed Dec 16 2020 - 20:13:24 EST


Prevent invalid (0, 0) inputs to hid-core's snto32() function.

Maybe it is just the dummy device here that is causing this, but
there are hundreds of calls to snto32(0, 0). Having n (bits count)
of 0 is causing the current UBSAN trap with a shift value of
0xffffffff (-1, or n - 1 in this function).

Either of the value to shift being 0 or the bits count being 0 can be
handled by just returning 0 to the caller, avoiding the following
complex shift + OR operations:

return value & (1 << (n - 1)) ? value | (~0U << n) : value;

Fixes: dde5845a529f ("[PATCH] Generic HID layer - code split")
Signed-off-by: Randy Dunlap <rdunlap@xxxxxxxxxxxxx>
Reported-by: syzbot+1e911ad71dd4ea72e04a@xxxxxxxxxxxxxxxxxxxxxxxxx
Cc: Jiri Kosina <jikos@xxxxxxxxxx>
Cc: Benjamin Tissoires <benjamin.tissoires@xxxxxxxxxx>
Cc: linux-input@xxxxxxxxxxxxxxx
---
drivers/hid/hid-core.c | 3 +++
1 file changed, 3 insertions(+)

--- lnx-510.orig/drivers/hid/hid-core.c
+++ lnx-510/drivers/hid/hid-core.c
@@ -1307,6 +1307,9 @@ EXPORT_SYMBOL_GPL(hid_open_report);

static s32 snto32(__u32 value, unsigned n)
{
+ if (!value || !n)
+ return 0;
+
switch (n) {
case 8: return ((__s8)value);
case 16: return ((__s16)value);