HID: core: Harden s32ton() against conversion to 0 bits

Testing by the syzbot fuzzer showed that the HID core gets a
shift-out-of-bounds exception when it tries to convert a 32-bit
quantity to a 0-bit quantity.  Ideally this should never occur, but
there are buggy devices and some might have a report field with size
set to zero; we shouldn't reject the report or the device just because
of that.

Instead, harden the s32ton() routine so that it returns a reasonable
result instead of crashing when it is called with the number of bits
set to 0 -- the same as what snto32() does.

Signed-off-by: Alan Stern <[email protected]>
Reported-by: [email protected]
Closes: https://lore.kernel.org/linux-usb/[email protected]/
Tested-by: [email protected]
Fixes: dde5845 ("[PATCH] Generic HID layer - code split")
Cc: [email protected]
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Benjamin Tissoires <[email protected]>

Author: AlanStern

drivers/hid/hid-core.c

@@ -66,8 +66,12 @@ static s32 snto32(__u32 value, unsigned int n)
 
 static u32 s32ton(__s32 value, unsigned int n)
 {
-	s32 a = value >> (n - 1);
+	s32 a;
 
+	if (!value || !n)
+		return 0;
+
+	a = value >> (n - 1);
 	if (a && a != -1)
 		return value < 0 ? 1 << (n - 1) : (1 << (n - 1)) - 1;
 	return value & ((1 << n) - 1);