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

jira VULN-136507
cve CVE-2025-39682
commit-author Alan Stern <stern@rowland.harvard.edu>
commit a6b87bf

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 <stern@rowland.harvard.edu>
	Reported-by: syzbot+b63d677d63bcac06cf90@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/linux-usb/68753a08.050a0220.33d347.0008.GAE@google.com/
	Tested-by: syzbot+b63d677d63bcac06cf90@syzkaller.appspotmail.com
Fixes: dde5845 ("[PATCH] Generic HID layer - code split")
	Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/613a66cd-4309-4bce-a4f7-2905f9bce0c9@rowland.harvard.edu
	Signed-off-by: Benjamin Tissoires <bentiss@kernel.org>
(cherry picked from commit a6b87bf)
	Signed-off-by: Brett Mastbergen <bmastbergen@ciq.com>

# Conflicts:
#	drivers/hid/hid-core.c

Author: bmastbergen

drivers/hid/hid-core.c

@@ -1338,7 +1338,12 @@ EXPORT_SYMBOL_GPL(hid_snto32);
 
 static u32 s32ton(__s32 value, unsigned 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);