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

jira VULN-131254 cve CVE-2025-38556 commit-author Alan Stern <stern@rowland.harvard.edu> commit a6b87bf upstream-diff This function is in a different place in this kernel, so there was a conflict. Also, when this function was moved in the upstream kernel, a newline was added afer 's32 a = value....'. Since that newline doesn't exist in this kernel this commit adds it. 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>

Author: bmastbergen

drivers/hid/hid-core.c

@@ -1052,7 +1052,12 @@ 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);