Re: [PATCH v2] hex2bin: make the function hex_to_bin constant-time

["Jason A. Donenfeld" <Jason@zx2c4.com>]

On Wed, May 04, 2022 at 05:38:28PM +0900, Stafford Horne wrote:
> Just a heads up it seems this patch is causing some instability with crypto self
> tests on OpenRISC when using a PREEMPT kernel (no SMP).
> 
> This was reported by Jason A. Donenfeld as it came up in wireguard testing.
> 
> I am trying to figure out if this is an OpenRISC PREEMPT issue or something
> else.

The code of this commit looks fine. And actually the bug goes away if
you just add a `pr_err("hello!\n");` to the function. Plus, the function
is never called by that test kernel.

Actually, the bug even goes away if you change the sign of the input
back to naked char (which might be semantically better anyway) and then
let the function itself do the sign change (see below).

So more likely is that this patch just helps unmask a real issue
elsewhere -- linker, compiler, or register restoration after preemption.
I don't think there's anything to do with regards to the patch of this
thread, as it's clearly fine. Unless you want that sign thing below, but
even then, who cares. We should keep digging in on the OpenRISC front.

Jason

diff --git a/include/linux/kernel.h b/include/linux/kernel.h
index fe6efb24d151..a890428bcc1a 100644
--- a/include/linux/kernel.h
+++ b/include/linux/kernel.h
@@ -285,7 +285,7 @@ static inline char *hex_byte_pack_upper(char *buf, u8 byte)
 	return buf;
 }

-extern int hex_to_bin(unsigned char ch);
+extern int hex_to_bin(char ch);
 extern int __must_check hex2bin(u8 *dst, const char *src, size_t count);
 extern char *bin2hex(char *dst, const void *src, size_t count);

diff --git a/lib/hexdump.c b/lib/hexdump.c
index 06833d404398..b636b4dcabe9 100644
--- a/lib/hexdump.c
+++ b/lib/hexdump.c
@@ -43,9 +43,9 @@ EXPORT_SYMBOL(hex_asc_upper);
  *	uppercase and lowercase letters, so we use (ch & 0xdf), which converts
  *	lowercase to uppercase
  */
-int hex_to_bin(unsigned char ch)
+int hex_to_bin(char ch)
 {
-	unsigned char cu = ch & 0xdf;
+	unsigned char cu = ch & 0xdfU;
 	return -1 +
 		((ch - '0' +  1) & (unsigned)((ch - '9' - 1) & ('0' - 1 - ch)) >> 8) +
 		((cu - 'A' + 11) & (unsigned)((cu - 'F' - 1) & ('A' - 1 - cu)) >> 8);