[PATCH v3] hex2bin: fix access beyond string end

[Mikulas Patocka]

If we pass too short string to "hex2bin" (and the string size without the
terminating NUL character is even), "hex2bin" reads one byte after the
terminating NUL character. This patch fixes it.

Note that hex_to_bin returns -1 on error and hex2bin return -EINVAL on
error - so we can't just return the variable "hi" or "lo" on error. This
inconsistency may be fixed in the next merge window, but for the purpose
of fixing this bug, we just preserve the existing behavior and return -1
and -EINVAL.

Signed-off-by: Mikulas Patocka <mpatocka@xxxxxxxxxx>
Reviewed-by: Andy Shevchenko <andy.shevchenko@xxxxxxxxx>
Fixes: b78049831ffe ("lib: add error checking to hex2bin")
Cc: stable@xxxxxxxxxxxxxxx

---
 lib/hexdump.c |    9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

Index: linux-2.6/lib/hexdump.c
===================================================================
--- linux-2.6.orig/lib/hexdump.c	2022-04-24 18:51:16.000000000 +0200
+++ linux-2.6/lib/hexdump.c	2022-04-27 17:16:38.000000000 +0200
@@ -45,10 +45,13 @@ EXPORT_SYMBOL(hex_to_bin);
 int hex2bin(u8 *dst, const char *src, size_t count)
 {
 	while (count--) {
-		int hi = hex_to_bin(*src++);
-		int lo = hex_to_bin(*src++);
+		int hi, lo;
 
-		if ((hi < 0) || (lo < 0))
+		hi = hex_to_bin(*src++);
+		if (unlikely(hi < 0))
+			return -EINVAL;
+		lo = hex_to_bin(*src++);
+		if (unlikely(lo < 0))
 			return -EINVAL;
 
 		*dst++ = (hi << 4) | lo;