[S] Change in flashrom[1.2.x]: helpers.c: Fix undefined behavior in strndup()

[Felix Singer (Code Review)]

Felix Singer has submitted this change. ( https://review.coreboot.org/c/flashrom/+/67870 )

Change subject: helpers.c: Fix undefined behavior in strndup()
......................................................................

helpers.c: Fix undefined behavior in strndup()

Using strlen() or strdup() inside strndup() is problematic: if the
input string is not null-terminated, these functions can read past the
end of the buffer, which triggers undefined behavior. Rewrite the
function to never read past the provided `maxlen` bound.

Change-Id: Id34127024085879228626fbad59af03268ec5255
Signed-off-by: Xiang Wang 
Reviewed-on: https://review.coreboot.org/c/flashrom/+/49741
Reviewed-by: Angel Pons 
Reviewed-by: Edward O'Callaghan 
Tested-by: build bot (Jenkins) 
Reviewed-on: https://review.coreboot.org/c/flashrom/+/67870
Reviewed-by: Felix Singer 
---
M helpers.c
1 file changed, 30 insertions(+), 8 deletions(-)

Approvals:
  build bot (Jenkins): Verified
  Felix Singer: Looks good to me, approved
  Angel Pons: Looks good to me, approved

diff --git a/helpers.c b/helpers.c
index c83cd2c..289848d 100644
--- a/helpers.c
+++ b/helpers.c
@@ -106,15 +106,16 @@
 /* strndup is a POSIX function not present in MinGW */
 char *strndup(const char *src, size_t maxlen)
 {
-	if (strlen(src) > maxlen) {
-		char *retbuf;
-		if ((retbuf = malloc(1 + maxlen)) != NULL) {
-			memcpy(retbuf, src, maxlen);
-			retbuf[maxlen] = '\0';
-		}
-		return retbuf;
+	char *retbuf;
+	size_t len;
+	for (len = 0; len < maxlen; len++)
+		if (src[len] == '\0')
+			break;
+	if ((retbuf = malloc(1 + len)) != NULL) {
+		memcpy(retbuf, src, len);
+		retbuf[len] = '\0';
    }
-	return strdup(src);
+	return retbuf;
 }
 #endif