Fix -V to fail invalid input safely

This change fixes CVE-2018-14879.

get_next_file() did not check the return value of strlen() and
underflowed an array index if the line read by fgets() from the file
started with \0. This caused an out-of-bounds read and could cause a
write. Add the missing check.

This vulnerability was discovered by Brian Carpenter & Geeknik Labs.

Cherry picked from 9ba91381954ad325ea4fd26b9c65a8bd9a2a85b6
in 4.9 branch.

1 files changed, 4 insertions, 2 deletions

diff --git a/tcpdump.c b/tcpdump.c
index a9c2b7dc..bf4f2ef5 100644
--- a/tcpdump.c
+++ b/tcpdump.c
@@ -854,13 +854,15 @@ static char *
 get_next_file(FILE *VFile, char *ptr)
 {
 	char *ret;
+	size_t len;
 
 	ret = fgets(ptr, PATH_MAX, VFile);
 	if (!ret)
 		return NULL;
 
-	if (ptr[strlen(ptr) - 1] == '\n')
-		ptr[strlen(ptr) - 1] = '\0';
+	len = strlen (ptr);
+	if (len > 0 && ptr[len - 1] == '\n')
+		ptr[len - 1] = '\0';
 
 	return ret;
 }