initramfs: Check negative timestamp to prevent broken cpio archive

Similar to commit 4c9d410f ("initramfs: Check timestamp to prevent
broken cpio archive"), except asserts that the timestamp is
non-negative. This can happen when the KBUILD_BUILD_TIMESTAMP is a value
before UNIX epoch, which may be set when making reproducible builds that
don't want to look like they use a valid date.

While support for dates before 1970 might not be supported, this is more
about preventing undetected CPIO corruption. The printf's use a minimum
length format specifier, and will happily make the field longer than 8
characters if they need to.
Signed-off-by: Benjamin Gray <bgray@linux.ibm.com>
Reviewed-by: Andrew Donnellan <ajd@linux.ibm.com>
Tested-by: Andrew Donnellan <ajd@linux.ibm.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

usr/gen_init_cpio.c

@@ -353,6 +353,12 @@ static int cpio_mkfile(const char *name, const char *location,
 		buf.st_mtime = 0xffffffff;
 	}
 
+	if (buf.st_mtime < 0) {
+		fprintf(stderr, "%s: Timestamp negative, clipping.\n",
+			location);
+		buf.st_mtime = 0;
+	}
+
 	if (buf.st_size > 0xffffffff) {
 		fprintf(stderr, "%s: Size exceeds maximum cpio file size\n",
 			location);
@@ -602,10 +608,10 @@ int main (int argc, char *argv[])
 	/*
 	 * Timestamps after 2106-02-07 06:28:15 UTC have an ascii hex time_t
 	 * representation that exceeds 8 chars and breaks the cpio header
-	 * specification.
+	 * specification. Negative timestamps similarly exceed 8 chars.
 	 */
-	if (default_mtime > 0xffffffff) {
-		fprintf(stderr, "ERROR: Timestamp too large for cpio format\n");
+	if (default_mtime > 0xffffffff || default_mtime < 0) {
+		fprintf(stderr, "ERROR: Timestamp out of range for cpio format\n");
 		exit(1);
 	}