initramfs: Check timestamp to prevent broken cpio archive

Cpio format reserves 8 bytes for an ASCII representation of a time_t timestamp.
While 2106-02-07 06:28:15 UTC (time_t = 0xffffffff) is still some years in the
future, a poorly chosen date string for KBUILD_BUILD_TIMESTAMP, converted into
seconds since the epoch, might lead to exceeded cpio timestamp limits that
result in a broken cpio archive.  Add timestamp checks to prevent overrun of
the 8-byte cpio header field.

My colleague Thomas Kühnel discovered the behaviour, when we accidentally fed
SOURCE_DATE_EPOCH to KBUILD_BUILD_TIMESTAMP as is: some timestamps (e.g.
1607420928 = 2021-12-08 9:48:48 UTC) will be interpreted by `date` as a valid
date specification of science fictional times (here: year 160742).  Even though
this is bad input for KBUILD_BUILD_TIMESTAMP, it should not break the initramfs
cpio format.
Signed-off-by: Nicolas Schier <nicolas@fjasle.eu>
Cc: Thomas Kühnel <thomas.kuehnel@avm.de>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

usr/gen_init_cpio.c

@@ -320,6 +320,12 @@ static int cpio_mkfile(const char *name, const char *location,
 		goto error;
 	}
 
+	if (buf.st_mtime > 0xffffffff) {
+		fprintf(stderr, "%s: Timestamp exceeds maximum cpio timestamp, clipping.\n",
+			location);
+		buf.st_mtime = 0xffffffff;
+	}
+
 	filebuf = malloc(buf.st_size);
 	if (!filebuf) {
 		fprintf (stderr, "out of memory\n");
@@ -551,6 +557,16 @@ 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.
+	 */
+	if (default_mtime > 0xffffffff) {
+		fprintf(stderr, "ERROR: Timestamp too large for cpio format\n");
+		exit(1);
+	}
+
 	if (argc - optind != 1) {
 		usage(argv[0]);
 		exit(1);