mtd: Avoid boot crash in RedBoot partition table parser

commit 8e2f802 upstream.

Given CONFIG_FORTIFY_SOURCE=y and a recent compiler,
commit 439a1bc ("fortify: Use __builtin_dynamic_object_size() when
available") produces the warning below and an oops.

    Searching for RedBoot partition table in 50000000.flash at offset 0x7e0000
    ------------[ cut here ]------------
    WARNING: lib/string_helpers.c:1035 at 0xc029e04c, CPU#0: swapper/0/1
    memcmp: detected buffer overflow: 15 byte read of buffer size 14
    Modules linked in:
    CPU: 0 UID: 0 PID: 1 Comm: swapper/0 Not tainted 6.19.0 #1 NONE

As Kees said, "'names' is pointing to the final 'namelen' many bytes
of the allocation ... 'namelen' could be basically any length at all.
This fortify warning looks legit to me -- this code used to be reading
beyond the end of the allocation."

Since the size of the dynamic allocation is calculated with strlen()
we can use strcmp() instead of memcmp() and remain within bounds.

Cc: Kees Cook <kees@kernel.org>
Cc: stable@vger.kernel.org
Cc: linux-hardening@vger.kernel.org
Link: https://lore.kernel.org/all/202602151911.AD092DFFCD@keescook/
Fixes: 1da177e ("Linux-2.6.12-rc2")
Suggested-by: Kees Cook <kees@kernel.org>
Signed-off-by: Finn Thain <fthain@linux-m68k.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: fthain

drivers/mtd/parsers/redboot.c

@@ -270,9 +270,9 @@ static int parse_redboot_partitions(struct mtd_info *master,
 
 		strcpy(names, fl->img->name);
 #ifdef CONFIG_MTD_REDBOOT_PARTS_READONLY
-		if (!memcmp(names, "RedBoot", 8) ||
-		    !memcmp(names, "RedBoot config", 15) ||
-		    !memcmp(names, "FIS directory", 14)) {
+		if (!strcmp(names, "RedBoot") ||
+		    !strcmp(names, "RedBoot config") ||
+		    !strcmp(names, "FIS directory")) {
 			parts[i].mask_flags = MTD_WRITEABLE;
 		}
 #endif