linux-kernel - [PATCH 5.9 208/255] bootconfig: Extend the magic check range to the preceding 3 bytes

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Date:   Tue, 17 Nov 2020 14:05:48 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Chen Yu <yu.chen.surf@...il.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        "Steven Rostedt (VMware)" <rostedt@...dmis.org>
Subject: [PATCH 5.9 208/255] bootconfig: Extend the magic check range to the preceding 3 bytes

From: Masami Hiramatsu <mhiramat@...nel.org>

commit 50b8a742850fce7293bed45753152c425f7e931b upstream.

Since Grub may align the size of initrd to 4 if user pass
initrd from cpio, we have to check the preceding 3 bytes as well.

Link: https://lkml.kernel.org/r/160520205132.303174.4876760192433315429.stgit@devnote2

Cc: stable@...r.kernel.org
Fixes: 85c46b78da58 ("bootconfig: Add bootconfig magic word for indicating bootconfig explicitly")
Reported-by: Chen Yu <yu.chen.surf@...il.com>
Tested-by: Chen Yu <yu.chen.surf@...il.com>
Signed-off-by: Masami Hiramatsu <mhiramat@...nel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@...dmis.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 init/main.c |   14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

--- a/init/main.c
+++ b/init/main.c
@@ -267,14 +267,24 @@ static void * __init get_boot_config_fro
 	u32 size, csum;
 	char *data;
 	u32 *hdr;
+	int i;
 
 	if (!initrd_end)
 		return NULL;
 
 	data = (char *)initrd_end - BOOTCONFIG_MAGIC_LEN;
-	if (memcmp(data, BOOTCONFIG_MAGIC, BOOTCONFIG_MAGIC_LEN))
-		return NULL;
+	/*
+	 * Since Grub may align the size of initrd to 4, we must
+	 * check the preceding 3 bytes as well.
+	 */
+	for (i = 0; i < 4; i++) {
+		if (!memcmp(data, BOOTCONFIG_MAGIC, BOOTCONFIG_MAGIC_LEN))
+			goto found;
+		data--;
+	}
+	return NULL;
 
+found:
 	hdr = (u32 *)(data - 8);
 	size = hdr[0];
 	csum = hdr[1];