vfs: fs_context: fix up param length parsing in legacy_parse_param

mainline inclusion
from mainline-v5.17-rc1
commit 722d9484
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I4YD3S
CVE: CVE-2022-0185

--------------------------------

The "PAGE_SIZE - 2 - size" calculation in legacy_parse_param() is an
unsigned type so a large value of "size" results in a high positive
value instead of a negative value as expected.  Fix this by getting rid
of the subtraction.
Signed-off-by: NJamie Hill-Daniel <jamie@hill-daniel.co.uk>
Signed-off-by: NWilliam Liu <willsroot@protonmail.com>
Tested-by: NSalvatore Bonaccorso <carnil@debian.org>
Tested-by: NThadeu Lima de Souza Cascardo <cascardo@canonical.com>
Acked-by: NDan Carpenter <dan.carpenter@oracle.com>
Acked-by: NAl Viro <viro@zeniv.linux.org.uk>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: NLuo Meng <luomeng12@huawei.com>
Reviewed-by: NZhang Yi <yi.zhang@huawei.com>
Signed-off-by: NZheng Zengkai <zhengzengkai@huawei.com>

fs/fs_context.c

@@ -548,7 +548,7 @@ static int legacy_parse_param(struct fs_context *fc, struct fs_parameter *param)
 			      param->key);
 	}
 
-	if (len > PAGE_SIZE - 2 - size)
+	if (size + len + 2 > PAGE_SIZE)
 		return invalf(fc, "VFS: Legacy: Cumulative options too large");
 	if (strchr(param->key, ',') ||
 	    (param->type == fs_value_is_string &&