nfs: fix possible null-ptr-deref when parsing param

According to commit "vfs: parse: deal with zero length string value",
kernel will set the param->string to null pointer in vfs_parse_fs_string()
if fs string has zero length.

Yet the problem is that, nfs_fs_context_parse_param() will dereferences the
param->string, without checking whether it is a null pointer, which may
trigger a null-ptr-deref bug.

This patch solves it by adding sanity check on param->string
in nfs_fs_context_parse_param().
Signed-off-by: Hawkins Jiawei <yin31149@gmail.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>

fs/nfs/fs_context.c

@@ -684,6 +684,8 @@ static int nfs_fs_context_parse_param(struct fs_context *fc,
 			return ret;
 		break;
 	case Opt_vers:
+		if (!param->string)
+			goto out_invalid_value;
 		trace_nfs_mount_assign(param->key, param->string);
 		ret = nfs_parse_version_string(fc, param->string);
 		if (ret < 0)
@@ -696,6 +698,8 @@ static int nfs_fs_context_parse_param(struct fs_context *fc,
 		break;
 
 	case Opt_proto:
+		if (!param->string)
+			goto out_invalid_value;
 		trace_nfs_mount_assign(param->key, param->string);
 		protofamily = AF_INET;
 		switch (lookup_constant(nfs_xprt_protocol_tokens, param->string, -1)) {
@@ -732,6 +736,8 @@ static int nfs_fs_context_parse_param(struct fs_context *fc,
 		break;
 
 	case Opt_mountproto:
+		if (!param->string)
+			goto out_invalid_value;
 		trace_nfs_mount_assign(param->key, param->string);
 		mountfamily = AF_INET;
 		switch (lookup_constant(nfs_xprt_protocol_tokens, param->string, -1)) {