[PATCH] fs: fs_parser: avoid NULL param->string to kstrtouint

[Yin Fengwei <nh26223.lmm@gmail.com>]

syzbot reported general protection fault in kstrtouint:
https://lkml.org/lkml/2019/7/18/328

From the log, if the mount option is something like:
   fd,XXXXXXXXXXXXXXXXXXXX

The default parameter (which has NULL param->string) will be
passed to vfs_parse_fs_param. Finally, this NULL param->string
is passed to kstrtouint and trigger NULL pointer access.

Reported-by: syzbot+398343b7c1b1b989228d@syzkaller.appspotmail.com
Fixes: 71cbb7570a9a ("vfs: Move the subtype parameter into fuse")

Signed-off-by: Yin Fengwei <nh26223.lmm@gmail.com>
---
 fs/fs_parser.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/fs/fs_parser.c b/fs/fs_parser.c
index d13fe7d797c2..578e6880ac67 100644
--- a/fs/fs_parser.c
+++ b/fs/fs_parser.c
@@ -210,6 +210,10 @@ int fs_parse(struct fs_context *fc,
 	case fs_param_is_fd: {
 		switch (param->type) {
 		case fs_value_is_string:
+			if (result->has_value) {
+				goto bad_value;
+			}
+
 			ret = kstrtouint(param->string, 0, &result->uint_32);
 			break;
 		case fs_value_is_file:
-- 
2.17.1