[PATCH -next 4/5] gfs2: fix possible null-ptr-deref when parsing param

[Hawkins Jiawei]

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, gfs2_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 gfs2_parse_param().

Reported-by: syzbot+da97a57c5b742d05db51@xxxxxxxxxxxxxxxxxxxxxxxxx
Tested-by: syzbot+da97a57c5b742d05db51@xxxxxxxxxxxxxxxxxxxxxxxxx
Cc: agruenba@xxxxxxxxxx
Cc: cluster-devel@xxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
Cc: rpeterso@xxxxxxxxxx
Cc: syzkaller-bugs@xxxxxxxxxxxxxxxx
Signed-off-by: Hawkins Jiawei <yin31149@xxxxxxxxx>
---
 fs/gfs2/ops_fstype.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/fs/gfs2/ops_fstype.c b/fs/gfs2/ops_fstype.c
index c0cf1d2d0ef5..934746f18c25 100644
--- a/fs/gfs2/ops_fstype.c
+++ b/fs/gfs2/ops_fstype.c
@@ -1446,12 +1446,18 @@ static int gfs2_parse_param(struct fs_context *fc, struct fs_parameter *param)
 
 	switch (o) {
 	case Opt_lockproto:
+		if (!param->string)
+			goto bad_val;
 		strscpy(args->ar_lockproto, param->string, GFS2_LOCKNAME_LEN);
 		break;
 	case Opt_locktable:
+		if (!param->string)
+			goto bad_val;
 		strscpy(args->ar_locktable, param->string, GFS2_LOCKNAME_LEN);
 		break;
 	case Opt_hostdata:
+		if (!param->string)
+			goto bad_val;
 		strscpy(args->ar_hostdata, param->string, GFS2_LOCKNAME_LEN);
 		break;
 	case Opt_spectator:
@@ -1535,6 +1541,10 @@ static int gfs2_parse_param(struct fs_context *fc, struct fs_parameter *param)
 		return invalfc(fc, "invalid mount option: %s", param->key);
 	}
 	return 0;
+
+bad_val:
+	return invalfc(fc, "Bad value '%s' for mount option '%s'\n",
+		       param->string, param->key);
 }
 
 static int gfs2_reconfigure(struct fs_context *fc)
-- 
2.25.1