erofs: fix use-after-free of fsid and domain_id string

When erofs instance is remounted with fsid or domain_id mount option
specified, the original fsid and domain_id string pointer in sbi->opt
is directly overridden with the fsid and domain_id string in the new
fs_context, without freeing the original fsid and domain_id string.
What's worse, when the new fsid and domain_id string is transferred to
sbi, they are not reset to NULL in fs_context, and thus they are freed
when remount finishes, while sbi is still referring to these strings.

Reconfiguration for fsid and domain_id seems unusual. Thus clarify this
restriction explicitly and dump a warning when users are attempting to
do this.

Besides, to fix the use-after-free issue, move fsid and domain_id from
erofs_mount_opts to outside.

Fixes: c6be2bd0a5dd ("erofs: register fscache volume")
Fixes: 8b7adf1dff3d ("erofs: introduce fscache-based domain")
Signed-off-by: Jingbo Xu <jefflexu@linux.alibaba.com>
Reviewed-by: Jia Zhu <zhujia.zj@bytedance.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Link: https://lore.kernel.org/r/20221021023153.1330-1-jefflexu@linux.alibaba.com
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>

1 files changed, 4 insertions, 4 deletions

diff --git a/fs/erofs/sysfs.c b/fs/erofs/sysfs.c
index 783bb7b21b51..fd476961f742 100644
--- a/fs/erofs/sysfs.c
+++ b/fs/erofs/sysfs.c
@@ -210,14 +210,14 @@ int erofs_register_sysfs(struct super_block *sb)
 	int err;
 
 	if (erofs_is_fscache_mode(sb)) {
-		if (sbi->opt.domain_id) {
-			str = kasprintf(GFP_KERNEL, "%s,%s", sbi->opt.domain_id,
-					sbi->opt.fsid);
+		if (sbi->domain_id) {
+			str = kasprintf(GFP_KERNEL, "%s,%s", sbi->domain_id,
+					sbi->fsid);
 			if (!str)
 				return -ENOMEM;
 			name = str;
 		} else {
-			name = sbi->opt.fsid;
+			name = sbi->fsid;
 		}
 	} else {
 		name = sb->s_id;