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, 22 insertions, 17 deletions

diff --git a/fs/erofs/super.c b/fs/erofs/super.c
index 2cf96ce1c32e..1c7dcca702b3 100644
--- a/fs/erofs/super.c
+++ b/fs/erofs/super.c
@@ -579,9 +579,9 @@ static int erofs_fc_parse_param(struct fs_context *fc,
 		break;
 	case Opt_fsid:
 #ifdef CONFIG_EROFS_FS_ONDEMAND
-		kfree(ctx->opt.fsid);
-		ctx->opt.fsid = kstrdup(param->string, GFP_KERNEL);
-		if (!ctx->opt.fsid)
+		kfree(ctx->fsid);
+		ctx->fsid = kstrdup(param->string, GFP_KERNEL);
+		if (!ctx->fsid)
 			return -ENOMEM;
 #else
 		errorfc(fc, "fsid option not supported");
@@ -589,9 +589,9 @@ static int erofs_fc_parse_param(struct fs_context *fc,
 		break;
 	case Opt_domain_id:
 #ifdef CONFIG_EROFS_FS_ONDEMAND
-		kfree(ctx->opt.domain_id);
-		ctx->opt.domain_id = kstrdup(param->string, GFP_KERNEL);
-		if (!ctx->opt.domain_id)
+		kfree(ctx->domain_id);
+		ctx->domain_id = kstrdup(param->string, GFP_KERNEL);
+		if (!ctx->domain_id)
 			return -ENOMEM;
 #else
 		errorfc(fc, "domain_id option not supported");
@@ -728,10 +728,12 @@ static int erofs_fc_fill_super(struct super_block *sb, struct fs_context *fc)
 
 	sb->s_fs_info = sbi;
 	sbi->opt = ctx->opt;
-	ctx->opt.fsid = NULL;
-	ctx->opt.domain_id = NULL;
 	sbi->devs = ctx->devs;
 	ctx->devs = NULL;
+	sbi->fsid = ctx->fsid;
+	ctx->fsid = NULL;
+	sbi->domain_id = ctx->domain_id;
+	ctx->domain_id = NULL;
 
 	if (erofs_is_fscache_mode(sb)) {
 		sb->s_blocksize = EROFS_BLKSIZ;
@@ -820,7 +822,7 @@ static int erofs_fc_get_tree(struct fs_context *fc)
 {
 	struct erofs_fs_context *ctx = fc->fs_private;
 
-	if (IS_ENABLED(CONFIG_EROFS_FS_ONDEMAND) && ctx->opt.fsid)
+	if (IS_ENABLED(CONFIG_EROFS_FS_ONDEMAND) && ctx->fsid)
 		return get_tree_nodev(fc, erofs_fc_fill_super);
 
 	return get_tree_bdev(fc, erofs_fc_fill_super);
@@ -834,6 +836,9 @@ static int erofs_fc_reconfigure(struct fs_context *fc)
 
 	DBG_BUGON(!sb_rdonly(sb));
 
+	if (ctx->fsid || ctx->domain_id)
+		erofs_info(sb, "ignoring reconfiguration for fsid|domain_id.");
+
 	if (test_opt(&ctx->opt, POSIX_ACL))
 		fc->sb_flags |= SB_POSIXACL;
 	else
@@ -873,8 +878,8 @@ static void erofs_fc_free(struct fs_context *fc)
 	struct erofs_fs_context *ctx = fc->fs_private;
 
 	erofs_free_dev_context(ctx->devs);
-	kfree(ctx->opt.fsid);
-	kfree(ctx->opt.domain_id);
+	kfree(ctx->fsid);
+	kfree(ctx->domain_id);
 	kfree(ctx);
 }
 
@@ -944,8 +949,8 @@ static void erofs_kill_sb(struct super_block *sb)
 	erofs_free_dev_context(sbi->devs);
 	fs_put_dax(sbi->dax_dev, NULL);
 	erofs_fscache_unregister_fs(sb);
-	kfree(sbi->opt.fsid);
-	kfree(sbi->opt.domain_id);
+	kfree(sbi->fsid);
+	kfree(sbi->domain_id);
 	kfree(sbi);
 	sb->s_fs_info = NULL;
 }
@@ -1098,10 +1103,10 @@ static int erofs_show_options(struct seq_file *seq, struct dentry *root)
 	if (test_opt(opt, DAX_NEVER))
 		seq_puts(seq, ",dax=never");
 #ifdef CONFIG_EROFS_FS_ONDEMAND
-	if (opt->fsid)
-		seq_printf(seq, ",fsid=%s", opt->fsid);
-	if (opt->domain_id)
-		seq_printf(seq, ",domain_id=%s", opt->domain_id);
+	if (sbi->fsid)
+		seq_printf(seq, ",fsid=%s", sbi->fsid);
+	if (sbi->domain_id)
+		seq_printf(seq, ",domain_id=%s", sbi->domain_id);
 #endif
 	return 0;
 }