linux-kernel - Re: [PATCH 07/18] fs_context: fix double free of legacy_fs

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Date:   Tue, 10 Jul 2018 09:02:04 +0100
From:   David Howells <dhowells@...hat.com>
To:     Eric Biggers <ebiggers3@...il.com>
Cc:     dhowells@...hat.com, Alexander Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        Eric Biggers <ebiggers@...gle.com>
Subject: Re: [PATCH 07/18] fs_context: fix double free of legacy_fs_context data

Eric Biggers <ebiggers3@...il.com> wrote:

> Why isn't this done in the same place that ->init_fs_context() would otherwise
> be called?  It logically does the same thing, right?

Fair point.  How about the attached incremental change?  It breaks the legacy
context initialisation out into its own init function and just sets that as
the default if the fs doesn't supply its own.

It also makes the freeing conditional.

David
---
diff --git a/fs/fs_context.c b/fs/fs_context.c
index 8af0542ab8b6..f388ab29d37d 100644
--- a/fs/fs_context.c
+++ b/fs/fs_context.c
@@ -42,6 +42,7 @@ struct legacy_fs_context {
 	enum legacy_fs_param	param_type;
 };
 
+static int legacy_init_fs_context(struct fs_context *fc, struct dentry *dentry);
 static const struct fs_context_operations legacy_fs_context_ops;
 
 static const match_table_t common_set_sb_flag = {
@@ -239,6 +240,7 @@ struct fs_context *vfs_new_fs_context(struct file_system_type *fs_type,
 				      unsigned int sb_flags,
 				      enum fs_context_purpose purpose)
 {
+	int (*init_fs_context)(struct fs_context *, struct dentry *);
 	struct fs_context *fc;
 	int ret = -ENOMEM;
 
@@ -246,15 +248,6 @@ struct fs_context *vfs_new_fs_context(struct file_system_type *fs_type,
 	if (!fc)
 		return ERR_PTR(-ENOMEM);
 
-	if (!fs_type->init_fs_context) {
-		fc->fs_private = kzalloc(sizeof(struct legacy_fs_context),
-					 GFP_KERNEL);
-		if (!fc->fs_private)
-			goto err_fc;
-
-		fc->ops = &legacy_fs_context_ops;
-	}
-
 	fc->purpose	= purpose;
 	fc->sb_flags	= sb_flags;
 	fc->fs_type	= get_filesystem(fs_type);
@@ -285,11 +278,13 @@ struct fs_context *vfs_new_fs_context(struct file_system_type *fs_type,
 
 
 	/* TODO: Make all filesystems support this unconditionally */
-	if (fc->fs_type->init_fs_context) {
-		ret = fc->fs_type->init_fs_context(fc, reference);
-		if (ret < 0)
-			goto err_fc;
-	}
+	init_fs_context = fc->fs_type->init_fs_context;
+	if (!init_fs_context)
+		init_fs_context = legacy_init_fs_context;
+
+	ret = (*init_fs_context)(fc, reference);
+	if (ret < 0)
+		goto err_fc;
 
 	/* Do the security check last because ->init_fs_context may change the
 	 * namespace subscriptions.
@@ -499,19 +494,21 @@ static void legacy_fs_context_free(struct fs_context *fc)
 {
 	struct legacy_fs_context *ctx = fc->fs_private;
 
-	free_secdata(ctx->secdata);
-	switch (ctx->param_type) {
-	case LEGACY_FS_UNSET_PARAMS:
-	case LEGACY_FS_NO_PARAMS:
-		break;
-	case LEGACY_FS_MAGIC_PARAMS:
-		break; /* ctx->data is a weird pointer */
-	default:
-		kfree(ctx->legacy_data);
-		break;
-	}
+	if (ctx) {
+		free_secdata(ctx->secdata);
+		switch (ctx->param_type) {
+		case LEGACY_FS_UNSET_PARAMS:
+		case LEGACY_FS_NO_PARAMS:
+			break;
+		case LEGACY_FS_MAGIC_PARAMS:
+			break; /* ctx->data is a weird pointer */
+		default:
+			kfree(ctx->legacy_data);
+			break;
+		}
 
-	kfree(ctx);
+		kfree(ctx);
+	}
 }
 
 /*
@@ -707,3 +704,18 @@ static const struct fs_context_operations legacy_fs_context_ops = {
 	.validate		= legacy_validate,
 	.get_tree		= legacy_get_tree,
 };
+
+/*
+ * Initialise a legacy context for a filesystem that doesn't support
+ * fs_context.
+ */
+static int legacy_init_fs_context(struct fs_context *fc, struct dentry *dentry)
+{
+
+	fc->fs_private = kzalloc(sizeof(struct legacy_fs_context), GFP_KERNEL);
+	if (!fc->fs_private)
+		return -ENOMEM;
+
+	fc->ops = &legacy_fs_context_ops;
+	return 0;
+}