linux-kernel - [PATCH] ramfs: add support for "mode=" mount option, fix

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Date:	Tue, 7 Apr 2009 08:20:59 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Wu Fengguang <fengguang.wu@...el.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Avan Anishchuk <matimatik@...il.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Pekka Enberg <penberg@...helsinki.fi>,
	Steven Rostedt <rostedt@...dmis.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Eduard - Gabriel Munteanu <eduard.munteanu@...ux360.ro>
Subject: [PATCH] ramfs: add support for "mode=" mount option, fix




>From 7baa5532398708976ada2502ad11b37f872f6e9e Mon Sep 17 00:00:00 2001
From: Ingo Molnar <mingo@...e.hu>
Date: Tue, 7 Apr 2009 07:28:01 +0200
Subject: [PATCH] ramfs: add support for "mode=" mount option, fix

Linus reported odd boot failures that showed memory corruption
patterns in the SLUB code and bisected it down to
c3b1b1cbf002e65a3cabd479e68b5f35886a26db: 'ramfs: add support
for "mode=" mount option'.

That commit does have a couple of genuinely odd looking lines.

For example:

+       sb->s_fs_info = fsi;
+
+       err = ramfs_parse_options(data, &fsi->mount_opts);
+       if (err)
+               goto fail;

Say we fail in ramfs_parse_options() and do the 'fail' pattern:

+fail:
+       kfree(fsi);
+       iput(inode);
+       return err;

so we have 'fsi' kfree()'d but dont clear sb->s_fs_info! That's
almost always a bad practice. And indeed, in the kill_super
callback:

+static void ramfs_kill_sb(struct super_block *sb)
+{
+       kfree(sb->s_fs_info);

What ensures that this cannot be a double kfree() memory corruption?

->kill_super() gets called even if ->fill_super() fails, so this
is a double kfree() when there are no ramfs mount options - which
results in memory corruption.

And there's also another, probably just theoretical worry about
another failure path:

+       fsi = kzalloc(sizeof(struct ramfs_fs_info), GFP_KERNEL);
+       if (!fsi) {
+               err = -ENOMEM;
+               goto fail;
+       }
+       sb->s_fs_info = fsi;

leaves sb->s_fs_info uninitialized in the failure case, and might
hit this code unconditionally:

+static void ramfs_kill_sb(struct super_block *sb)
+{
+       kfree(sb->s_fs_info);
+       kill_litter_super(sb);
+}

Leaving this code at the mercy of the external call environment
initializing sb->s_fs_info. Which if it does not do (or stops
doing in the future), can trigger a kfree of a random pointer.

So improve this code a bit too, and improve the readability of other
initializations here a bit.

Reported-by: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Avan Anishchuk <matimatik@...il.com>
Acked-by: Wu Fengguang <fengguang.wu@...el.com>
Signed-off-by: Ingo Molnar <mingo@...e.hu>
---
 fs/ramfs/inode.c |   19 +++++++++++--------
 1 files changed, 11 insertions(+), 8 deletions(-)

diff --git a/fs/ramfs/inode.c b/fs/ramfs/inode.c
index a404fb8..3a6b193 100644
--- a/fs/ramfs/inode.c
+++ b/fs/ramfs/inode.c
@@ -221,22 +221,23 @@ static int ramfs_fill_super(struct super_block * sb, void * data, int silent)
 	save_mount_options(sb, data);
 
 	fsi = kzalloc(sizeof(struct ramfs_fs_info), GFP_KERNEL);
+	sb->s_fs_info = fsi;
 	if (!fsi) {
 		err = -ENOMEM;
 		goto fail;
 	}
-	sb->s_fs_info = fsi;
 
 	err = ramfs_parse_options(data, &fsi->mount_opts);
 	if (err)
 		goto fail;
 
-	sb->s_maxbytes = MAX_LFS_FILESIZE;
-	sb->s_blocksize = PAGE_CACHE_SIZE;
-	sb->s_blocksize_bits = PAGE_CACHE_SHIFT;
-	sb->s_magic = RAMFS_MAGIC;
-	sb->s_op = &ramfs_ops;
-	sb->s_time_gran = 1;
+	sb->s_maxbytes		= MAX_LFS_FILESIZE;
+	sb->s_blocksize		= PAGE_CACHE_SIZE;
+	sb->s_blocksize_bits	= PAGE_CACHE_SHIFT;
+	sb->s_magic		= RAMFS_MAGIC;
+	sb->s_op		= &ramfs_ops;
+	sb->s_time_gran		= 1;
+
 	inode = ramfs_get_inode(sb, S_IFDIR | fsi->mount_opts.mode, 0);
 	if (!inode) {
 		err = -ENOMEM;
@@ -244,14 +245,16 @@ static int ramfs_fill_super(struct super_block * sb, void * data, int silent)
 	}
 
 	root = d_alloc_root(inode);
+	sb->s_root = root;
 	if (!root) {
 		err = -ENOMEM;
 		goto fail;
 	}
-	sb->s_root = root;
+
 	return 0;
 fail:
 	kfree(fsi);
+	sb->s_fs_info = NULL;
 	iput(inode);
 	return err;
 }
--
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