linux-kernel - Re: [PATCH v2] ext4: Prevent stack overrun in ext4_file_open when recording last known mountpoint

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Message-ID: <20110930194655.GX12086@tux1.beaverton.ibm.com>
Date:	Fri, 30 Sep 2011 12:46:55 -0700
From:	"Darrick J. Wong" <djwong@...ibm.com>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	"Theodore Ts'o" <tytso@....edu>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	linux-ext4 <linux-ext4@...r.kernel.org>
Subject: Re: [PATCH v2] ext4: Prevent stack overrun in ext4_file_open when
	recording last known mountpoint

On Fri, Sep 16, 2011 at 11:40:05AM -0700, Darrick J. Wong wrote:
> In ext4_file_open, the filesystem records the mountpoint of the first file that
> is opened after mounting the filesystem.  It does this by allocating a 64-byte
> stack buffer, calling d_path() to grab the mount point through which this file
> was accessed, and then memcpy()ing 64 bytes into the superblock's
> s_last_mounted field, starting from the return value of d_path(), which is
> stored as "cp".  However, if cp > buf (which it frequently is since path
> components are prepended starting at the end of buf) then we can end up copying
> stack data into the superblock.
> 
> Writing stack variables into the superblock doesn't sound like a great idea, so
> use strlcpy instead.  Andi Kleen suggested using strlcpy instead of strncpy.

Ok, it's been a couple of weeks.... any thoughts, Ted?

--D
> 
> Signed-off-by: Darrick J. Wong <djwong@...ibm.com>
> ---
> 
>  fs/ext4/file.c |    4 ++--
>  1 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/ext4/file.c b/fs/ext4/file.c
> index e4095e9..9781099 100644
> --- a/fs/ext4/file.c
> +++ b/fs/ext4/file.c
> @@ -181,8 +181,8 @@ static int ext4_file_open(struct inode * inode, struct file * filp)
>  		path.dentry = mnt->mnt_root;
>  		cp = d_path(&path, buf, sizeof(buf));
>  		if (!IS_ERR(cp)) {
> -			memcpy(sbi->s_es->s_last_mounted, cp,
> -			       sizeof(sbi->s_es->s_last_mounted));
> +			strlcpy(sbi->s_es->s_last_mounted, cp,
> +				sizeof(sbi->s_es->s_last_mounted));
>  			ext4_mark_super_dirty(sb);
>  		}
>  	}
> --
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