linux-kernel - Re: [PATCH 01/13] sysfs: Update sysfs_setxattr so it updates secdata under the sysfs

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Message-ID: <20091103134823.GB31981@us.ibm.com>
Date:	Tue, 3 Nov 2009 07:48:23 -0600
From:	"Serge E. Hallyn" <serue@...ibm.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Greg Kroah-Hartman <gregkh@...e.de>,
	Kay Sievers <kay.sievers@...y.org>, Greg KH <greg@...ah.com>,
	linux-kernel@...r.kernel.org, Tejun Heo <tj@...nel.org>,
	Cornelia Huck <cornelia.huck@...ibm.com>,
	linux-fsdevel@...r.kernel.org,
	Eric Dumazet <eric.dumazet@...il.com>,
	Benjamin LaHaise <bcrl@...et.ca>,
	"Eric W. Biederman" <ebiederm@...stanetworks.com>
Subject: Re: [PATCH 01/13] sysfs: Update sysfs_setxattr so it updates
	secdata under the sysfs_mutex

Quoting Eric W. Biederman (ebiederm@...ssion.com):
> From: Eric W. Biederman <ebiederm@...stanetworks.com>
> 
> The sysfs_mutex is required to ensure updates are and will remain
> atomic with respect to other inode iattr updates, that do not happen
> through the filesystem.
> 
> Signed-off-by: Eric W. Biederman <ebiederm@...stanetworks.com>
> ---
>  fs/sysfs/inode.c |   41 +++++++++++++++++++++++++++++------------
>  1 files changed, 29 insertions(+), 12 deletions(-)
> 
> diff --git a/fs/sysfs/inode.c b/fs/sysfs/inode.c
> index e28cecf..8a08250 100644
> --- a/fs/sysfs/inode.c
> +++ b/fs/sysfs/inode.c
> @@ -122,23 +122,39 @@ int sysfs_setattr(struct dentry * dentry, struct iattr * iattr)
>  	return error;
>  }
> 
> +static int sysfs_sd_setsecdata(struct sysfs_dirent *sd, void **secdata, u32 *secdata_len)
> +{
> +	struct sysfs_inode_attrs *iattrs;
> +	void *old_secdata;
> +	size_t old_secdata_len;
> +
> +	iattrs = sd->s_iattr;
> +	if (!iattrs)
> +		iattrs = sysfs_init_inode_attrs(sd);
> +	if (!iattrs)
> +		return -ENOMEM;
> +
> +	old_secdata = iattrs->ia_secdata;
> +	old_secdata_len = iattrs->ia_secdata_len;
> +
> +	iattrs->ia_secdata = *secdata;
> +	iattrs->ia_secdata_len = *secdata_len;
> +
> +	*secdata = old_secdata;
> +	*secdata_len = old_secdata_len;
> +	return 0;
> +}
> +
>  int sysfs_setxattr(struct dentry *dentry, const char *name, const void *value,
>  		size_t size, int flags)
>  {
>  	struct sysfs_dirent *sd = dentry->d_fsdata;
> -	struct sysfs_inode_attrs *iattrs;
>  	void *secdata;
>  	int error;
>  	u32 secdata_len = 0;
> 
>  	if (!sd)
>  		return -EINVAL;
> -	if (!sd->s_iattr)
> -		sd->s_iattr = sysfs_init_inode_attrs(sd);
> -	if (!sd->s_iattr)
> -		return -ENOMEM;
> -
> -	iattrs = sd->s_iattr;
> 
>  	if (!strncmp(name, XATTR_SECURITY_PREFIX, XATTR_SECURITY_PREFIX_LEN)) {
>  		const char *suffix = name + XATTR_SECURITY_PREFIX_LEN;
> @@ -150,12 +166,13 @@ int sysfs_setxattr(struct dentry *dentry, const char *name, const void *value,
>  						&secdata, &secdata_len);
>  		if (error)
>  			goto out;
> -		if (iattrs->ia_secdata)
> -			security_release_secctx(iattrs->ia_secdata,
> -						iattrs->ia_secdata_len);
> -		iattrs->ia_secdata = secdata;
> -		iattrs->ia_secdata_len = secdata_len;
> 
> +		mutex_lock(&sysfs_mutex);
> +		error = sysfs_sd_setsecdata(sd, &secdata, &secdata_len);

You're ignoring the potential -ENOMEM return value here?

Worse, if -ENOMEM, then secdata was never set, so you call
security_release_secctx() on a random value left on the stack...

> +		mutex_unlock(&sysfs_mutex);
> +
> +		if (secdata)
> +			security_release_secctx(secdata, secdata_len);
>  	} else
>  		return -EINVAL;
>  out:
> -- 
> 1.6.5.2.143.g8cc62
> 
> --
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