linux-kernel - RE: Re: [PATCH 1/1] Fix memory leak in kernfs_security_xattr_set and kernfs_security_xattr

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Date:   Fri, 01 Jun 2018 14:26:09 +0530
From:   CHANDAN VN <chandan.vn@...sung.com>
To:     Tejun Heo <tj@...nel.org>, Casey Schaufler <casey@...aufler-ca.com>
CC:     "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "bfields@...ldses.org" <bfields@...ldses.org>,
        "jlayton@...nel.org" <jlayton@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-nfs@...r.kernel.org" <linux-nfs@...r.kernel.org>,
        CPGS <cpgs@...sung.com>,
        Sireesha Talluri <sireesha.t@...sung.com>,
        Chris Wright <chrisw@...s-sol.org>,
        "linux-security-module@...r.kernel.org" 
        <linux-security-module@...r.kernel.org>
Subject: RE: Re: [PATCH 1/1] Fix memory leak in kernfs_security_xattr_set
 and kernfs_security_xattr_set

Hi
 

>On 5/31/2018 9:11 AM, Tejun Heo wrote:
> On Thu, May 31, 2018 at 09:04:25AM -0700, Casey Schaufler wrote:
>>> On 5/31/2018 8:39 AM, Tejun Heo wrote:
>>>> (cc'ing more security folks and copying whole body)
>>>>
>>>> So, I'm sure the patch fixes the memory leak but API wise it looks
>>>> super confusing.  Can security folks chime in here?  Is this the right
>>>> fix?
>>>> security_inode_getsecctx() provides a security context. Technically,
>>>> this is a data blob, although both provider provide a null terminated
>>>> string. security_inode_getsecurity(), on the other hand, provides a
>>>> string to match an attribute name. The former releases the security
>>>> context with security_release_secctx(), where the later releases the
>>>> string with kfree().
>>>>
>>>> When the Smack hook smack_inode_getsecctx() was added in 2009
>>>> for use by labeled NFS the alloc value passed to
>>> smack_inode_getsecurity() was set incorrectly. This wasn't a
>>> major issue, since labeled NFS is a fringe case. When kernfs
>>> started using the hook, it became the issue you discovered.
>>>
>>> The reason that we have all this confusion is that SELinux
>>> generates security contexts as needed, while Smack keeps them
>>> around all the time. Releasing an SELinux context frees memory,
>>> while releasing a Smack context is a null operation.
>> Any chance this detail can be hidden behind security api?  This looks
>> pretty error-prone, no?
 
>>It *is* hidden behind the security API. The problem is strictly
>>within the Smack code, where the implementer of smack_inode_getsecctx()
>>made an error.

I agree that the fix can be done simply by using "false" for 
smack_inode_getsecurity(), but what happens with kernfs_node_setsecdata()
and smack_inode_notifysecctx(). kernfs_node_setsecdata() is probably ignorable
but smack_inode_notifysecctx() is sending the "ctx" to smack_inode_setsecurity()
and since "ctx" would be NULL because we used "false", smack_inode_setsecurity()
becomes dummy.