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

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Date:   Fri, 1 Jun 2018 09:22:28 -0700
From:   Casey Schaufler <casey@...aufler-ca.com>
To:     chandan.vn@...sung.com, Tejun Heo <tj@...nel.org>
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>,
        Casey Schaufler <casey@...aufler-ca.com>
Subject: Re: [PATCH 1/1] Fix memory leak in kernfs_security_xattr_set and
 kernfs_security_xattr_set

On 6/1/2018 1:56 AM, CHANDAN VN wrote:
> 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.

Thank you for pointing this out. You're right, there's more
at issue here than changing the alloc flag will fix. I think
that calling smack_inode_getsecurity() from smack_inode_getsecctx()
is making the code more complicated than it needs to be. I will
have a patch shortly.