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

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Date:   Thu, 31 May 2018 09:22:18 -0700
From:   Casey Schaufler <casey@...aufler-ca.com>
To:     Tejun Heo <tj@...nel.org>
Cc:     CHANDAN VN <chandan.vn@...sung.com>, gregkh@...uxfoundation.org,
        bfields@...ldses.org, jlayton@...nel.org,
        linux-kernel@...r.kernel.org, linux-nfs@...r.kernel.org,
        cpgs@...sung.com, sireesha.t@...sung.com,
        Chris Wright <chrisw@...s-sol.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 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.

>
> Thanks.
>