linux-kernel - Re: [kernel-hardening] Re: [PATCH v1 1/1] Add Trusted Path Execution as a stackable LSM

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Message-ID: <6ffe74f5-f431-68a4-b34c-f6e623722f9d@nmatt.com>
Date:   Sun, 4 Jun 2017 08:43:22 -0400
From:   Matt Brown <matt@...tt.com>
To:     Eric Biggers <ebiggers3@...il.com>
Cc:     Al Viro <viro@...IV.linux.org.uk>, james.l.morris@...cle.com,
        serge@...lyn.com, linux-kernel@...r.kernel.org,
        linux-security-module@...r.kernel.org,
        kernel-hardening@...ts.openwall.com
Subject: Re: [kernel-hardening] Re: [PATCH v1 1/1] Add Trusted Path Execution
 as a stackable LSM

On 06/04/2017 01:47 AM, Eric Biggers wrote:
> On Sun, Jun 04, 2017 at 01:24:13AM -0400, Matt Brown wrote:
>> On 06/03/2017 02:33 AM, Al Viro wrote:
>>> On Sat, Jun 03, 2017 at 01:53:51AM -0400, Matt Brown wrote:
>>>
>>>> +static int tpe_bprm_set_creds(struct linux_binprm *bprm)
>>>> +{
>>>> +	struct file *file = bprm->file;
>>>> +	struct inode *inode = d_backing_inode(file->f_path.dentry->d_parent);
>>>> +	struct inode *file_inode = d_backing_inode(file->f_path.dentry);
>>>
>>> Bloody wonderful.  Do tell, what *does* prevent a race with rename(2) here,
>>> somehow making sure that your 'inode' won't get freed right under you?
>>>
>>
>> Good catch. How does this look:
>>
>> spin_lock(&inode->i_lock);
>> spin_lock(&file_inode->i_lock);
>> if (global_nonroot(inode->i_uid) && !uid_eq(inode->i_uid, cred->uid))
>> 	reason1 = "directory not owned by user";
>> else if (inode->i_mode & 0002)
>> 	reason1 = "file in world-writable directory";
>> else if ((inode->i_mode & 0020) && global_nonroot_gid(inode->i_gid))
>> 	reason1 = "file in group-writable directory";
>> else if (file_inode->i_mode & 0002)
>> 	reason1 = "file is world-writable";
>> spin_unlock(&inode->i_lock);
>> spin_unlock(&file_inode->i_lock);
>>
>> and likewise for other places in the code?
>
> No, it needs to take a reference on the parent dentry before using it, using
> dget_parent(), I think, and then dropping it later with dput().  Taking i_lock
> isn't needed.
>
> Eric
>

Got it. Thank you!

Matt Brown