Bug#770492: [RFC PATCH RESEND] vfs: Move security_inode_killpriv() after permission checks

[Stephen Smalley <sds@tycho.nsa.gov>]

On 01/20/2015 06:17 PM, James Morris wrote:
> On Sat, 17 Jan 2015, Ben Hutchings wrote:
> 
>> chown() and write() should clear all privilege attributes on
>> a file - setuid, setgid, setcap and any other extended
>> privilege attributes.
>>
>> However, any attributes beyond setuid and setgid are managed by the
>> LSM and not directly by the filesystem, so they cannot be set along
>> with the other attributes.
>>
>> Currently we call security_inode_killpriv() in notify_change(),
>> but in case of a chown() this is too early - we have not called
>> inode_change_ok() or made any filesystem-specific permission/sanity
>> checks.
>>
>> Add a new function setattr_killpriv() which calls
>> security_inode_killpriv() if necessary, and change the setattr()
>> implementation to call this in each filesystem that supports xattrs.
>> This assumes that extended privilege attributes are always stored in
>> xattrs.
> 
> It'd be useful to get some input from LSM module maintainers on this. 
> 
> e.g. doesn't SELinux already handle this via policy directives?

There have been a couple postings of a similar patch set [1] by Jan
Kara, although I don't believe that series addressed chown().

If I am reading the patches correctly, they (correctly) don't affect
SELinux or Smack labels; they are just calling the existing
security_inode_killpriv() hook, which is only implemented for the
capability module to remove the security.capability xattr.

[1] http://marc.info/?l=linux-security-module&m=141890696325054&w=2