Re: [Patch] allow file truncations when both suid and write permissions set

[Eric Paris <eparis@parisplace.org>]

On Wed, Jul 1, 2009 at 3:27 PM, Eric Sandeen<sandeen@redhat.com> wrote:
> Eric Sandeen wrote:
>> Amerigo Wang wrote:
>>> When suid is set and the non-owner user has write permission,
>>> any writing into this file should be allowed and suid should be
>>> removed after that.
>>>
>>> However, current kernel only allows writing without truncations,
>>> when we do truncations on that file, we get EPERM. This is a bug.
>
> ...
>
>> So I think the main problem here is simply that we didn't set
>> ATTR_FORCE, right...
>>
>> Seems a little odd to |= with ret, -then- check if it's non-0.  Maybe:
>>
>>       /* Remove suid/sgid on truncate too */
>> -     newattrs.ia_valid |= should_remove_suid(dentry);
>> +     ret = should_remove_suid(dentry);
>> +     if (ret)
>> +             newattrs.ia_valid |= (ret | ATTR_FORCE);
>>
>
> On second thought, and after talking w/ eparis, I think this probably
> needs a security_inode_killpriv() too... it seems like it might be best
> to change file_remove_suid(*file) to dentry_remove_suid(*dentry) and
> just call that from do_truncate()?
>
> -Eric

All of this stuff seems horribly complex....   I'm trying to wrap my
head around everything going on here as well....

-Eric (Paris)