[PATCHESv2] Add vfs_admin vfs module

[Uri Simchoni]

On 10/06/2015 07:13 PM, Jeremy Allison wrote:
> On Tue, Oct 06, 2015 at 08:41:12AM -0700, Jeremy Allison wrote:
>> On Tue, Oct 06, 2015 at 08:36:34AM -0700, Jeremy Allison wrote:
>>> For directories, the only "safe" POSIX way to do this as
>>> root is :
>>>
>>> a). mkdir(name)
>>> b). fd = open(name, O_RDONLY);
>>> c). fstat(fd);
>>> d). Ensure owner/primary group match current POSIX
>>> creds.
>>> e). fchown(fd, newcreds);
>>>
>>> That's not race-free, but at least it can detect
>>> any races.
>> Correction ! That should be:
>>
>> a). mkdir(name)
>> a1). chdir(name);
>> b). fd = open(".", O_RDONLY);
>> c). fstat(fd);
>> d). Ensure owner/primary group match current POSIX
>> creds.
>> e). fchown(fd, newcreds);
>>
>> This stuff is tricky :-).
> And the above is *also* wrong :-).
>
> The correct (*) way is :
>
> parent_name = get_parent_name(name);
> final_component = get_final_component(name);
> chdir(parent_name);
> check_name = realpath(".");
> ensure check_name == parent_name;
> mkdir(final_component);
> chdir(final_component);
> check_name = realpath(".");
> ensure check_name == name;
> fd = open(".", O_RDONLY);
> fstat(fd);
> ensure stat owner matches current posix creds.
> fchown(fd, new_creds);
>
> (*) until I or someone else spots another flaw :-).
>
> This of course won't work with pathnames
> deliberatly containing symlinks, but is
> as safe as I can think of on the spur-of
> the moment.
UNIX. Worse is Better(tm).

https://www.jwz.org/doc/worse-is-better.html

I'll incorporate that in the next version. Would you say the first 
version of your raceless chown is sufficient for symlinks and "mknod"'s?

Thanks,
Uri.