Re: [RFC v2 00/10] snet: Security for NETwork syscalls

[Samir Bellabes <sam@synack.fr>]

Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp> writes:

> Hello.
>
> Samir Bellabes wrote:
>> Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp> writes:
>> 
>> > Regarding [RFC v2 09/10] snet: introduce snet_ticket
>> > +enum snet_verdict snet_ticket_check(struct snet_info *info)
>> > +{
>> > +       struct snet_ticket *st = NULL;
>> > +       unsigned int h = 0, verdict = SNET_VERDICT_NONE;
>> > +       struct list_head *l = NULL;
>> > +       struct snet_task_security *tsec = NULL;
>> > +
>> > +       if (snet_ticket_mode == SNET_TICKET_OFF)
>> > +               goto out;
>> > +
>> > +       tsec = (struct snet_task_security*) current_security();
>> > +
>> > +       h = jhash_2words(info->syscall, info->protocol, 0) % HSIZE;
>> > +       l = &tsec->hash[h];
>> > +
>> > +       read_lock_bh(&tsec->lock);
>> >
>> > Credentials are allocated for copy-on-write basis.
>> > Sharing "tsec" among multiple "struct task_struct" is what you intended?
>> 
>> No, there is no shared "tsec".
>> snet_ticket_check() is called from the process context. So "tsec" is
>> a pointer to the "void *security" pointer from its own "struct
>> task_struct".
>>
> Until 2.6.28:
>
>   "void *security" is directory attached to "struct task_struct".
>   copy_process() calls security_task_alloc().
>
>   Therefore, task1->security != task2->security is guaranteed as long as you do
>
>     task->security = kmalloc();
>
>   at security_task_alloc().
>
> Since 2.6.29:
>
>   "void *security" is attached to "struct cred", and "struct cred *" is
>   attached to "struct task_struct". copy_process() calls copy_creds() and
>   prepare_creds() calls security_prepare_creds(). But copy_creds() does not
>   call prepare_creds() for clone(CLONE_THREAD) case.
>
>   Therefore, task1->cred->security != task2->cred->security is not guaranteed
>   even if you do
>
>     cred->security = kmalloc();
>
>   at security_prepare_creds().
>
>> every task_struct have a "tsec" allocated to its "void *security"
>> pointer. 
>
> You meant to have assigned "void *security" dedicated to "struct task_struct".
> But "void *security" is no longer directly attached to "struct task_struct".
> I couldn't find code that checks whether "current->cred" is used by only
> current thread or not. "current->cred" being used by only current thread is
> a requirement for having a "tsec" allocated to every "struct task_struct".
>
> Your code will share "tsec" among multiple threads if a process created
> threads using clone(CLONE_THREAD | CLONE_SIGHAND | CLONE_VM). Each thread has
> its own "struct task_struct" but they share "cred->security".
> Sharing "tsec" among multiple threads is what you intended?

Hello Tetsuo,

ok, I get your point now.
It seems that this is the common behaviour of credentials, so yes, "tsec"
will be shared between this kind of threads, and it makes sense.

Maybe, I need to protect read-write access to this "tsec" structure.

thank you for this comment Tetsuo,

sam