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


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