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    SubjectRe: [PATCH v11 11/11] seccomp: implement SECCOMP_FILTER_FLAG_TSYNC
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    On Thu, Jul 17, 2014 at 8:45 AM, Kees Cook <keescook@chromium.org> wrote:
    > On Thu, Jul 17, 2014 at 8:04 AM, David Drysdale <drysdale@google.com> wrote:
    >> On Wed, Jul 16, 2014 at 10:50 PM, Kees Cook <keescook@chromium.org> wrote:
    >>> diff --git a/kernel/seccomp.c b/kernel/seccomp.c
    >>> index 9065d2c79c56..2125b83ccfd4 100644
    >>> +/**
    >>> + * seccomp_can_sync_threads: checks if all threads can be synchronized
    >>> + *
    >>> + * Expects sighand and cred_guard_mutex locks to be held.
    >>> + *
    >>> + * Returns 0 on success, -ve on error, or the pid of a thread which was
    >>> + * either not in the correct seccomp mode or it did not have an ancestral
    >>> + * seccomp filter.
    >>> + */
    >>> +static inline pid_t seccomp_can_sync_threads(void)
    >>> +{
    >>> + struct task_struct *thread, *caller;
    >>> +
    >>> + BUG_ON(!mutex_is_locked(&current->signal->cred_guard_mutex));
    >>> + BUG_ON(!spin_is_locked(&current->sighand->siglock));
    >>> +
    >>> + if (current->seccomp.mode != SECCOMP_MODE_FILTER)
    >>> + return -EACCES;
    >>
    >> Quick question -- is it possible to apply the first filter and also synchronize
    >> it across threads in the same operation? If so, does this arm also need to
    >> cope with seccomp.mode being SECCOMP_MODE_DISABLED?
    >>
    >> [seccomp_set_mode_filter() looks to call this via seccomp_attach_filter()
    >> before it does seccomp_assign_mode()]
    >
    > I don't entirely understand what you're asking. The threads gain the
    > filter and the mode before the current thread may gain the mode (if
    > it's the first time this has been called). Due to all the locks,
    > though, this isn't a problem. Is there a situation you see where there
    > might be a problem?

    Just to follow up for posterity on lkml: the problem was that mode was
    being set in "current" _after_ sync, so the mode check in can_sync
    would fail if "current" was not yet in filter mode. (i.e. the first
    attached filter could not have the TSYNC flag.) This check was
    redundant with the attach_filter entry point checks, and protected
    nothing, so it has been removed and a new test added to the seccomp
    regression test suite. :)

    I sent it as a new patch on top of v11, instead of respinning
    everything as v12. If that's not preferred, I can send v12 with this
    fix incorporated.

    Thanks!

    -Kees

    --
    Kees Cook
    Chrome OS Security


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