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    SubjectRe: [PATCH 2/2] Yama: add PTRACE exception tracking
    Hi Tetsuo,

    On Wed, Jun 30, 2010 at 10:09:54AM +0900, Tetsuo Handa wrote:
    > Kees Cook wrote:
    > > +static spinlock_t ptracer_relations_lock;
    > static DEFINE_SPINLOCK(ptracer_relations_lock);

    Ah, very cool, I missed that while reading through spinlock code. :)

    > > + relation = kzalloc(sizeof(*relation), GFP_KERNEL);
    > You can use kmalloc() since all fields are initialized within this function.

    I wasn't sure if list_add needed a zeroed ->node, so I opted for safety
    here. Is list_add safe to use on an uninitialized ->node? (Looks like it
    is on code review, I'll just use regular kmalloc.)

    > > +static int ptracer_exception_found(struct task_struct *tracer,
    > > + struct task_struct *tracee)
    > > +{
    > > + int rc = 0;
    > > + struct ptrace_relation *relation;
    > > + struct task_struct *parent = NULL;
    > > +
    > > + spin_lock(&ptracer_relations_lock);
    > > + list_for_each_entry(relation, &ptracer_relations, node)
    > > + if (relation->tracee == tracee) {
    > > + parent = relation->tracer;
    > > + break;
    > > + }
    > > + if (task_is_descendant(parent, tracer))
    > > + rc = 1;
    > > + spin_unlock(&ptracer_relations_lock);
    >
    > Can't we release ptracer_relations_lock before calling
    > task_is_descendant() since task_is_descendant() won't
    > access "struct ptrace_relation" on ptracer_relations list.

    This is where it gets a little funny. I need to keep that lock so that
    task_is_descendant isn't racing yama_task_free. I don't want to be in
    the position where I've left the lock only to have another CPU free the
    task_struct that was just located, so I have to keep the lock until I've
    finished using "parent". (And I can't take the task with get_task since
    it's already too late, and if I take it during _add, the task will never
    be freed.)

    > > @@ -32,27 +204,20 @@ static int yama_ptrace_access_check(struct task_struct *child,
    > > {
    > > int rc;
    > >
    > > + /* If standard caps disallows it, so does Yama. We should
    > > + * should only tighten restrictions further.
    > s/should should/should/

    Agh, thanks.

    -Kees

    --
    Kees Cook
    Ubuntu Security Team


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