Re: + syscalls-x86-add-__nr_kcmp-syscall-v8.patch added to -mm tree

Oleg Nesterov
2012-02-15T18:51:27Z

On 02/15, Cyrill Gorcunov wrote:
>
> On Wed, Feb 15, 2012 at 05:22:22PM +0100, Oleg Nesterov wrote:
> >
> > > So I don't understand how it's different from what
> > > is provided in this patch. What I'm missing?
> >
> > environ_read() does
> >
> > mm = mm_access(task);
> > if (mm)
> > do_something(mm);
> >
> > even if it races with, say, execve(setuid_app) we can't read the
> > new ->mm.
>
> Wait, I'm confused
>
> process 1 (reader) process 2 ("task" itself)
> mm = mm_access(task);
> task changes own credentials
> so reader can't access on next
> read if it would try, but since
> access already granted... it
> continues do_something(mm)
> if (mm)
> do_something(mm);
>
> So in the patch I tried the same, once access is granted it
> belongs to a caller.

See the "execve(setuid_app)", this is what I meant. Even if we
race with execve() and the task raises its privileges we can't
read the new ->mm, we will read the old mm for which we have
(had) the rights to access.

> > while your code (very roughly) does something like
> >
> > mm = mm_access(task);
> > if (mm)
> > do_something(task->mm);
> >
> > while it is quite possible that mm != task->mm.
>
> Oleg, could you please explain me where it happens
> that task->mm (I've got access to) will be changed
> to some new -mm while I'm inspecting it.

Cough... this is question I am trying to ask ;)

Let me try again. To simplify, lets discuss the KCMP_VM case
only.

I do not really understand why do we need ptrace_may_access().
I do not see any security problems with kcmp_ptr(task->mm), but
I am not expert.

However, you added this check so I assume you have some reason.
But this can race with execve(setuid_app) and KCMP_VM can play
with task->mm after this task raises its caps. If this is fine,
then why do we need ptrace_may_access?

OK, please ignore. I sent the initial email just becase KCMP_FILE
is buggy.

> > > + for (i = 0; i < KCMP_TYPES; i++)
> > > + cookies[i][1] |= (~(~0UL >> 1) | 1);
> >
> > I am puzzled, help ;) this is equal to
> >
> > cookies[i][1] |= -LONG_MAX;
> > or
> > cookies[i][1] |= (LONG_MIN | 1);
> >
> > for what? why do we want to set these 2 bits (MSB and LSB) ?
>
> Letme quote hpa@ here :)
>
> | This code is wrong. You will have a zero cookie, legitimately, once in
> | 2^32 or 2^64 attempts, depending on the bitness.
> |
> | The other thing is that for the multiplicative cookie you should OR in
> | the value (~(~0UL >> 1) | 1) in order to make sure that the value is (a)
> | large and (b) odd.

OK, thanks.

Oleg.