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

[Oleg Nesterov <oleg@redhat.com>]

On 02/15, Cyrill Gorcunov wrote:
>
> On Wed, Feb 15, 2012 at 04:38:16PM +0100, Oleg Nesterov wrote:
> ...
> > >
> > > Wait, how it's differ from other ptrace_may_access calls all over
> > > the kernel? I suppose I'm missing something obvious?
> >
> > For example? Say, mm_access() is fine because it returns ->mm
> > which we are going to play with.
>
> So, say we have
>
> environ_read
>   mm_for_maps
>    mm_access
>     success, and first reader continue here
>
> then say task change own credentials and all
> this sequence fails because access is not granted
> anymore (say for second reader), but first reader
> still able to continue reading because access was
> graned earlier.

Can't understand... Yes, environ_read() can succeed for the
first reader, and then later it can fail for the same/another
reader. And?

> 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.

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.

> > Once again, I am not saying that this code really has the security
> > problems. I simply do not know. But it looks wrong without the
> > comment. I do not really understand why do we need ptrace_may_access(),
> > but whatever reason we have how we can trust it? Say, when KCMP_VM
> > checks ->mm, all we know is that PTRACE_MODE_READ succeed in the
> > past. This looks confusing, imho.
>
> Adding the comment is not a problem. The problem is that I
> dont understand if there error in patch or not, can we stick
> with ptrace_may_access or need something different here?
> The idea was exactly like -- if you have enough rights to
> proceed ptrace_may_access then syscall should continue
> executing and return comparision result.

My only point is: this check is obviously racy, and thus it looks
confusing. Whether this is fine or not, I do not know. Personally
I see no reason for ptrace_may_access(), but I am not security
expert.

Oleg.