Re: çåï[PATCH] perf core: Use KSTK_ESP() instead of pt_regs->sp while output user regs

[Andy Lutomirski]

On Jan 4, 2015 8:11 AM, "Jiri Olsa" <jolsa@xxxxxxxxxx> wrote:
>
> On Tue, Dec 30, 2014 at 08:03:27PM +0100, Peter Zijlstra wrote:
> > On Thu, Dec 25, 2014 at 07:48:28AM -0800, Andy Lutomirski wrote:
> > > On a quick look, there are plenty of other bugs in there besides just
> > > the stack pointer issue.  The ABI check that uses TIF_IA32 in the perf
> > > core is completely wrong.  TIF_IA32 may be equal to the actual
> > > userspace bitness by luck, but, if so, that's more or less just luck.
> > > And there's a user_mode test that should be user_mode_vm.
> > >
> > > Also, it's not just sp that's wrong.  There are various places that
> > > you can interrupt in which many of the registers have confusing
> > > locations.  You could try using the cfi unwind data, but that's
> > > unlikely to work for regs like cs and ss, and, during context switch,
> > > this has very little chance of working.
> > >
> > > What's the point of this feature?  Honestly, my suggestion would be to
> > > delete it instead of trying to fix it.  It's also not clear to me that
> > > there aren't serious security problems here -- it's entirely possible
> > > for sensitive *kernel* values to and up in task_pt_regs at certain
> > > times, and if you run during context switch and there's no code to
> > > suppress this dump during context switch, then you could be showing
> > > regs that belong to the wrong task.
> >
> > Of course the people who actually wrote the code are not on CC :/
> >
> > There's two users of this iirc;
> >
> >  1) the dwarf stack unwinder thingy, which basically dumps the userspace
> >  regs and the top of userspace stack on 'event'.
>
> looks like this solves the issue I was trying to fix
> long time ago:
>   http://marc.info/?l=linux-kernel&m=134934717011451&w=2
>
> this seems a lot simpler ;-) I'll test..

I suspect that, if it works, then it's pure luck.
(task)->thread.usersp in KSTK_ESP is bogus -- your code was more
accurate.

I think we should seriously consider making use of this feature by
non-root users require an explicit sysctl.  Sending values to user
code that are, at best, free of sensitive kernel data most of the time
is IMO inappropriate for an unprivileged API.

I'm currently working on a patch to try to clean this up better.

--Andy

>
> jirka
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