[PATCH v2 1/1] mm/madvise: replace ptrace attach requirement for process_madvise

[Michal Hocko]

On Wed 20-01-21 14:17:39, Jann Horn wrote:
> On Wed, Jan 13, 2021 at 3:22 PM Michal Hocko <mhocko at suse.com> wrote:
> > On Tue 12-01-21 09:51:24, Suren Baghdasaryan wrote:
> > > On Tue, Jan 12, 2021 at 9:45 AM Oleg Nesterov <oleg at redhat.com> wrote:
> > > >
> > > > On 01/12, Michal Hocko wrote:
> > > > >
> > > > > On Mon 11-01-21 09:06:22, Suren Baghdasaryan wrote:
> > > > >
> > > > > > What we want is the ability for one process to influence another process
> > > > > > in order to optimize performance across the entire system while leaving
> > > > > > the security boundary intact.
> > > > > > Replace PTRACE_MODE_ATTACH with a combination of PTRACE_MODE_READ
> > > > > > and CAP_SYS_NICE. PTRACE_MODE_READ to prevent leaking ASLR metadata
> > > > > > and CAP_SYS_NICE for influencing process performance.
> > > > >
> > > > > I have to say that ptrace modes are rather obscure to me. So I cannot
> > > > > really judge whether MODE_READ is sufficient. My understanding has
> > > > > always been that this is requred to RO access to the address space. But
> > > > > this operation clearly has a visible side effect. Do we have any actual
> > > > > documentation for the existing modes?
> > > > >
> > > > > I would be really curious to hear from Jann and Oleg (now Cced).
> > > >
> > > > Can't comment, sorry. I never understood these security checks and never tried.
> > > > IIUC only selinux/etc can treat ATTACH/READ differently and I have no idea what
> > > > is the difference.
> 
> Yama in particular only does its checks on ATTACH and ignores READ,
> that's the difference you're probably most likely to encounter on a
> normal desktop system, since some distros turn Yama on by default.
> Basically the idea there is that running "gdb -p $pid" or "strace -p
> $pid" as a normal user will usually fail, but reading /proc/$pid/maps
> still works; so you can see things like detailed memory usage
> information and such, but you're not supposed to be able to directly
> peek into a running SSH client and inject data into the existing SSH
> connection, or steal the cryptographic keys for the current
> connection, or something like that.
> 
> > > I haven't seen a written explanation on ptrace modes but when I
> > > consulted Jann his explanation was:
> > >
> > > PTRACE_MODE_READ means you can inspect metadata about processes with
> > > the specified domain, across UID boundaries.
> > > PTRACE_MODE_ATTACH means you can fully impersonate processes with the
> > > specified domain, across UID boundaries.
> >
> > Maybe this would be a good start to document expectations. Some more
> > practical examples where the difference is visible would be great as
> > well.
> 
> Before documenting the behavior, it would be a good idea to figure out
> what to do with perf_event_open(). That one's weird in that it only
> requires PTRACE_MODE_READ, but actually allows you to sample stuff
> like userspace stack and register contents (if perf_event_paranoid is
> 1 or 2). Maybe for SELinux things (and maybe also for Yama), there
> should be a level in between that allows fully inspecting the process
> (for purposes like profiling) but without the ability to corrupt its
> memory or registers or things like that. Or maybe perf_event_open()
> should just use the ATTACH mode.

Thanks for the clarification. I still cannot say I would have a good
mental picture. Having something in Documentation/core-api/ sounds
really needed. Wrt to perf_event_open it sounds really odd it can do
more than other places restrict indeed. Something for the respective
maintainer but I strongly suspect people simply copy the pattern from
other places because the expected semantic is not really clear.

-- 
Michal Hocko
SUSE Labs