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

[Suren Baghdasaryan]

On Wed, Jan 20, 2021 at 8:57 AM Suren Baghdasaryan <surenb at google.com> wrote:
>
> On Wed, Jan 20, 2021 at 5:18 AM Jann Horn <jannh at google.com> 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 additional clarifications, Jann!
> Just to clarify, the documentation I'm preparing is a man page for
> process_madvise(2) which will list the required capabilities but won't
> dive into all the security details.
> I believe the above suggestions are for documenting different PTRACE
> modes and will not be included in that man page. Maybe a separate
> document could do that but I'm definitely not qualified to write it.

Folks, I posted the man page here:
https://lore.kernel.org/linux-mm/20210120202337.1481402-1-surenb@google.com/

Also I realized that this patch is not changing at all and if I send a
new version, the only difference would be CC'ing it to stable and
linux-security-module.
I'm CC'ing stable (James already CC'ed LSM), but if I should re-post
it please let me know.

Cc: stable at vger.kernel.org # 5.10+