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

[Suren Baghdasaryan]

On Thu, Jan 28, 2021 at 11:51 AM Suren Baghdasaryan <surenb at google.com> wrote:
>
> On Tue, Jan 26, 2021 at 5:52 AM 'Michal Hocko' via kernel-team
> <kernel-team at android.com> wrote:
> >
> > 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.
> >
>
> Sorry, back to the matters of this patch. Are there any actionable
> items for me to take care of before it can be accepted? The only
> request from Andrew to write a man page is being worked on at
> https://lore.kernel.org/linux-mm/20210120202337.1481402-1-surenb@google.com/
> and I'll follow up with the next version. I also CC'ed stable@ for
> this to be included into 5.10 per Andrew's request. That CC was lost
> at some point, so CC'ing again.
>
> I do not see anything else on this patch to fix. Please chime in if
> there are any more concerns, otherwise I would ask Andrew to take it
> into mm-tree and stable@ to apply it to 5.10.
> Thanks!

process_madvise man page V2 is posted at:
https://lore.kernel.org/linux-mm/20210129070340.566340-1-surenb@google.com/

>
>
> > --
> > Michal Hocko
> > SUSE Labs
> >
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