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

28 Jan 2021


      On Tue, Jan 26, 2021 at 5:52 AM 'Michal Hocko' via kernel-team
kernel-team@android.com wrote:
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On Wed 20-01-21 14:17:39, Jann Horn wrote:
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On Wed, Jan 13, 2021 at 3:22 PM Michal Hocko mhocko@suse.com wrote:
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On Tue 12-01-21 09:51:24, Suren Baghdasaryan wrote:
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On Tue, Jan 12, 2021 at 9:45 AM Oleg Nesterov oleg@redhat.com wrote:
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On 01/12, Michal Hocko wrote:
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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.
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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!
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Michal Hocko
SUSE Labs
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Re: [PATCH v2 1/1] mm/madvise: replace ptrace attach requirement for process_madvise