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Re: [kernel-hardening] [PATCH 2/2] security,perf: Allow further restriction of perf_event_open

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  Re: [kernel-hardening] [PATCH 2/2] security,perf: Allow further  restriction of perf_event_open Kees Cook <keescook@chromium.org> - 2016-06-17 00:30 +0200
    Re: [kernel-hardening] [PATCH 2/2] security,perf: Allow further  restriction of perf_event_open Peter Zijlstra <peterz@infradead.org> - 2016-06-17 09:00 +0200
      Re: [kernel-hardening] [PATCH 2/2] security,perf: Allow further  restriction of perf_event_open Daniel Micay <danielmicay@gmail.com> - 2016-06-17 18:20 +0200
        Re: [kernel-hardening] [PATCH 2/2] security,perf: Allow further  restriction of perf_event_open Arnaldo Carvalho de Melo <acme@kernel.org> - 2016-06-17 22:10 +0200
          Re: [kernel-hardening] [PATCH 2/2] security,perf: Allow further  restriction of perf_event_open Daniel Micay <danielmicay@gmail.com> - 2016-06-18 03:00 +0200

#1424506 — Re: [kernel-hardening] [PATCH 2/2] security,perf: Allow further restriction of perf_event_open

Hi guys,

This patch wasn't originally CCed to you (I'm fixing that now). Would
you consider taking this into the perf tree? It's been in active use
in both Debian and Android for a while now.

(If need be, I can resend it.)

Thanks!

-Kees

On Sat, Jun 4, 2016 at 1:49 PM, Jeffrey Vander Stoep <jeffv@google.com> wrote:
> Acked-by: Jeff Vander Stoep <jeffv@google.com>
>
> In addition to Debian, this patch has been merged into AOSP and is a
> requirement for Android:
> https://android-review.googlesource.com/#/q/topic:CONFIG_SECURITY_PERF_EVENTS_RESTRICT
>
>
> On Wed, Apr 13, 2016 at 9:12 AM, Kees Cook <keescook@chromium.org> wrote:
>> On Mon, Jan 11, 2016 at 7:23 AM, Ben Hutchings <ben@decadent.org.uk>
>> wrote:
>>> When kernel.perf_event_open is set to 3 (or greater), disallow all
>>> access to performance events by users without CAP_SYS_ADMIN.
>>> Add a Kconfig symbol CONFIG_SECURITY_PERF_EVENTS_RESTRICT that
>>> makes this value the default.
>>>
>>> This is based on a similar feature in grsecurity
>>> (CONFIG_GRKERNSEC_PERF_HARDEN).  This version doesn't include making
>>> the variable read-only.  It also allows enabling further restriction
>>> at run-time regardless of whether the default is changed.
>>>
>>> Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
>>
>> Whoops, I entirely missed this email! Just found it now.
>>
>> Ben, can you resend this with Perf maintainers in CC? This seems
>> sensible enough to me.
>>
>> -Kees
>>
>>> ---
>>> I made a similar change to Debian's kernel packages in August,
>>> including the more restrictive default, and no-one has complained yet.
>>>
>>> Ben.
>>>
>>>  Documentation/sysctl/kernel.txt | 4 +++-
>>>  include/linux/perf_event.h      | 5 +++++
>>>  kernel/events/core.c            | 8 ++++++++
>>>  security/Kconfig                | 9 +++++++++
>>>  4 files changed, 25 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/Documentation/sysctl/kernel.txt
>>> b/Documentation/sysctl/kernel.txt
>>> index 88a2c8e..76e2ca8 100644
>>> --- a/Documentation/sysctl/kernel.txt
>>> +++ b/Documentation/sysctl/kernel.txt
>>> @@ -629,12 +629,14 @@ allowed to execute.
>>>  perf_event_paranoid:
>>>
>>>  Controls use of the performance events system by unprivileged
>>> -users (without CAP_SYS_ADMIN).  The default value is 1.
>>> +users (without CAP_SYS_ADMIN).  The default value is 3 if
>>> +CONFIG_SECURITY_PERF_EVENTS_RESTRICT is set, or 1 otherwise.
>>>
>>>   -1: Allow use of (almost) all events by all users
>>>  >=0: Disallow raw tracepoint access by users without CAP_IOC_LOCK
>>>  >=1: Disallow CPU event access by users without CAP_SYS_ADMIN
>>>  >=2: Disallow kernel profiling by users without CAP_SYS_ADMIN
>>> +>=3: Disallow all event access by users without CAP_SYS_ADMIN
>>>
>>>  ==============================================================
>>>
>>> diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
>>> index f9828a4..aa72940 100644
>>> --- a/include/linux/perf_event.h
>>> +++ b/include/linux/perf_event.h
>>> @@ -989,6 +989,11 @@ extern int perf_cpu_time_max_percent_handler(struct
>>> ctl_table *table, int write,
>>>                 loff_t *ppos);
>>>
>>>
>>> +static inline bool perf_paranoid_any(void)
>>> +{
>>> +       return sysctl_perf_event_paranoid > 2;
>>> +}
>>> +
>>>  static inline bool perf_paranoid_tracepoint_raw(void)
>>>  {
>>>         return sysctl_perf_event_paranoid > -1;
>>> diff --git a/kernel/events/core.c b/kernel/events/core.c
>>> index cfc227c..85bc810 100644
>>> --- a/kernel/events/core.c
>>> +++ b/kernel/events/core.c
>>> @@ -175,8 +175,13 @@ static struct srcu_struct pmus_srcu;
>>>   *   0 - disallow raw tracepoint access for unpriv
>>>   *   1 - disallow cpu events for unpriv
>>>   *   2 - disallow kernel profiling for unpriv
>>> + *   3 - disallow all unpriv perf event use
>>>   */
>>> +#ifdef CONFIG_SECURITY_PERF_EVENTS_RESTRICT
>>> +int sysctl_perf_event_paranoid __read_mostly = 3;
>>> +#else
>>>  int sysctl_perf_event_paranoid __read_mostly = 1;
>>> +#endif
>>>
>>>  /* Minimum for 512 kiB + 1 user control page */
>>>  int sysctl_perf_event_mlock __read_mostly = 512 + (PAGE_SIZE / 1024); /*
>>> 'free' kiB per user */
>>> @@ -8265,6 +8270,9 @@ SYSCALL_DEFINE5(perf_event_open,
>>>         if (flags & ~PERF_FLAG_ALL)
>>>                 return -EINVAL;
>>>
>>> +       if (perf_paranoid_any() && !capable(CAP_SYS_ADMIN))
>>> +               return -EACCES;
>>> +
>>>         err = perf_copy_attr(attr_uptr, &attr);
>>>         if (err)
>>>                 return err;
>>> diff --git a/security/Kconfig b/security/Kconfig
>>> index e452378..30a2603 100644
>>> --- a/security/Kconfig
>>> +++ b/security/Kconfig
>>> @@ -18,6 +18,15 @@ config SECURITY_DMESG_RESTRICT
>>>
>>>           If you are unsure how to answer this question, answer N.
>>>
>>> +config SECURITY_PERF_EVENTS_RESTRICT
>>> +       bool "Restrict unprivileged use of performance events"
>>> +       depends on PERF_EVENTS
>>> +       help
>>> +         If you say Y here, the kernel.perf_event_paranoid sysctl
>>> +         will be set to 3 by default, and no unprivileged use of the
>>> +         perf_event_open syscall will be permitted unless it is
>>> +         changed.
>>> +
>>>  config SECURITY
>>>         bool "Enable different security models"
>>>         depends on SYSFS
>>
>>
>>
>> --
>> Kees Cook
>> Chrome OS & Brillo Security
>



-- 
Kees Cook
Chrome OS & Brillo Security

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#1424731

On Thu, Jun 16, 2016 at 03:27:55PM -0700, Kees Cook wrote:
> Hi guys,
> 
> This patch wasn't originally CCed to you (I'm fixing that now). Would
> you consider taking this into the perf tree? 

No.

> It's been in active use
> in both Debian and Android for a while now.

Very nice of you all to finally inform us I suppose :/

> >>> When kernel.perf_event_open is set to 3 (or greater), disallow all
> >>> access to performance events by users without CAP_SYS_ADMIN.
> >>> Add a Kconfig symbol CONFIG_SECURITY_PERF_EVENTS_RESTRICT that
> >>> makes this value the default.
> >>>
> >>> This is based on a similar feature in grsecurity
> >>> (CONFIG_GRKERNSEC_PERF_HARDEN).  This version doesn't include making
> >>> the variable read-only.  It also allows enabling further restriction
> >>> at run-time regardless of whether the default is changed.

This Changelog is completely devoid of information. _WHY_ are you doing
this?

Also, hate the CONFIG.

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#1425284

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On Fri, 2016-06-17 at 08:54 +0200, Peter Zijlstra wrote:
> On Thu, Jun 16, 2016 at 03:27:55PM -0700, Kees Cook wrote:
> > Hi guys,
> > 
> > This patch wasn't originally CCed to you (I'm fixing that now).
> > Would
> > you consider taking this into the perf tree? 
> 
> No.
> 
> > It's been in active use
> > in both Debian and Android for a while now.
> 
> Very nice of you all to finally inform us I suppose :/

It was in Debian a lot longer than Android, although the Android feature
came from a downstream variant where it was done much earlier:

https://android-review.googlesource.com/#/c/233736/

> > > > > 
> > > > > access to performance events by users without CAP_SYS_ADMIN.
> > > > > Add a Kconfig symbol CONFIG_SECURITY_PERF_EVENTS_RESTRICT that
> > > > > makes this value the default.
> > > > > 
> > > > > This is based on a similar feature in grsecurity
> > > > > (CONFIG_GRKERNSEC_PERF_HARDEN).  This version doesn't include
> > > > > making
> > > > > the variable read-only.  It also allows enabling further
> > > > > restriction
> > > > > at run-time regardless of whether the default is changed.
> 
> This Changelog is completely devoid of information. _WHY_ are you
> doing
> this?

Attack surface reduction. It's possible to use seccomp-bpf for some
limited cases, but it's not flexible enough. There are lots of
information leaks and local privilege escalation vulnerabilities via
perf events, yet on most Linux installs it's not ever being used. So
turning it off by default on those installs is an easy win. The holes
are reduced to root -> kernel (and that's not a meaningful boundary in
mainline right now - although as is the case here, Debian has a bunch of
securelevel patches for that).

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#1425437

Em Fri, Jun 17, 2016 at 12:16:47PM -0400, Daniel Micay escreveu:
> On Fri, 2016-06-17 at 08:54 +0200, Peter Zijlstra wrote:
> > This Changelog is completely devoid of information. _WHY_ are you
> > doing this?
 
> Attack surface reduction. It's possible to use seccomp-bpf for some
> limited cases, but it's not flexible enough. There are lots of
> information leaks and local privilege escalation vulnerabilities via
> perf events, yet on most Linux installs it's not ever being used. So
> turning it off by default on those installs is an easy win. The holes
> are reduced to root -> kernel (and that's not a meaningful boundary in
> mainline right now - although as is the case here, Debian has a bunch of
> securelevel patches for that).

Is ptrace also disabled on such systems, or any of the other more recent
syscalls? The same arguments could probably be used to disable those:
reduce attack surface, possibly the new ones have bugs as they are
relatively new and it takes a long time for new syscalls to be more
generally used, if we go on disabling them in such a way, they will
probably never get used :-\

Wouldn't the recent bump in perf_event_paranoid to 2 enough? I.e. only
allow profiling of user tasks?

Or is there something more specific that we should disable/constrain to
reduce such surface contact without using such a big hammer?

- Arnaldo

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#1425587

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On Fri, 2016-06-17 at 17:00 -0300, Arnaldo Carvalho de Melo wrote:
> Em Fri, Jun 17, 2016 at 12:16:47PM -0400, Daniel Micay escreveu:
> > On Fri, 2016-06-17 at 08:54 +0200, Peter Zijlstra wrote:
> > > This Changelog is completely devoid of information. _WHY_ are you
> > > doing this?
>  
> > Attack surface reduction. It's possible to use seccomp-bpf for some
> > limited cases, but it's not flexible enough. There are lots of
> > information leaks and local privilege escalation vulnerabilities via
> > perf events, yet on most Linux installs it's not ever being used. So
> > turning it off by default on those installs is an easy win. The
> > holes
> > are reduced to root -> kernel (and that's not a meaningful boundary
> > in
> > mainline right now - although as is the case here, Debian has a
> > bunch of
> > securelevel patches for that).
> 
> Is ptrace also disabled on such systems, or any of the other more
> recent
> syscalls? The same arguments could probably be used to disable those:
> reduce attack surface, possibly the new ones have bugs as they are
> relatively new and it takes a long time for new syscalls to be more
> generally used, if we go on disabling them in such a way, they will
> probably never get used :-\

ptrace is allowed for third party apps within their SELinux domain, but
they all run as different users (so the kernel attack surface is there).
It's now disabled for privileged platform apps and most other domains. A
bit backwards, but removing it for third party apps would break
compatibility so it would have to be done at an API level bump. At
least, without deciding it is worth the cost like hidepid=2 in Android N
(which exposes no mechanism for exceptions in 3rd party apps, only the
base system).

If seccomp-bpf didn't imply such high system call overhead outside of
x86_64, Android would probably be walling off some new system calls. It
needs 2-phase lookup similar to x86 on ARM. Android kernels do avoid
enabling lots of kernel functionality from System V IPC to USERNS
though. New features wouldn't end up enabled if they were behind config
options without an explicit choice.

> Wouldn't the recent bump in perf_event_paranoid to 2 enough? I.e. only
> allow profiling of user tasks?

Most of the vulnerabilities are still exposed at 2. That prevents
leaking information about the kernel WITHOUT vulnerabilities though, and
would be an improvement when perf is disabled - but that doesn't really
matter much (Android kernel debugging and profiling would also still
work with 2).

> Or is there something more specific that we should disable/constrain
> to
> reduce such surface contact without using such a big hammer?

It's desired to have it globally disabled by default. Could use seccomp-
bpf to globally disable it, but that's a bigger hammer because it can't
be globally turned off without a reboot (could only profile newly
spawned processes after disabling it). Since it's only going to be
enabled by developers, trying to make it more fine-grained wouldn't
really pay off.

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