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

[Kees Cook <keescook@chromium.org>]

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