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Date: Thu, 10 Sep 2020 16:14:38 -0700
From: Kees Cook <keescook@...omium.org>
To: kernel-hardening@...ts.openwall.com
Cc: John Wood <john.wood@....com>, Matthew Wilcox <willy@...radead.org>,
Jonathan Corbet <corbet@....net>,
Alexander Viro <viro@...iv.linux.org.uk>,
Ingo Molnar <mingo@...hat.com>,
Peter Zijlstra <peterz@...radead.org>,
Juri Lelli <juri.lelli@...hat.com>,
Vincent Guittot <vincent.guittot@...aro.org>,
Dietmar Eggemann <dietmar.eggemann@....com>,
Steven Rostedt <rostedt@...dmis.org>,
Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
Luis Chamberlain <mcgrof@...nel.org>,
Iurii Zaikin <yzaikin@...gle.com>, James Morris <jmorris@...ei.org>,
"Serge E. Hallyn" <serge@...lyn.com>, linux-doc@...r.kernel.org,
linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
linux-security-module@...r.kernel.org
Subject: Re: [RFC PATCH 4/6] security/fbfam: Add a new sysctl to control the
crashing rate threshold
On Thu, Sep 10, 2020 at 01:21:05PM -0700, Kees Cook wrote:
> From: John Wood <john.wood@....com>
>
> This is a previous step to add the detection feature.
>
> A fork brute force attack will be detected when an application crashes
> quickly. Since, a rate can be defined as a time per fault, add a new
> sysctl to control the crashing rate threshold.
>
> This way, each system can tune the detection's sensibility adjusting the
> milliseconds per fault. So, if the application's crashing rate falls
> under this threshold an attack will be detected. So, the higher this
> value, the faster an attack will be detected.
>
> Signed-off-by: John Wood <john.wood@....com>
> ---
> include/fbfam/fbfam.h | 4 ++++
> kernel/sysctl.c | 9 +++++++++
> security/fbfam/Makefile | 1 +
> security/fbfam/fbfam.c | 11 +++++++++++
> security/fbfam/sysctl.c | 20 ++++++++++++++++++++
> 5 files changed, 45 insertions(+)
> create mode 100644 security/fbfam/sysctl.c
>
> diff --git a/include/fbfam/fbfam.h b/include/fbfam/fbfam.h
> index b5b7d1127a52..2cfe51d2b0d5 100644
> --- a/include/fbfam/fbfam.h
> +++ b/include/fbfam/fbfam.h
> @@ -3,8 +3,12 @@
> #define _FBFAM_H_
>
> #include <linux/sched.h>
> +#include <linux/sysctl.h>
>
> #ifdef CONFIG_FBFAM
> +#ifdef CONFIG_SYSCTL
> +extern struct ctl_table fbfam_sysctls[];
> +#endif
Instead of doing the extern and adding to sysctl.c, this can all be done
directly (dynamically) from the fbfam.c file instead.
> int fbfam_fork(struct task_struct *child);
> int fbfam_execve(void);
> int fbfam_exit(void);
> diff --git a/kernel/sysctl.c b/kernel/sysctl.c
> index 09e70ee2332e..c3b4d737bef3 100644
> --- a/kernel/sysctl.c
> +++ b/kernel/sysctl.c
> @@ -77,6 +77,8 @@
> #include <linux/uaccess.h>
> #include <asm/processor.h>
>
> +#include <fbfam/fbfam.h>
> +
> #ifdef CONFIG_X86
> #include <asm/nmi.h>
> #include <asm/stacktrace.h>
> @@ -2660,6 +2662,13 @@ static struct ctl_table kern_table[] = {
> .extra1 = SYSCTL_ZERO,
> .extra2 = SYSCTL_ONE,
> },
> +#endif
> +#ifdef CONFIG_FBFAM
> + {
> + .procname = "fbfam",
> + .mode = 0555,
> + .child = fbfam_sysctls,
> + },
> #endif
> { }
> };
> diff --git a/security/fbfam/Makefile b/security/fbfam/Makefile
> index f4b9f0b19c44..b8d5751ecea4 100644
> --- a/security/fbfam/Makefile
> +++ b/security/fbfam/Makefile
> @@ -1,2 +1,3 @@
> # SPDX-License-Identifier: GPL-2.0
> obj-$(CONFIG_FBFAM) += fbfam.o
> +obj-$(CONFIG_SYSCTL) += sysctl.o
> diff --git a/security/fbfam/fbfam.c b/security/fbfam/fbfam.c
> index 0387f95f6408..9be4639b72eb 100644
> --- a/security/fbfam/fbfam.c
> +++ b/security/fbfam/fbfam.c
> @@ -7,6 +7,17 @@
> #include <linux/refcount.h>
> #include <linux/slab.h>
>
> +/**
> + * sysctl_crashing_rate_threshold - Crashing rate threshold.
> + *
> + * The rate's units are in milliseconds per fault.
> + *
> + * A fork brute force attack will be detected if the application's crashing rate
> + * falls under this threshold. So, the higher this value, the faster an attack
> + * will be detected.
> + */
> +unsigned long sysctl_crashing_rate_threshold = 30000;
I would move the sysctls here, instead. (Also, the above should be
const.)
> +
> /**
> * struct fbfam_stats - Fork brute force attack mitigation statistics.
> * @refc: Reference counter.
> diff --git a/security/fbfam/sysctl.c b/security/fbfam/sysctl.c
> new file mode 100644
> index 000000000000..430323ad8e9f
> --- /dev/null
> +++ b/security/fbfam/sysctl.c
> @@ -0,0 +1,20 @@
> +// SPDX-License-Identifier: GPL-2.0
> +#include <linux/sysctl.h>
> +
> +extern unsigned long sysctl_crashing_rate_threshold;
> +static unsigned long ulong_one = 1;
> +static unsigned long ulong_max = ULONG_MAX;
> +
> +struct ctl_table fbfam_sysctls[] = {
> + {
> + .procname = "crashing_rate_threshold",
> + .data = &sysctl_crashing_rate_threshold,
> + .maxlen = sizeof(sysctl_crashing_rate_threshold),
> + .mode = 0644,
> + .proc_handler = proc_doulongvec_minmax,
> + .extra1 = &ulong_one,
> + .extra2 = &ulong_max,
> + },
> + { }
> +};
I wouldn't bother splitting this into a separate file. (Just leave it in
fbfam.c)
--
Kees Cook
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