Re: [RFC PATCH 4/6] security/fbfam: Add a new sysctl to control the crashing rate threshold

[Kees Cook]

On Thu, Sep 10, 2020 at 01:21:05PM -0700, Kees Cook wrote:
> From: John Wood <john.wood@xxxxxxx>
> 
> 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@xxxxxxx>
> ---
>  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