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

[John Wood <john.wood@gmx.com>]

Hi, more inline.

On Thu, Sep 10, 2020 at 04:14:38PM -0700, Kees Cook wrote:
> > 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.

Like Yama do in the yama_init_sysctl() function? As a reference code.

> >  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.)

If the above variable is const how the sysctl interface can modify it?
I think it would be better to declare it as __read_mostly instead. What
do you think?

unsigned long sysctl_crashing_rate_threshold __read_mostly = 30000;

> > +
> >  /**
> >   * 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

Thanks,
John Wood