Re: [RFC PATCH 5/6] security/fbfam: Detect a fork brute force attack

[Kees Cook <keescook@chromium.org>]

On Thu, Sep 10, 2020 at 01:21:06PM -0700, Kees Cook wrote:
> From: John Wood <john.wood@gmx.com>
> 
> To detect a fork brute force attack it is necessary to compute the
> crashing rate of the application. This calculation is performed in each
> fatal fail of a task, or in other words, when a core dump is triggered.
> If this rate shows that the application is crashing quickly, there is a
> clear signal that an attack is happening.
> 
> Since the crashing rate is computed in milliseconds per fault, if this
> rate goes under a certain threshold a warning is triggered.
> 
> Signed-off-by: John Wood <john.wood@gmx.com>
> ---
>  fs/coredump.c          |  2 ++
>  include/fbfam/fbfam.h  |  2 ++
>  security/fbfam/fbfam.c | 39 +++++++++++++++++++++++++++++++++++++++
>  3 files changed, 43 insertions(+)
> 
> diff --git a/fs/coredump.c b/fs/coredump.c
> index 76e7c10edfc0..d4ba4e1828d5 100644
> --- a/fs/coredump.c
> +++ b/fs/coredump.c
> @@ -51,6 +51,7 @@
>  #include "internal.h"
>  
>  #include <trace/events/sched.h>
> +#include <fbfam/fbfam.h>
>  
>  int core_uses_pid;
>  unsigned int core_pipe_limit;
> @@ -825,6 +826,7 @@ void do_coredump(const kernel_siginfo_t *siginfo)
>  fail_creds:
>  	put_cred(cred);
>  fail:
> +	fbfam_handle_attack(siginfo->si_signo);

I don't think this is the right place for detecting a crash -- isn't
this only for the "dumping core" condition? In other words, don't you
want to do this in get_signal()'s "fatal" block? (i.e. very close to the
do_coredump, but without the "should I dump?" check?)

Hmm, but maybe I'm wrong? It looks like you're looking at noticing the
process taking a signal from SIG_KERNEL_COREDUMP_MASK ?

(Better yet: what are fatal conditions that do NOT match
SIG_KERNEL_COREDUMP_MASK, and should those be covered?)

Regardless, *this* looks like the only place without an LSM hook. And it
doesn't seem unreasonable to add one here. I assume it would probably
just take the siginfo pointer, which is also what you're checking.

e.g. for include/linux/lsm_hook_defs.h:

LSM_HOOK(int, 0, task_coredump, const kernel_siginfo_t *siginfo);


>  	return;
>  }
>  
> diff --git a/include/fbfam/fbfam.h b/include/fbfam/fbfam.h
> index 2cfe51d2b0d5..9ac8e33d8291 100644
> --- a/include/fbfam/fbfam.h
> +++ b/include/fbfam/fbfam.h
> @@ -12,10 +12,12 @@ extern struct ctl_table fbfam_sysctls[];
>  int fbfam_fork(struct task_struct *child);
>  int fbfam_execve(void);
>  int fbfam_exit(void);
> +int fbfam_handle_attack(int signal);
>  #else
>  static inline int fbfam_fork(struct task_struct *child) { return 0; }
>  static inline int fbfam_execve(void) { return 0; }
>  static inline int fbfam_exit(void) { return 0; }
> +static inline int fbfam_handle_attack(int signal) { return 0; }
>  #endif
>  
>  #endif /* _FBFAM_H_ */
> diff --git a/security/fbfam/fbfam.c b/security/fbfam/fbfam.c
> index 9be4639b72eb..3aa669e4ea51 100644
> --- a/security/fbfam/fbfam.c
> +++ b/security/fbfam/fbfam.c
> @@ -4,7 +4,9 @@
>  #include <linux/errno.h>
>  #include <linux/gfp.h>
>  #include <linux/jiffies.h>
> +#include <linux/printk.h>
>  #include <linux/refcount.h>
> +#include <linux/signal.h>
>  #include <linux/slab.h>
>  
>  /**
> @@ -172,3 +174,40 @@ int fbfam_exit(void)
>  	return 0;
>  }
>  
> +/**
> + * fbfam_handle_attack() - Fork brute force attack detection.
> + * @signal: Signal number that causes the core dump.
> + *
> + * The crashing rate of an application is computed in milliseconds per fault in
> + * each crash. So, if this rate goes under a certain threshold there is a clear
> + * signal that the application is crashing quickly. At this moment, a fork brute
> + * force attack is happening.
> + *
> + * Return: -EFAULT if the current task doesn't have statistical data. Zero
> + *         otherwise.
> + */
> +int fbfam_handle_attack(int signal)
> +{
> +	struct fbfam_stats *stats = current->fbfam_stats;
> +	u64 delta_jiffies, delta_time;
> +	u64 crashing_rate;
> +
> +	if (!stats)
> +		return -EFAULT;
> +
> +	if (!(signal == SIGILL || signal == SIGBUS || signal == SIGKILL ||
> +	      signal == SIGSEGV || signal == SIGSYS))
> +		return 0;

This will only be called for:

#define SIG_KERNEL_COREDUMP_MASK (\
        rt_sigmask(SIGQUIT)   |  rt_sigmask(SIGILL)    | \
        rt_sigmask(SIGTRAP)   |  rt_sigmask(SIGABRT)   | \
        rt_sigmask(SIGFPE)    |  rt_sigmask(SIGSEGV)   | \
        rt_sigmask(SIGBUS)    |  rt_sigmask(SIGSYS)    | \
        rt_sigmask(SIGXCPU)   |  rt_sigmask(SIGXFSZ)   | \
        SIGEMT_MASK                                    )

So you're skipping:

	SIGQUIT
	SIGTRAP
	SIGABRT
	SIGFPE
	SIGXCPU
	SIGXFSZ
	SIGEMT_MASK

I would include SIGABRT (e.g. glibc will call abort() for stack
canary, malloc, etc failures, which may indicate a mitigation has
fired).

-- 
Kees Cook