[kernel-hardening] Re: [PATCH 4/5] bug: Provide toggle for BUG on data corruption

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Tue, Aug 16, 2016 at 02:11:04PM -0700, Kees Cook wrote:
> The kernel checks for several cases of data structure corruption under
> either normal runtime, or under various CONFIG_DEBUG_* settings. When
> corruption is detected, some systems may want to BUG() immediately instead
> of letting the corruption continue. Many of these manipulation primitives
> can be used by security flaws to gain arbitrary memory write control. This
> provides CONFIG_BUG_ON_CORRUPTION to control newly added BUG() locations.
> 
> This is inspired by similar hardening in PaX and Grsecurity, and by
> Stephen Boyd in MSM kernels.
> 
> Signed-off-by: Kees Cook <keescook@chromium.org>

OK, I will bite...  Why both the WARN() and the BUG_ON()?

								Thanx, Paul

> ---
>  include/linux/bug.h             |  7 +++++++
>  kernel/locking/spinlock_debug.c |  1 +
>  kernel/workqueue.c              |  2 ++
>  lib/Kconfig.debug               | 10 ++++++++++
>  lib/list_debug.c                |  7 +++++++
>  5 files changed, 27 insertions(+)
> 
> diff --git a/include/linux/bug.h b/include/linux/bug.h
> index e51b0709e78d..7e69758dd798 100644
> --- a/include/linux/bug.h
> +++ b/include/linux/bug.h
> @@ -118,4 +118,11 @@ static inline enum bug_trap_type report_bug(unsigned long bug_addr,
>  }
> 
>  #endif	/* CONFIG_GENERIC_BUG */
> +
> +#ifdef CONFIG_BUG_ON_CORRUPTION
> +# define CORRUPTED_DATA_STRUCTURE	true
> +#else
> +# define CORRUPTED_DATA_STRUCTURE	false
> +#endif
> +
>  #endif	/* _LINUX_BUG_H */
> diff --git a/kernel/locking/spinlock_debug.c b/kernel/locking/spinlock_debug.c
> index 0374a596cffa..d5f833769feb 100644
> --- a/kernel/locking/spinlock_debug.c
> +++ b/kernel/locking/spinlock_debug.c
> @@ -64,6 +64,7 @@ static void spin_dump(raw_spinlock_t *lock, const char *msg)
>  		owner ? owner->comm : "<none>",
>  		owner ? task_pid_nr(owner) : -1,
>  		lock->owner_cpu);
> +	BUG_ON(CORRUPTED_DATA_STRUCTURE);
>  	dump_stack();
>  }
> 
> diff --git a/kernel/workqueue.c b/kernel/workqueue.c
> index ef071ca73fc3..ea0132b55eca 100644
> --- a/kernel/workqueue.c
> +++ b/kernel/workqueue.c
> @@ -48,6 +48,7 @@
>  #include <linux/nodemask.h>
>  #include <linux/moduleparam.h>
>  #include <linux/uaccess.h>
> +#include <linux/bug.h>
> 
>  #include "workqueue_internal.h"
> 
> @@ -2108,6 +2109,7 @@ __acquires(&pool->lock)
>  		       current->comm, preempt_count(), task_pid_nr(current),
>  		       worker->current_func);
>  		debug_show_held_locks(current);
> +		BUG_ON(CORRUPTED_DATA_STRUCTURE);
>  		dump_stack();
>  	}
> 
> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> index 2307d7c89dac..d64bd6b6fd45 100644
> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -1987,6 +1987,16 @@ config TEST_STATIC_KEYS
> 
>  	  If unsure, say N.
> 
> +config BUG_ON_CORRUPTION
> +	bool "Trigger a BUG when data corruption is detected"
> +	help
> +	  Select this option if the kernel should BUG when it encounters
> +	  data corruption in various kernel memory structures during checks
> +	  added by options like CONFIG_DEBUG_LIST, CONFIG_DEBUG_SPINLOCK,
> +	  etc.
> +
> +	  If unsure, say N.
> +
>  source "samples/Kconfig"
> 
>  source "lib/Kconfig.kgdb"
> diff --git a/lib/list_debug.c b/lib/list_debug.c
> index 80e2e40a6a4e..161c7e7d3478 100644
> --- a/lib/list_debug.c
> +++ b/lib/list_debug.c
> @@ -26,16 +26,19 @@ bool __list_add_debug(struct list_head *new,
>  	if (unlikely(next->prev != prev)) {
>  		WARN(1, "list_add corruption. next->prev should be prev (%p), but was %p. (next=%p).\n",
>  			prev, next->prev, next);
> +		BUG_ON(CORRUPTED_DATA_STRUCTURE);
>  		return false;
>  	}
>  	if (unlikely(prev->next != next)) {
>  		WARN(1, "list_add corruption. prev->next should be next (%p), but was %p. (prev=%p).\n",
>  			next, prev->next, prev);
> +		BUG_ON(CORRUPTED_DATA_STRUCTURE);
>  		return false;
>  	}
>  	if (unlikely(new == prev || new == next)) {
>  		WARN(1, "list_add double add: new=%p, prev=%p, next=%p.\n",
>  			new, prev, next);
> +		BUG_ON(CORRUPTED_DATA_STRUCTURE);
>  		return false;
>  	}
>  	return true;
> @@ -52,21 +55,25 @@ bool __list_del_entry_debug(struct list_head *entry)
>  	if (unlikely(next == LIST_POISON1)) {
>  		WARN(1, "list_del corruption, %p->next is LIST_POISON1 (%p)\n",
>  			entry, LIST_POISON1);
> +		BUG_ON(CORRUPTED_DATA_STRUCTURE);
>  		return false;
>  	}
>  	if (unlikely(prev == LIST_POISON2)) {
>  		WARN(1, "list_del corruption, %p->prev is LIST_POISON2 (%p)\n",
>  			entry, LIST_POISON2);
> +		BUG_ON(CORRUPTED_DATA_STRUCTURE);
>  		return false;
>  	}
>  	if (unlikely(prev->next != entry)) {
>  		WARN(1, "list_del corruption. prev->next should be %p, but was %p\n",
>  			entry, prev->next);
> +		BUG_ON(CORRUPTED_DATA_STRUCTURE);
>  		return false;
>  	}
>  	if (unlikely(next->prev != entry)) {
>  		WARN(1, "list_del corruption. next->prev should be %p, but was %p\n",
>  			entry, next->prev);
> +		BUG_ON(CORRUPTED_DATA_STRUCTURE);
>  		return false;
>  	}
>  	return true;
> -- 
> 2.7.4
>