Re: [PATCH v2 1/5] list: Split list_add() debug checking into separate function

[Steven Rostedt]

On Tue, 16 Aug 2016 17:20:25 -0700
Kees Cook  wrote:

> Right now, __list_add() code is repeated either in list.h or in
> list_debug.c, but only the debug checks are the different part. This
> extracts the checking into a separate function and consolidates
> __list_add(). Additionally this __list_add_debug() will stop list
> manipulations if a corruption is detected, instead of allowing for further
> corruption that may lead to even worse conditions.
> 
> This is slight refactoring of the same hardening done in PaX and Grsecurity.
> 
> Signed-off-by: Kees Cook 
> ---
>  include/linux/list.h | 22 --
>  lib/list_debug.c | 48 +++-
>  2 files changed, 39 insertions(+), 31 deletions(-)
> 
> diff --git a/include/linux/list.h b/include/linux/list.h
> index 5183138aa932..0ed58591538e 100644
> --- a/include/linux/list.h
> +++ b/include/linux/list.h
> @@ -28,27 +28,37 @@ static inline void INIT_LIST_HEAD(struct list_head *list)
>   list->prev = list;
>  }
>  
> +#ifdef CONFIG_DEBUG_LIST
> +extern bool __list_add_valid(struct list_head *new,
> +   struct list_head *prev,
> +   struct list_head *next);
> +#else
> +static inline bool __list_add_valid(struct list_head *new,
> + struct list_head *prev,
> + struct list_head *next)
> +{
> + return true;
> +}
> +#endif
> +
>  /*
>   * Insert a new entry between two known consecutive entries.
>   *
>   * This is only for internal list manipulation where we know
>   * the prev/next entries already!
>   */
> -#ifndef CONFIG_DEBUG_LIST
>  static inline void __list_add(struct list_head *new,
> struct list_head *prev,
> struct list_head *next)
>  {
> + if (!__list_add_valid(new, prev, next))
> + return;
> +
>   next->prev = new;
>   new->next = next;
>   new->prev = prev;
>   WRITE_ONCE(prev->next, new);
>  }
> -#else
> -extern void __list_add(struct list_head *new,
> -   struct list_head *prev,
> -   struct list_head *next);
> -#endif
>  
>  /**
>   * list_add - add a new entry
> diff --git a/lib/list_debug.c b/lib/list_debug.c
> index 3859bf63561c..149dd57b583b 100644
> --- a/lib/list_debug.c
> +++ b/lib/list_debug.c
> @@ -2,8 +2,7 @@
>   * Copyright 2006, Red Hat, Inc., Dave Jones
>   * Released under the General Public License (GPL).
>   *
> - * This file contains the linked list implementations for
> - * DEBUG_LIST.
> + * This file contains the linked list validation for DEBUG_LIST.
>   */
>  
>  #include 
> @@ -13,33 +12,32 @@
>  #include 
>  
>  /*
> - * Insert a new entry between two known consecutive entries.
> - *
> - * This is only for internal list manipulation where we know
> - * the prev/next entries already!
> + * Check that the data structures for the list manipulations are reasonably
> + * valid. Failures here indicate memory corruption (and possibly an exploit
> + * attempt).
>   */
>  
> -void __list_add(struct list_head *new,
> -   struct list_head *prev,
> -   struct list_head *next)
> +bool __list_add_valid(struct list_head *new, struct list_head *prev,
> +   struct list_head *next)
>  {
> - WARN(next->prev != prev,
> - "list_add corruption. next->prev should be "
> - "prev (%p), but was %p. (next=%p).\n",
> - prev, next->prev, next);
> - WARN(prev->next != next,
> - "list_add corruption. prev->next should be "
> - "next (%p), but was %p. (prev=%p).\n",
> - next, prev->next, prev);
> - WARN(new == prev || new == next,
> -  "list_add double add: new=%p, prev=%p, next=%p.\n",
> -  new, prev, next);
> - next->prev = new;
> - new->next = next;
> - new->prev = prev;
> - WRITE_ONCE(prev->next, 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);
> + return false;

BTW, WARN() does return the result, thus you could have just wrapped the
if () around them:

if (WARN(next->prev != prev,
"list_add corruption. next->prev should be prev (%p), 
but was %p. (next=%p).\n",
prev, next->prev, next))
return;

Just FYI.

-- Steve


> + }
> + 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);
> + return false;
> + }
> + if (unlikely(new == prev || new == next)) {
> + WARN(1, "list_add double add: new=%p, prev=%p, next=%p.\n",
> + 

Re: [PATCH v2 1/5] list: Split list_add() debug checking into separate function

[Steven Rostedt]

On Tue, 16 Aug 2016 17:20:25 -0700
Kees Cook  wrote:

> Right now, __list_add() code is repeated either in list.h or in
> list_debug.c, but only the debug checks are the different part. This
> extracts the checking into a separate function and consolidates
> __list_add(). Additionally this __list_add_debug() will stop list
> manipulations if a corruption is detected, instead of allowing for further
> corruption that may lead to even worse conditions.
> 
> This is slight refactoring of the same hardening done in PaX and Grsecurity.
> 
> Signed-off-by: Kees Cook 
> ---
>  include/linux/list.h | 22 --
>  lib/list_debug.c | 48 +++-
>  2 files changed, 39 insertions(+), 31 deletions(-)
> 
> diff --git a/include/linux/list.h b/include/linux/list.h
> index 5183138aa932..0ed58591538e 100644
> --- a/include/linux/list.h
> +++ b/include/linux/list.h
> @@ -28,27 +28,37 @@ static inline void INIT_LIST_HEAD(struct list_head *list)
>   list->prev = list;
>  }
>  
> +#ifdef CONFIG_DEBUG_LIST
> +extern bool __list_add_valid(struct list_head *new,
> +   struct list_head *prev,
> +   struct list_head *next);
> +#else
> +static inline bool __list_add_valid(struct list_head *new,
> + struct list_head *prev,
> + struct list_head *next)
> +{
> + return true;
> +}
> +#endif
> +
>  /*
>   * Insert a new entry between two known consecutive entries.
>   *
>   * This is only for internal list manipulation where we know
>   * the prev/next entries already!
>   */
> -#ifndef CONFIG_DEBUG_LIST
>  static inline void __list_add(struct list_head *new,
> struct list_head *prev,
> struct list_head *next)
>  {
> + if (!__list_add_valid(new, prev, next))
> + return;
> +
>   next->prev = new;
>   new->next = next;
>   new->prev = prev;
>   WRITE_ONCE(prev->next, new);
>  }
> -#else
> -extern void __list_add(struct list_head *new,
> -   struct list_head *prev,
> -   struct list_head *next);
> -#endif
>  
>  /**
>   * list_add - add a new entry
> diff --git a/lib/list_debug.c b/lib/list_debug.c
> index 3859bf63561c..149dd57b583b 100644
> --- a/lib/list_debug.c
> +++ b/lib/list_debug.c
> @@ -2,8 +2,7 @@
>   * Copyright 2006, Red Hat, Inc., Dave Jones
>   * Released under the General Public License (GPL).
>   *
> - * This file contains the linked list implementations for
> - * DEBUG_LIST.
> + * This file contains the linked list validation for DEBUG_LIST.
>   */
>  
>  #include 
> @@ -13,33 +12,32 @@
>  #include 
>  
>  /*
> - * Insert a new entry between two known consecutive entries.
> - *
> - * This is only for internal list manipulation where we know
> - * the prev/next entries already!
> + * Check that the data structures for the list manipulations are reasonably
> + * valid. Failures here indicate memory corruption (and possibly an exploit
> + * attempt).
>   */
>  
> -void __list_add(struct list_head *new,
> -   struct list_head *prev,
> -   struct list_head *next)
> +bool __list_add_valid(struct list_head *new, struct list_head *prev,
> +   struct list_head *next)
>  {
> - WARN(next->prev != prev,
> - "list_add corruption. next->prev should be "
> - "prev (%p), but was %p. (next=%p).\n",
> - prev, next->prev, next);
> - WARN(prev->next != next,
> - "list_add corruption. prev->next should be "
> - "next (%p), but was %p. (prev=%p).\n",
> - next, prev->next, prev);
> - WARN(new == prev || new == next,
> -  "list_add double add: new=%p, prev=%p, next=%p.\n",
> -  new, prev, next);
> - next->prev = new;
> - new->next = next;
> - new->prev = prev;
> - WRITE_ONCE(prev->next, 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);
> + return false;

BTW, WARN() does return the result, thus you could have just wrapped the
if () around them:

if (WARN(next->prev != prev,
"list_add corruption. next->prev should be prev (%p), 
but was %p. (next=%p).\n",
prev, next->prev, next))
return;

Just FYI.

-- Steve


> + }
> + 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);
> + return false;
> + }
> + if (unlikely(new == prev || new == next)) {
> + WARN(1, "list_add double add: new=%p, prev=%p, next=%p.\n",
> + new, prev, next);
> +