[Devel] Re: [PATCH v3 13/13] protect architectures where THREAD_SIZE >= PAGE_SIZE against fork bombs

[Michal Hocko]

On Tue 18-09-12 18:04:10, Glauber Costa wrote:
> Because those architectures will draw their stacks directly from the
> page allocator, rather than the slab cache, we can directly pass
> __GFP_KMEMCG flag, and issue the corresponding free_pages.
> 
> This code path is taken when the architecture doesn't define
> CONFIG_ARCH_THREAD_INFO_ALLOCATOR (only ia64 seems to), and has
> THREAD_SIZE >= PAGE_SIZE. Luckily, most - if not all - of the remaining
> architectures fall in this category.
> 
> This will guarantee that every stack page is accounted to the memcg the
> process currently lives on, and will have the allocations to fail if
> they go over limit.
> 
> For the time being, I am defining a new variant of THREADINFO_GFP, not
> to mess with the other path. Once the slab is also tracked by memcg, we
> can get rid of that flag.
> 
> Tested to successfully protect against :(){ :|:& };:

OK. Although I was complaining that this is not the full truth the last
time, I do not insist on gravy details about the slaughter this will
cause to the rest of the group and that who-ever could fork in the group
can easily DOS the whole hierarchy. It has some interesting side effects
as well but let's keep this to a careful reader ;)

The patch, as is, is still useful and an improvement because it reduces
the impact.

> 
> Signed-off-by: Glauber Costa <glommer at parallels.com>
> Acked-by: Frederic Weisbecker <fweisbec at redhat.com>
> Acked-by: Kamezawa Hiroyuki <kamezawa.hiroyu at jp.fujitsu.com>
> CC: Christoph Lameter <cl at linux.com>
> CC: Pekka Enberg <penberg at cs.helsinki.fi>
> CC: Michal Hocko <mhocko at suse.cz>
> CC: Johannes Weiner <hannes at cmpxchg.org>
> CC: Suleiman Souhlal <suleiman at google.com>

Reviewed-by: Michal Hocko <mhocko at suse.cz>

> ---
>  include/linux/thread_info.h | 2 ++
>  kernel/fork.c               | 4 ++--
>  2 files changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/thread_info.h b/include/linux/thread_info.h
> index ccc1899..e7e0473 100644
> --- a/include/linux/thread_info.h
> +++ b/include/linux/thread_info.h
> @@ -61,6 +61,8 @@ extern long do_no_restart_syscall(struct restart_block *parm);
>  # define THREADINFO_GFP		(GFP_KERNEL | __GFP_NOTRACK)
>  #endif
>  
> +#define THREADINFO_GFP_ACCOUNTED (THREADINFO_GFP | __GFP_KMEMCG)
> +
>  /*
>   * flag set/clear/test wrappers
>   * - pass TIF_xxxx constants to these functions
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 0ff2bf7..897e89c 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -146,7 +146,7 @@ void __weak arch_release_thread_info(struct thread_info *ti)
>  static struct thread_info *alloc_thread_info_node(struct task_struct *tsk,
>  						  int node)
>  {
> -	struct page *page = alloc_pages_node(node, THREADINFO_GFP,
> +	struct page *page = alloc_pages_node(node, THREADINFO_GFP_ACCOUNTED,
>  					     THREAD_SIZE_ORDER);
>  
>  	return page ? page_address(page) : NULL;
> @@ -154,7 +154,7 @@ static struct thread_info *alloc_thread_info_node(struct task_struct *tsk,
>  
>  static inline void free_thread_info(struct thread_info *ti)
>  {
> -	free_pages((unsigned long)ti, THREAD_SIZE_ORDER);
> +	free_accounted_pages((unsigned long)ti, THREAD_SIZE_ORDER);
>  }
>  # else
>  static struct kmem_cache *thread_info_cache;
> -- 
> 1.7.11.4
> 
> --
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-- 
Michal Hocko
SUSE Labs