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    SubjectRe: [PATCH v2 11/11] protect architectures where THREAD_SIZE >= PAGE_SIZE against fork bombs
    On Thu 09-08-12 17:01:19, 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.

    quick git grep "define *THREAD_SIZE\>" arch says that there is no such
    architecture.

    > 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 :(){ :|:& };:

    I guess there were no other tasks in the same group (except for the
    parent shell), right? I am asking because this should trigger memcg-oom
    but that one will usually pick up something else than the fork bomb
    which would have a small memory footprint. But that needs to be handled
    on the oom level obviously.

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

    Reviewed-by: Michal Hocko <mhocko@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 dc3ff16..b0b90c3 100644
    > --- a/kernel/fork.c
    > +++ b/kernel/fork.c
    > @@ -142,7 +142,7 @@ 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;
    > @@ -151,7 +151,7 @@ static struct thread_info *alloc_thread_info_node(struct task_struct *tsk,
    > static inline void free_thread_info(struct thread_info *ti)
    > {
    > arch_release_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;
    > --
    > 1.7.11.2
    >
    > --
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    --
    Michal Hocko
    SUSE Labs


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