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    Subject[PATCH v3 13/13] protect architectures where THREAD_SIZE >= PAGE_SIZE against fork bombs
    Date
    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 :(){ :|:& };:

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