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    SubjectRe: [PATCH v11 19/19] kasan: enable instrumentation of global variables
    On 02/16/2015 05:58 AM, Rusty Russell wrote:
    > Andrey Ryabinin <a.ryabinin@samsung.com> writes:
    >> This feature let us to detect accesses out of bounds of
    >> global variables. This will work as for globals in kernel
    >> image, so for globals in modules. Currently this won't work
    >> for symbols in user-specified sections (e.g. __init, __read_mostly, ...)
    >>
    >> The idea of this is simple. Compiler increases each global variable
    >> by redzone size and add constructors invoking __asan_register_globals()
    >> function. Information about global variable (address, size,
    >> size with redzone ...) passed to __asan_register_globals() so we could
    >> poison variable's redzone.
    >>
    >> This patch also forces module_alloc() to return 8*PAGE_SIZE aligned
    >> address making shadow memory handling ( kasan_module_alloc()/kasan_module_free() )
    >> more simple. Such alignment guarantees that each shadow page backing
    >> modules address space correspond to only one module_alloc() allocation.
    >
    > Hmm, I understand why you only fixed x86, but it's weird.
    >
    > I think MODULE_ALIGN belongs in linux/moduleloader.h, and every arch
    > should be fixed up to use it (though you could leave that for later).
    >
    > Might as well fix the default implementation at least.
    >
    >> @@ -49,8 +49,15 @@ void kasan_krealloc(const void *object, size_t new_size);
    >> void kasan_slab_alloc(struct kmem_cache *s, void *object);
    >> void kasan_slab_free(struct kmem_cache *s, void *object);
    >>
    >> +#define MODULE_ALIGN (PAGE_SIZE << KASAN_SHADOW_SCALE_SHIFT)
    >> +
    >> +int kasan_module_alloc(void *addr, size_t size);
    >> +void kasan_module_free(void *addr);
    >> +
    >> #else /* CONFIG_KASAN */
    >>
    >> +#define MODULE_ALIGN 1
    >
    > Hmm, that should be PAGE_SIZE (we assume that in several places).
    >
    >> @@ -1807,6 +1808,7 @@ static void unset_module_init_ro_nx(struct module *mod) { }
    >> void __weak module_memfree(void *module_region)
    >> {
    >> vfree(module_region);
    >> + kasan_module_free(module_region);
    >> }
    >
    > This looks racy (memory reuse?). Perhaps try other order?
    >

    You are right, it's racy. Concurrent kasan_module_alloc() could fail because
    kasan_module_free() wasn't called/finished yet, so whole module_alloc() will fail
    and module loading will fail.
    However, I just find out that this race is not the worst problem here.
    When vfree(addr) called in interrupt context, memory at addr will be reused for
    storing 'struct llist_node':

    void vfree(const void *addr)
    {
    ...
    if (unlikely(in_interrupt())) {
    struct vfree_deferred *p = this_cpu_ptr(&vfree_deferred);
    if (llist_add((struct llist_node *)addr, &p->list))
    schedule_work(&p->wq);


    In this case we have to free shadow *after* freeing 'module_region', because 'module_region'
    is still used in llist_add() and in free_work() latter.
    free_work() (in mm/vmalloc.c) processes list in LIFO order, so to free shadow after freeing
    'module_region' kasan_module_free(module_region); should be called before vfree(module_region);

    It will be racy still, but this is not so bad as potential crash that we have now.
    Honestly, I have no idea how to fix this race nicely. Any suggestions?





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