Re: [PATCH RFC v1 18/26] kmsan: mm: call KMSAN hooks from SLUB code

[Alexander Potapenko <glider@google.com>]

On Fri, Oct 18, 2019 at 3:22 PM Qian Cai <cai@lca.pw> wrote:
>
> On Fri, 2019-10-18 at 11:42 +0200, glider@google.com wrote:
> > In order to report uninitialized memory coming from heap allocations
> > KMSAN has to poison them unless they're created with __GFP_ZERO.
> >
> > It's handy that we need KMSAN hooks in the places where
> > init_on_alloc/init_on_free initialization is performed.
>
> Well, there is SLUB debug which has red zoning and poisoning checks. What's
> value of this patch?
Sorry, are you talking about the whole patchset or just this patch?
Note that SLUB debug is unable to detect uninitialized values with
bit-to-bit precision, neither have I heard of anyone using it for
detecting uses of uninitialized memory in the kernel at all.
The purpose of SLUB debug is to detect corruptions of freed memory.
> >
> > Signed-off-by: Alexander Potapenko <glider@google.com>
> > To: Alexander Potapenko <glider@google.com>
> > Cc: Andrew Morton <akpm@linux-foundation.org>
> > Cc: Vegard Nossum <vegard.nossum@oracle.com>
> > Cc: Dmitry Vyukov <dvyukov@google.com>
> > Cc: linux-mm@kvack.org
> > ---
> >
> > Change-Id: I51103b7981d3aabed747d0c85cbdc85568665871
> > ---
> >  mm/slub.c | 37 +++++++++++++++++++++++++++++++------
> >  1 file changed, 31 insertions(+), 6 deletions(-)
> >
> > diff --git a/mm/slub.c b/mm/slub.c
> > index 3d63ae320d31..3d6d4c63446e 100644
> > --- a/mm/slub.c
> > +++ b/mm/slub.c
> > @@ -21,6 +21,8 @@
> >  #include <linux/proc_fs.h>
> >  #include <linux/seq_file.h>
> >  #include <linux/kasan.h>
> > +#include <linux/kmsan.h>
> > +#include <linux/kmsan-checks.h> /* KMSAN_INIT_VALUE */
> >  #include <linux/cpu.h>
> >  #include <linux/cpuset.h>
> >  #include <linux/mempolicy.h>
> > @@ -285,17 +287,27 @@ static void prefetch_freepointer(const struct kmem_cache *s, void *object)
> >       prefetch(object + s->offset);
> >  }
> >
> > +/*
> > + * When running under KMSAN, get_freepointer_safe() may return an uninitialized
> > + * pointer value in the case the current thread loses the race for the next
> > + * memory chunk in the freelist. In that case this_cpu_cmpxchg_double() in
> > + * slab_alloc_node() will fail, so the uninitialized value won't be used, but
> > + * KMSAN will still check all arguments of cmpxchg because of imperfect
> > + * handling of inline assembly.
> > + * To work around this problem, use KMSAN_INIT_VALUE() to force initialize the
> > + * return value of get_freepointer_safe().
> > + */
> >  static inline void *get_freepointer_safe(struct kmem_cache *s, void *object)
> >  {
> >       unsigned long freepointer_addr;
> >       void *p;
> >
> >       if (!debug_pagealloc_enabled())
> > -             return get_freepointer(s, object);
> > +             return KMSAN_INIT_VALUE(get_freepointer(s, object));
> >
> >       freepointer_addr = (unsigned long)object + s->offset;
> >       probe_kernel_read(&p, (void **)freepointer_addr, sizeof(p));
> > -     return freelist_ptr(s, p, freepointer_addr);
> > +     return KMSAN_INIT_VALUE(freelist_ptr(s, p, freepointer_addr));
> >  }
> >
> >  static inline void set_freepointer(struct kmem_cache *s, void *object, void *fp)
> > @@ -1390,6 +1402,7 @@ static inline void *kmalloc_large_node_hook(void *ptr, size_t size, gfp_t flags)
> >       ptr = kasan_kmalloc_large(ptr, size, flags);
> >       /* As ptr might get tagged, call kmemleak hook after KASAN. */
> >       kmemleak_alloc(ptr, size, 1, flags);
> > +     kmsan_kmalloc_large(ptr, size, flags);
> >       return ptr;
> >  }
> >
> > @@ -1397,6 +1410,7 @@ static __always_inline void kfree_hook(void *x)
> >  {
> >       kmemleak_free(x);
> >       kasan_kfree_large(x, _RET_IP_);
> > +     kmsan_kfree_large(x);
> >  }
> >
> >  static __always_inline bool slab_free_hook(struct kmem_cache *s, void *x)
> > @@ -1453,6 +1467,12 @@ static inline bool slab_free_freelist_hook(struct kmem_cache *s,
> >               } while (object != old_tail);
> >       }
> >
> > +     do {
> > +             object = next;
> > +             next = get_freepointer(s, object);
> > +             kmsan_slab_free(s, object);
> > +     } while (object != old_tail);
> > +
> >  /*
> >   * Compiler cannot detect this function can be removed if slab_free_hook()
> >   * evaluates to nothing.  Thus, catch all relevant config debug options here.
> > @@ -2769,6 +2789,7 @@ static __always_inline void *slab_alloc_node(struct kmem_cache *s,
> >       if (unlikely(slab_want_init_on_alloc(gfpflags, s)) && object)
> >               memset(object, 0, s->object_size);
> >
> > +     kmsan_slab_alloc(s, object, gfpflags);
> >       slab_post_alloc_hook(s, gfpflags, 1, &object);
> >
> >       return object;
> > @@ -2797,6 +2818,7 @@ void *kmem_cache_alloc_trace(struct kmem_cache *s, gfp_t gfpflags, size_t size)
> >       void *ret = slab_alloc(s, gfpflags, _RET_IP_);
> >       trace_kmalloc(_RET_IP_, ret, size, s->size, gfpflags);
> >       ret = kasan_kmalloc(s, ret, size, gfpflags);
> > +
> >       return ret;
> >  }
> >  EXPORT_SYMBOL(kmem_cache_alloc_trace);
> > @@ -2809,7 +2831,6 @@ void *kmem_cache_alloc_node(struct kmem_cache *s, gfp_t gfpflags, int node)
> >
> >       trace_kmem_cache_alloc_node(_RET_IP_, ret,
> >                                   s->object_size, s->size, gfpflags, node);
> > -
> >       return ret;
> >  }
> >  EXPORT_SYMBOL(kmem_cache_alloc_node);
> > @@ -2825,6 +2846,7 @@ void *kmem_cache_alloc_node_trace(struct kmem_cache *s,
> >                          size, s->size, gfpflags, node);
> >
> >       ret = kasan_kmalloc(s, ret, size, gfpflags);
> > +
> >       return ret;
> >  }
> >  EXPORT_SYMBOL(kmem_cache_alloc_node_trace);
> > @@ -3150,7 +3172,7 @@ int kmem_cache_alloc_bulk(struct kmem_cache *s, gfp_t flags, size_t size,
> >                         void **p)
> >  {
> >       struct kmem_cache_cpu *c;
> > -     int i;
> > +     int i, j;
> >
> >       /* memcg and kmem_cache debug support */
> >       s = slab_pre_alloc_hook(s, flags);
> > @@ -3188,11 +3210,11 @@ int kmem_cache_alloc_bulk(struct kmem_cache *s, gfp_t flags, size_t size,
> >
> >       /* Clear memory outside IRQ disabled fastpath loop */
> >       if (unlikely(slab_want_init_on_alloc(flags, s))) {
> > -             int j;
> > -
> >               for (j = 0; j < i; j++)
> >                       memset(p[j], 0, s->object_size);
> >       }
> > +     for (j = 0; j < i; j++)
> > +             kmsan_slab_alloc(s, p[j], flags);
> >
> >       /* memcg and kmem_cache debug support */
> >       slab_post_alloc_hook(s, flags, size, p);
> > @@ -3793,6 +3815,7 @@ static int __init setup_slub_min_objects(char *str)
> >
> >  __setup("slub_min_objects=", setup_slub_min_objects);
> >
> > +__no_sanitize_memory
> >  void *__kmalloc(size_t size, gfp_t flags)
> >  {
> >       struct kmem_cache *s;
> > @@ -5698,6 +5721,7 @@ static char *create_unique_id(struct kmem_cache *s)
> >       p += sprintf(p, "%07u", s->size);
> >
> >       BUG_ON(p > name + ID_STR_LENGTH - 1);
> > +     kmsan_unpoison_shadow(name, p - name);
> >       return name;
> >  }
> >
> > @@ -5847,6 +5871,7 @@ static int sysfs_slab_alias(struct kmem_cache *s, const char *name)
> >       al->name = name;
> >       al->next = alias_list;
> >       alias_list = al;
> > +     kmsan_unpoison_shadow(al, sizeof(struct saved_alias));
> >       return 0;
> >  }
> >



-- 
Alexander Potapenko
Software Engineer

Google Germany GmbH
Erika-Mann-Straße, 33
80636 München

Geschäftsführer: Paul Manicle, Halimah DeLaine Prado
Registergericht und -nummer: Hamburg, HRB 86891
Sitz der Gesellschaft: Hamburg