On Fri, 2019-10-18 at 15:55 +0200, Alexander Potapenko wrote:
> On Fri, Oct 18, 2019 at 3:42 PM Qian Cai <cai@xxxxxx> wrote:
> >
> > On Fri, 2019-10-18 at 15:33 +0200, Alexander Potapenko wrote:
> > > On Fri, Oct 18, 2019 at 3:22 PM Qian Cai <cai@xxxxxx> wrote:
> > > >
> > > > On Fri, 2019-10-18 at 11:42 +0200, glider@xxxxxxxxxx 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?
> >
> > 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.
> >
> > The point is if developers have SLUB debug enabled, all the free objects will be
> > poisoned, so what's the point of checking uninitialized memory there?
>
> You are right, SLUB debug has to be handled separately. If I'm
> understanding correctly, right now KMSAN poisons freed memory before
> SLUB debug wipes it, therefore the memory will count as initialized.
> On the other hand, newly allocated memory is still marked as
> uninitialized, so a lot of bugs still remain detectable.
Yes, but newly allocated slub objects will be poisoned as well.
> TBH, I haven't tested KMSAN with SLUB debug good enough. Note that
> it's anyway a separate build that requires Clang, so right now it
> doesn't make much sense to combine KMSAN and SLUB debug together.
Can't you just build a debug kernel with SLUB debug enabled but dropping this
patch? If there is an uninitialized memory here leading to data corruption, SLUB
debug should be detected as well as this patch. If not, it needs to understand
why.
> > > > >
> > > > > Signed-off-by: Alexander Potapenko <glider@xxxxxxxxxx>
> > > > > To: Alexander Potapenko <glider@xxxxxxxxxx>
> > > > > Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> > > > > Cc: Vegard Nossum <vegard.nossum@xxxxxxxxxx>
> > > > > Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
> > > > > Cc: linux-mm@xxxxxxxxx
> > > > > ---
> > > > >
> > > > > 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;
> > > > > }
> > > > >
> > >
> > >
> > >
>
>
>
