On Tue, 26 Apr 2016 09:21:10 -0700 Thomas Garnier <thgarnie@xxxxxxxxxx> wrote: > Provides an optional config (CONFIG_FREELIST_RANDOM) to randomize the > SLAB freelist. The list is randomized during initialization of a new set > of pages. The order on different freelist sizes is pre-computed at boot > for performance. Each kmem_cache has its own randomized freelist. Before > pre-computed lists are available freelists are generated > dynamically. This security feature reduces the predictability of the > kernel SLAB allocator against heap overflows rendering attacks much less > stable. > > For example this attack against SLUB (also applicable against SLAB) > would be affected: > https://jon.oberheide.org/blog/2010/09/10/linux-kernel-can-slub-overflow/ > > Also, since v4.6 the freelist was moved at the end of the SLAB. It means > a controllable heap is opened to new attacks not yet publicly discussed. > A kernel heap overflow can be transformed to multiple use-after-free. > This feature makes this type of attack harder too. > > To generate entropy, we use get_random_bytes_arch because 0 bits of > entropy is available in the boot stage. In the worse case this function > will fallback to the get_random_bytes sub API. We also generate a shift > random number to shift pre-computed freelist for each new set of pages. > > The config option name is not specific to the SLAB as this approach will > be extended to other allocators like SLUB. > > Performance results highlighted no major changes: > > Hackbench (running 90 10 times): > > Before average: 0.0698 > After average: 0.0663 (-5.01%) > > slab_test 1 run on boot. Difference only seen on the 2048 size test > being the worse case scenario covered by freelist randomization. New > slab pages are constantly being created on the 10000 allocations. > Variance should be mainly due to getting new pages every few > allocations. > > ... > > --- a/include/linux/slab_def.h > +++ b/include/linux/slab_def.h > @@ -80,6 +80,10 @@ struct kmem_cache { > struct kasan_cache kasan_info; > #endif > > +#ifdef CONFIG_FREELIST_RANDOM > + void *random_seq; > +#endif > + > struct kmem_cache_node *node[MAX_NUMNODES]; > }; > > diff --git a/init/Kconfig b/init/Kconfig > index 0c66640..73453d0 100644 > --- a/init/Kconfig > +++ b/init/Kconfig > @@ -1742,6 +1742,15 @@ config SLOB > > endchoice > > +config FREELIST_RANDOM > + default n > + depends on SLAB > + bool "SLAB freelist randomization" > + help > + Randomizes the freelist order used on creating new SLABs. This > + security feature reduces the predictability of the kernel slab > + allocator against heap overflows. Against the v2 patch I didst observe: : CONFIG_FREELIST_RANDOM bugs me a bit - "freelist" is so vague. : CONFIG_SLAB_FREELIST_RANDOM would be better. I mean, what Kconfig : identifier could be used for implementing randomisation in : slub/slob/etc once CONFIG_FREELIST_RANDOM is used up? but this pearl appeared to pass unnoticed. > config SLUB_CPU_PARTIAL > default y > depends on SLUB && SMP > diff --git a/mm/slab.c b/mm/slab.c > index b82ee6b..0ed728a 100644 > --- a/mm/slab.c > +++ b/mm/slab.c > @@ -1230,6 +1230,61 @@ static void __init set_up_node(struct kmem_cache *cachep, int index) > } > } > > +#ifdef CONFIG_FREELIST_RANDOM > +static void freelist_randomize(struct rnd_state *state, freelist_idx_t *list, > + size_t count) > +{ > + size_t i; > + unsigned int rand; > + > + for (i = 0; i < count; i++) > + list[i] = i; > + > + /* Fisher-Yates shuffle */ > + for (i = count - 1; i > 0; i--) { > + rand = prandom_u32_state(state); > + rand %= (i + 1); > + swap(list[i], list[rand]); > + } > +} > + > +/* Create a random sequence per cache */ > +static int cache_random_seq_create(struct kmem_cache *cachep) > +{ > + unsigned int seed, count = cachep->num; > + struct rnd_state state; > + > + if (count < 2) > + return 0; > + > + /* If it fails, we will just use the global lists */ > + cachep->random_seq = kcalloc(count, sizeof(freelist_idx_t), GFP_KERNEL); > + if (!cachep->random_seq) > + return -ENOMEM; OK, no BUG. If this happens, kmem_cache_init_late() will go BUG instead ;) Questions for slab maintainers: What's going on with the gfp_flags in there? kmem_cache_init_late() passes GFP_NOWAIT into enable_cpucache(). a) why the heck does it do that? It's __init code! b) if there's a legit reason then your new cache_random_seq_create() should be getting its gfp_t from its caller, rather than blindly assuming GFP_KERNEL. c) kmem_cache_init_late() goes BUG on ENOMEM. Generally that's OK in __init code: we assume infinite memory during bootup. But it's really quite weird to use GFP_NOWAIT and then to go BUG if GFP_NOWAIT had its predictable outcome (ie: failure). Finally, all callers of enable_cpucache() (and hence of cache_random_seq_create()) are __init, so we're unnecessarily bloating up vmlinux. Could someone please take a look at this as a separate thing? > + /* Get best entropy at this stage */ > + get_random_bytes_arch(&seed, sizeof(seed)); > + prandom_seed_state(&state, seed); > + > + freelist_randomize(&state, cachep->random_seq, count); > + return 0; > +} > + > > ... > -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>