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Date: Tue, 10 May 2016 19:17:50 +0200 From: Alexander Potapenko <glider@...gle.com> To: Andrey Ryabinin <ryabinin.a.a@...il.com> Cc: Andrey Konovalov <adech.fo@...il.com>, Christoph Lameter <cl@...ux.com>, Dmitry Vyukov <dvyukov@...gle.com>, Andrew Morton <akpm@...ux-foundation.org>, Steven Rostedt <rostedt@...dmis.org>, Joonsoo Kim <iamjoonsoo.kim@....com>, Joonsoo Kim <js1304@...il.com>, Kostya Serebryany <kcc@...gle.com>, kasan-dev <kasan-dev@...glegroups.com>, LKML <linux-kernel@...r.kernel.org>, "linux-mm@...ck.org" <linux-mm@...ck.org> Subject: Re: [PATCH v8 7/7] mm: kasan: Initial memory quarantine implementation On Tue, May 10, 2016 at 5:39 PM, Andrey Ryabinin <ryabinin.a.a@...il.com> wrote: > 2016-03-15 13:10 GMT+03:00 Alexander Potapenko <glider@...gle.com>: > >> >> static inline int kasan_module_alloc(void *addr, size_t size) { return 0; } >> static inline void kasan_free_shadow(const struct vm_struct *vm) {} >> diff --git a/lib/test_kasan.c b/lib/test_kasan.c >> index 82169fb..799c98e 100644 >> --- a/lib/test_kasan.c >> +++ b/lib/test_kasan.c >> @@ -344,6 +344,32 @@ static noinline void __init kasan_stack_oob(void) >> *(volatile char *)p; >> } >> >> +#ifdef CONFIG_SLAB >> +static noinline void __init kasan_quarantine_cache(void) >> +{ >> + struct kmem_cache *cache = kmem_cache_create( >> + "test", 137, 8, GFP_KERNEL, NULL); >> + int i; >> + >> + for (i = 0; i < 100; i++) { >> + void *p = kmem_cache_alloc(cache, GFP_KERNEL); >> + >> + kmem_cache_free(cache, p); >> + p = kmalloc(sizeof(u64), GFP_KERNEL); >> + kfree(p); >> + } >> + kmem_cache_shrink(cache); >> + for (i = 0; i < 100; i++) { >> + u64 *p = kmem_cache_alloc(cache, GFP_KERNEL); >> + >> + kmem_cache_free(cache, p); >> + p = kmalloc(sizeof(u64), GFP_KERNEL); >> + kfree(p); >> + } >> + kmem_cache_destroy(cache); >> +} >> +#endif >> + > > Test looks quite useless. The kernel does allocations/frees all the > time, so I don't think that this test > adds something valuable. Agreed. > And what's the result that we expect from this test? No crashes? > I'm thinking it would better to remove it. Do you think it may make sense to improve it by introducing an actual use-after-free? Or perhaps we could insert a loop doing 1000 kmalloc()/kfree() calls into the existing UAF tests. > [...] > >> + >> +/* smp_load_acquire() here pairs with smp_store_release() in >> + * quarantine_reduce(). >> + */ >> +#define QUARANTINE_LOW_SIZE (smp_load_acquire(&quarantine_size) * 3 / 4) > > I'd prefer open coding barrier with a proper comment int place, > instead of sneaking it into macros. Ack. > [...] > >> + >> +void quarantine_reduce(void) >> +{ >> + size_t new_quarantine_size; >> + unsigned long flags; >> + struct qlist to_free = QLIST_INIT; >> + size_t size_to_free = 0; >> + void **last; >> + >> + /* smp_load_acquire() here pairs with smp_store_release() below. */ > > Besides pairing rules, the comment should also explain *why* we need > this and for what > load/stores it provides memory ordering guarantees. For example take a > look at other > comments near barriers in the kernel tree. Something along the lines of "We must load A before B, hence the barrier"? >> + if (likely(ACCESS_ONCE(global_quarantine.bytes) <= >> + smp_load_acquire(&quarantine_size))) >> + return; >> + >> -- Alexander Potapenko Software Engineer Google Germany GmbH Erika-Mann-Straße, 33 80636 München Geschäftsführer: Matthew Scott Sucherman, Paul Terence Manicle Registergericht und -nummer: Hamburg, HRB 86891 Sitz der Gesellschaft: Hamburg
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