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stronger encoding of free lists using two keys per page

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This commit is contained in:
daan 2019-12-27 23:33:50 -08:00
parent ce02986d56
commit e3391d9a53
8 changed files with 83 additions and 50 deletions
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58
include/mimalloc-internal.h
View file

@ -392,12 +392,28 @@ static inline void mi_page_set_has_aligned(mi_page_t* page, bool has_aligned) {
}
// -------------------------------------------------------------------
// Encoding/Decoding the free list next pointers
// Note: we pass a `null` value to be used as the `NULL` value for the
// end of a free list. This is to prevent the cookie itself to ever
// be present among user blocks (as `cookie^0==cookie`).
// -------------------------------------------------------------------
/* -------------------------------------------------------------------
Encoding/Decoding the free list next pointers
This is to protect against buffer overflow exploits where the
free list is mutated. Many hardened allocators xor the next pointer `p`
with a secret key `k1`, as `p^k1`, but if the attacker can guess
the pointer `p` this can reveal `k1` (since `p^k1^p == k1`).
Moreover, if multiple blocks can be read, the attacker can
xor both as `(p1^k1) ^ (p2^k1) == p1^p2` which may reveal a lot
about the pointers (and subsequently `k1`).
Instead mimalloc uses an extra key `k2` and encode as `rotl(p+k2,13)^k1`.
Since these operations are not associative, the above approaches do not
work so well any more even if the `p` can be guesstimated. (We include
the rotation since xor and addition are otherwise linear in the lowest bit)
Both keys are unique per page.
We also pass a separate `null` value to be used as `NULL` or otherwise
`rotl(k2,13)^k1` would appear (too) often as a sentinel value.
------------------------------------------------------------------- */
#define MI_ENCODE_ROTATE_BITS (13)
static inline bool mi_is_in_same_segment(const void* p, const void* q) {
return (_mi_ptr_segment(p) == _mi_ptr_segment(q));
@ -412,49 +428,55 @@ static inline bool mi_is_in_same_page(const void* p, const void* q) {
return (idxp == idxq);
}
static inline mi_block_t* mi_block_nextx( const void* null, const mi_block_t* block, uintptr_t cookie ) {
static inline uintptr_t mi_rotl(uintptr_t x, uintptr_t shift) {
return ((x << shift) | (x >> (MI_INTPTR_BITS - shift)));
}
static inline uintptr_t mi_rotr(uintptr_t x, uintptr_t shift) {
return ((x >> shift) | (x << (MI_INTPTR_BITS - shift)));
}
static inline mi_block_t* mi_block_nextx( const void* null, const mi_block_t* block, uintptr_t key1, uintptr_t key2 ) {
#ifdef MI_ENCODE_FREELIST
mi_block_t* b = (mi_block_t*)(block->next ^ cookie);
mi_block_t* b = (mi_block_t*)(mi_rotr(block->next ^ key1, MI_ENCODE_ROTATE_BITS) - key2);
if (mi_unlikely((void*)b==null)) { b = NULL; }
return b;
#else
UNUSED(cookie); UNUSED(null);
UNUSED(key1); UNUSED(key2); UNUSED(null);
return (mi_block_t*)block->next;
#endif
}
static inline void mi_block_set_nextx(const void* null, mi_block_t* block, const mi_block_t* next, uintptr_t cookie) {
static inline void mi_block_set_nextx(const void* null, mi_block_t* block, const mi_block_t* next, uintptr_t key1, uintptr_t key2) {
#ifdef MI_ENCODE_FREELIST
if (mi_unlikely(next==NULL)) { next = (mi_block_t*)null; }
block->next = (mi_encoded_t)next ^ cookie;
block->next = mi_rotl((mi_encoded_t)next + key2, MI_ENCODE_ROTATE_BITS) ^ key1;
#else
UNUSED(cookie); UNUSED(null);
UNUSED(key1); UNUSED(key2); UNUSED(null);
block->next = (mi_encoded_t)next;
#endif
}
static inline mi_block_t* mi_block_next(const mi_page_t* page, const mi_block_t* block) {
#ifdef MI_ENCODE_FREELIST
mi_block_t* next = mi_block_nextx(page,block,page->cookie);
// check for free list corruption: is `next` at least in our segment range?
mi_block_t* next = mi_block_nextx(page,block,page->key[0],page->key[1]);
// check for free list corruption: is `next` at least in the same page?
// TODO: check if `next` is `page->block_size` aligned?
if (next!=NULL && !mi_is_in_same_page(block, next)) {
if (mi_unlikely(next!=NULL && !mi_is_in_same_page(block, next))) {
_mi_fatal_error("corrupted free list entry of size %zub at %p: value 0x%zx\n", page->block_size, block, (uintptr_t)next);
next = NULL;
}
return next;
#else
UNUSED(page);
return mi_block_nextx(page,block,0);
return mi_block_nextx(page,block,0,0);
#endif
}
static inline void mi_block_set_next(const mi_page_t* page, mi_block_t* block, const mi_block_t* next) {
#ifdef MI_ENCODE_FREELIST
mi_block_set_nextx(page,block,next, page->cookie);
mi_block_set_nextx(page,block,next, page->key[0], page->key[1]);
#else
UNUSED(page);
mi_block_set_nextx(page,block, next,0);
mi_block_set_nextx(page,block, next,0,0);
#endif
}

17
include/mimalloc-types.h
View file

@ -191,7 +191,7 @@ typedef struct mi_page_s {
mi_block_t* free; // list of available free blocks (`malloc` allocates from this list)
#ifdef MI_ENCODE_FREELIST
uintptr_t cookie; // random cookie to encode the free lists
uintptr_t key[2]; // two random keys to encode the free lists (see `_mi_block_next`)
#endif
size_t used; // number of blocks in use (including blocks in `local_free` and `thread_free`)
@ -206,9 +206,9 @@ typedef struct mi_page_s {
struct mi_page_s* prev; // previous page owned by this thread with the same `block_size`
// improve page index calculation
// without padding: 10 words on 64-bit, 11 on 32-bit. Secure adds one word
#if (MI_INTPTR_SIZE==8 && defined(MI_ENCODE_FREELIST)) || (MI_INTPTR_SIZE==4 && !defined(MI_ENCODE_FREELIST))
void* padding[1]; // 12 words on 64-bit with cookie, 12 words on 32-bit plain
// without padding: 10 words on 64-bit, 11 on 32-bit. Secure adds two words
#if (MI_INTPTR_SIZE==4)
void* padding[1]; // 12/14 words on 32-bit plain
#endif
} mi_page_t;
@ -239,8 +239,8 @@ typedef struct mi_segment_s {
size_t capacity; // count of available pages (`#free + used`)
size_t segment_size;// for huge pages this may be different from `MI_SEGMENT_SIZE`
size_t segment_info_size; // space we are using from the first page for segment meta-data and possible guard pages.
uintptr_t cookie; // verify addresses in debug mode: `mi_ptr_cookie(segment) == segment->cookie`
uintptr_t cookie; // verify addresses in secure mode: `_mi_ptr_cookie(segment) == segment->cookie`
// layout like this to optimize access in `mi_free`
size_t page_shift; // `1 << page_shift` == the page sizes == `page->block_size * page->reserved` (unless the first page, then `-segment_info_size`).
volatile _Atomic(uintptr_t) thread_id; // unique id of the thread owning this segment
@ -289,8 +289,9 @@ struct mi_heap_s {
mi_page_queue_t pages[MI_BIN_FULL + 1]; // queue of pages for each size class (or "bin")
volatile _Atomic(mi_block_t*) thread_delayed_free;
uintptr_t thread_id; // thread this heap belongs too
uintptr_t cookie;
mi_random_ctx_t random; // random number used for secure allocation
uintptr_t cookie; // random cookie to verify pointers (see `_mi_ptr_cookie`)
uintptr_t key[2]; // twb random keys used to encode the `thread_delayed_free` list
mi_random_ctx_t random; // random number context used for secure allocation
size_t page_count; // total number of pages in the `pages` queues.
bool no_reclaim; // `true` if this heap should not reclaim abandoned pages
};

8
src/alloc.c
View file

@ -157,7 +157,7 @@ static mi_decl_noinline bool mi_check_is_double_freex(const mi_page_t* page, con
}
static inline bool mi_check_is_double_free(const mi_page_t* page, const mi_block_t* block) {
mi_block_t* n = mi_block_nextx(page, block, page->cookie); // pretend it is freed, and get the decoded first field
mi_block_t* n = mi_block_nextx(page, block, page->key[0], page->key[1]); // pretend it is freed, and get the decoded first field
if (((uintptr_t)n & (MI_INTPTR_SIZE-1))==0 && // quick check: aligned pointer?
(n==NULL || mi_is_in_same_segment(block, n))) // quick check: in same segment or NULL?
{
@ -242,7 +242,7 @@ static mi_decl_noinline void _mi_free_block_mt(mi_page_t* page, mi_block_t* bloc
mi_block_t* dfree;
do {
dfree = (mi_block_t*)heap->thread_delayed_free;
mi_block_set_nextx(heap,block,dfree, heap->cookie);
mi_block_set_nextx(heap,block,dfree, heap->key[0], heap->key[1]);
} while (!mi_atomic_cas_ptr_weak(mi_atomic_cast(void*,&heap->thread_delayed_free), block, dfree));
}
@ -266,7 +266,7 @@ static inline void _mi_free_block(mi_page_t* page, bool local, mi_block_t* block
// and push it on the free list
if (mi_likely(local)) {
// owning thread can free a block directly
if (mi_check_is_double_free(page, block)) return;
if (mi_unlikely(mi_check_is_double_free(page, block))) return;
mi_block_set_next(page, block, page->local_free);
page->local_free = block;
page->used--;
@ -341,7 +341,7 @@ void mi_free(void* p) mi_attr_noexcept
if (mi_likely(tid == segment->thread_id && page->flags.full_aligned == 0)) { // the thread id matches and it is not a full page, nor has aligned blocks
// local, and not full or aligned
mi_block_t* block = (mi_block_t*)p;
if (mi_check_is_double_free(page,block)) return;
if (mi_unlikely(mi_check_is_double_free(page,block))) return;
mi_block_set_next(page, block, page->local_free);
page->local_free = block;
page->used--;

2
src/heap.c
View file

@ -193,6 +193,8 @@ mi_heap_t* mi_heap_new(void) {
heap->thread_id = _mi_thread_id();
_mi_random_split(&bheap->random, &heap->random);
heap->cookie = _mi_heap_random_next(heap) | 1;
heap->key[0] = _mi_heap_random_next(heap);
heap->key[1] = _mi_heap_random_next(heap);
heap->no_reclaim = true; // don't reclaim abandoned pages or otherwise destroy is unsafe
return heap;
}

30
src/init.c
View file

@ -16,13 +16,13 @@ const mi_page_t _mi_page_empty = {
{ 0 }, false,
NULL, // free
#if MI_ENCODE_FREELIST
0,
{ 0, 0 },
#endif
0, // used
NULL,
ATOMIC_VAR_INIT(0), ATOMIC_VAR_INIT(0),
0, NULL, NULL, NULL
#if (MI_INTPTR_SIZE==8 && defined(MI_ENCODE_FREELIST)) || (MI_INTPTR_SIZE==4 && !defined(MI_ENCODE_FREELIST))
#if (MI_INTPTR_SIZE==4)
, { NULL } // padding
#endif
};
@ -83,8 +83,9 @@ const mi_heap_t _mi_heap_empty = {
MI_SMALL_PAGES_EMPTY,
MI_PAGE_QUEUES_EMPTY,
ATOMIC_VAR_INIT(NULL),
0,
0,
0, // tid
0, // cookie
{ 0, 0 }, // keys
{ {0}, {0}, 0 },
0,
false
@ -105,18 +106,21 @@ static mi_tld_t tld_main = {
{ MI_STATS_NULL } // stats
};
#if MI_INTPTR_SIZE==8
#define MI_INIT_COOKIE (0xCDCDCDCDCDCDCDCDUL)
#else
#define MI_INIT_COOKIE (0xCDCDCDCDUL)
#endif
mi_heap_t _mi_heap_main = {
&tld_main,
MI_SMALL_PAGES_EMPTY,
MI_PAGE_QUEUES_EMPTY,
NULL,
0, // thread id
#if MI_INTPTR_SIZE==8 // the cookie of the main heap can be fixed (unlike page cookies that need to be secure!)
0xCDCDCDCDCDCDCDCDUL,
#else
0xCDCDCDCDUL,
#endif
{ {0}, {0}, 0 }, // random
0, // thread id
MI_INIT_COOKIE, // initial cookie
{ MI_INIT_COOKIE, MI_INIT_COOKIE }, // the key of the main heap can be fixed (unlike page keys that need to be secure!)
{ {0}, {0}, 0 }, // random
0, // page count
false // can reclaim
};
@ -156,6 +160,8 @@ static bool _mi_heap_init(void) {
heap->thread_id = _mi_thread_id();
_mi_random_init(&heap->random);
heap->cookie = _mi_heap_random_next(heap) | 1;
heap->key[0] = _mi_heap_random_next(heap);
heap->key[1] = _mi_heap_random_next(heap);
heap->tld = tld;
memset(tld, 0, sizeof(*tld));
tld->heap_backing = heap;
@ -399,6 +405,8 @@ void mi_process_init(void) mi_attr_noexcept {
_mi_random_init(&_mi_heap_main.random);
#ifndef __APPLE__ // TODO: fix this? cannot update cookie if allocation already happened..
_mi_heap_main.cookie = _mi_heap_random_next(&_mi_heap_main);
_mi_heap_main.key[0] = _mi_heap_random_next(&_mi_heap_main);
_mi_heap_main.key[1] = _mi_heap_random_next(&_mi_heap_main);
#endif
mi_process_setup_auto_thread_done();
_mi_os_init();

14
src/page.c
View file

@ -103,7 +103,7 @@ static bool mi_page_is_valid_init(mi_page_t* page) {
bool _mi_page_is_valid(mi_page_t* page) {
mi_assert_internal(mi_page_is_valid_init(page));
#if MI_SECURE
mi_assert_internal(page->cookie != 0);
mi_assert_internal(page->key != 0);
#endif
if (page->heap!=NULL) {
mi_segment_t* segment = _mi_page_segment(page);
@ -284,7 +284,7 @@ void _mi_heap_delayed_free(mi_heap_t* heap) {
// and free them all
while(block != NULL) {
mi_block_t* next = mi_block_nextx(heap,block, heap->cookie);
mi_block_t* next = mi_block_nextx(heap,block, heap->key[0], heap->key[1]);
// use internal free instead of regular one to keep stats etc correct
if (!_mi_free_delayed_block(block)) {
// we might already start delayed freeing while another thread has not yet
@ -292,9 +292,8 @@ void _mi_heap_delayed_free(mi_heap_t* heap) {
mi_block_t* dfree;
do {
dfree = (mi_block_t*)heap->thread_delayed_free;
mi_block_set_nextx(heap, block, dfree, heap->cookie);
mi_block_set_nextx(heap, block, dfree, heap->key[0], heap->key[1]);
} while (!mi_atomic_cas_ptr_weak(mi_atomic_cast(void*,&heap->thread_delayed_free), block, dfree));
}
block = next;
}
@ -357,7 +356,7 @@ void _mi_page_abandon(mi_page_t* page, mi_page_queue_t* pq) {
#if MI_DEBUG>1
// check there are no references left..
for (mi_block_t* block = (mi_block_t*)pheap->thread_delayed_free; block != NULL; block = mi_block_nextx(pheap, block, pheap->cookie)) {
for (mi_block_t* block = (mi_block_t*)pheap->thread_delayed_free; block != NULL; block = mi_block_nextx(pheap, block, pheap->key[0], pheap->key[1])) {
mi_assert_internal(_mi_ptr_page(block) != page);
}
#endif
@ -608,7 +607,8 @@ static void mi_page_init(mi_heap_t* heap, mi_page_t* page, size_t block_size, mi
mi_assert_internal(page_size / block_size < (1L<<16));
page->reserved = (uint16_t)(page_size / block_size);
#ifdef MI_ENCODE_FREELIST
page->cookie = _mi_heap_random_next(heap) | 1;
page->key[0] = _mi_heap_random_next(heap);
page->key[1] = _mi_heap_random_next(heap);
#endif
page->is_zero = page->is_zero_init;
@ -621,7 +621,7 @@ static void mi_page_init(mi_heap_t* heap, mi_page_t* page, size_t block_size, mi
mi_assert_internal(page->prev == NULL);
mi_assert_internal(!mi_page_has_aligned(page));
#if (MI_ENCODE_FREELIST)
mi_assert_internal(page->cookie != 0);
mi_assert_internal(page->key != 0);
#endif
mi_assert_expensive(mi_page_is_valid_init(page));

2
src/random.c
View file

@ -231,9 +231,9 @@ void _mi_random_init(mi_random_ctx_t* ctx) {
if (!os_random_buf(key, sizeof(key))) {
// if we fail to get random data from the OS, we fall back to a
// weak random source based on the current time
_mi_warning_message("unable to use secure randomness\n");
uintptr_t x = os_random_weak(0);
for (size_t i = 0; i < 8; i++) { // key is eight 32-bit words.
_mi_warning_message("unable to use secure randomness\n");
x = _mi_random_shuffle(x);
((uint32_t*)key)[i] = (uint32_t)x;
}

2
src/segment.c
View file

@ -520,7 +520,7 @@ static mi_segment_t* mi_segment_alloc(size_t required, mi_page_kind_t page_kind,
segment->segment_size = segment_size;
segment->segment_info_size = pre_size;
segment->thread_id = _mi_thread_id();
segment->cookie = _mi_ptr_cookie(segment);
segment->cookie = _mi_ptr_cookie(segment);
// _mi_stat_increase(&tld->stats->page_committed, segment->segment_info_size);
// set protection