diff options
Diffstat (limited to 'third_party/bearssl/src/ssl_lru.c')
| -rw-r--r-- | third_party/bearssl/src/ssl_lru.c | 537 |
1 files changed, 537 insertions, 0 deletions
diff --git a/third_party/bearssl/src/ssl_lru.c b/third_party/bearssl/src/ssl_lru.c new file mode 100644 index 0000000..4c71011 --- /dev/null +++ b/third_party/bearssl/src/ssl_lru.c @@ -0,0 +1,537 @@ +/* + * Copyright (c) 2016 Thomas Pornin <[email protected]> + * + * Permission is hereby granted, free of charge, to any person obtaining + * a copy of this software and associated documentation files (the + * "Software"), to deal in the Software without restriction, including + * without limitation the rights to use, copy, modify, merge, publish, + * distribute, sublicense, and/or sell copies of the Software, and to + * permit persons to whom the Software is furnished to do so, subject to + * the following conditions: + * + * The above copyright notice and this permission notice shall be + * included in all copies or substantial portions of the Software. + * + * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, + * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF + * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND + * NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS + * BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN + * ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN + * CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE + * SOFTWARE. + */ + +#include "inner.h" + +/* + * Each entry consists in a fixed number of bytes. Entries are concatenated + * in the store block. "Addresses" are really offsets in the block, + * expressed over 32 bits (so the cache may have size at most 4 GB, which + * "ought to be enough for everyone"). The "null address" is 0xFFFFFFFF. + * Note that since the storage block alignment is in no way guaranteed, we + * perform only accesses that can handle unaligned data. + * + * Two concurrent data structures are maintained: + * + * -- Entries are organised in a doubly-linked list; saved entries are added + * at the head, and loaded entries are moved to the head. Eviction uses + * the list tail (this is the LRU algorithm). + * + * -- Entries are indexed with a binary tree: all left descendants of a + * node have a lower session ID (in lexicographic order), while all + * right descendants have a higher session ID. The tree is heuristically + * balanced. + * + * Entry format: + * + * session ID 32 bytes + * master secret 48 bytes + * protocol version 2 bytes (big endian) + * cipher suite 2 bytes (big endian) + * list prev 4 bytes (big endian) + * list next 4 bytes (big endian) + * tree left child 4 bytes (big endian) + * tree right child 4 bytes (big endian) + * + * If an entry has a protocol version set to 0, then it is "disabled": + * it was a session pushed to the cache at some point, but it has + * been explicitly removed. + * + * We need to keep the tree balanced because an attacker could make + * handshakes, selecting some specific sessions (by reusing them) to + * try to make us make an imbalanced tree that makes lookups expensive + * (a denial-of-service attack that would persist as long as the cache + * remains, i.e. even after the attacker made all his connections). + * To do that, we replace the session ID (or the start of the session ID) + * with a HMAC value computed over the replaced part; the hash function + * implementation and the key are obtained from the server context upon + * first save() call. + * + * Theoretically, an attacker could use the exact timing of the lookup + * to infer the current tree topology, and try to revive entries to make + * it as unbalanced as possible. However, since the session ID are + * chosen randomly by the server, and the attacker cannot see the + * indexing values and must thus rely on blind selection, it should be + * exponentially difficult for the attacker to maintain a large + * imbalance. + */ +#define SESSION_ID_LEN 32 +#define MASTER_SECRET_LEN 48 + +#define SESSION_ID_OFF 0 +#define MASTER_SECRET_OFF 32 +#define VERSION_OFF 80 +#define CIPHER_SUITE_OFF 82 +#define LIST_PREV_OFF 84 +#define LIST_NEXT_OFF 88 +#define TREE_LEFT_OFF 92 +#define TREE_RIGHT_OFF 96 + +#define LRU_ENTRY_LEN 100 + +#define ADDR_NULL ((uint32_t)-1) + +#define GETSET(name, off) \ +static inline uint32_t get_ ## name(br_ssl_session_cache_lru *cc, uint32_t x) \ +{ \ + return br_dec32be(cc->store + x + (off)); \ +} \ +static inline void set_ ## name(br_ssl_session_cache_lru *cc, \ + uint32_t x, uint32_t val) \ +{ \ + br_enc32be(cc->store + x + (off), val); \ +} + +GETSET(prev, LIST_PREV_OFF) +GETSET(next, LIST_NEXT_OFF) +GETSET(left, TREE_LEFT_OFF) +GETSET(right, TREE_RIGHT_OFF) + +/* + * Transform the session ID by replacing the first N bytes with a HMAC + * value computed over these bytes, using the random key K (the HMAC + * value is truncated if needed). HMAC will use the same hash function + * as the DRBG in the SSL server context, so with SHA-256, SHA-384, + * or SHA-1, depending on what is available. + * + * The risk of collision is considered too small to be a concern; and + * the impact of a collision is low (the handshake won't succeed). This + * risk is much lower than any transmission error, which would lead to + * the same consequences. + * + * Source and destination arrays msut be disjoint. + */ +static void +mask_id(br_ssl_session_cache_lru *cc, + const unsigned char *src, unsigned char *dst) +{ + br_hmac_key_context hkc; + br_hmac_context hc; + + memcpy(dst, src, SESSION_ID_LEN); + br_hmac_key_init(&hkc, cc->hash, cc->index_key, sizeof cc->index_key); + br_hmac_init(&hc, &hkc, SESSION_ID_LEN); + br_hmac_update(&hc, src, SESSION_ID_LEN); + br_hmac_out(&hc, dst); +} + +/* + * Find a node by ID. Returned value is the node address, or ADDR_NULL if + * the node is not found. + * + * If addr_link is not NULL, then '*addr_link' is set to the address of the + * last followed link. If the found node is the root, or if the tree is + * empty, then '*addr_link' is set to ADDR_NULL. + */ +static uint32_t +find_node(br_ssl_session_cache_lru *cc, const unsigned char *id, + uint32_t *addr_link) +{ + uint32_t x, y; + + x = cc->root; + y = ADDR_NULL; + while (x != ADDR_NULL) { + int r; + + r = memcmp(id, cc->store + x + SESSION_ID_OFF, SESSION_ID_LEN); + if (r < 0) { + y = x + TREE_LEFT_OFF; + x = get_left(cc, x); + } else if (r == 0) { + if (addr_link != NULL) { + *addr_link = y; + } + return x; + } else { + y = x + TREE_RIGHT_OFF; + x = get_right(cc, x); + } + } + if (addr_link != NULL) { + *addr_link = y; + } + return ADDR_NULL; +} + +/* + * For node x, find its replacement upon removal. + * + * -- If node x has no child, then this returns ADDR_NULL. + * -- Otherwise, if node x has a left child, then the replacement is the + * rightmost left-descendent. + * -- Otherwise, the replacement is the leftmost right-descendent. + * + * If a node is returned, then '*al' is set to the address of the field + * that points to that node. Otherwise (node x has no child), '*al' is + * set to ADDR_NULL. + * + * Note that the replacement node, when found, is always a descendent + * of node 'x', so it cannot be the tree root. Thus, '*al' can be set + * to ADDR_NULL only when no node is found and ADDR_NULL is returned. + */ +static uint32_t +find_replacement_node(br_ssl_session_cache_lru *cc, uint32_t x, uint32_t *al) +{ + uint32_t y1, y2; + + y1 = get_left(cc, x); + if (y1 != ADDR_NULL) { + y2 = x + TREE_LEFT_OFF; + for (;;) { + uint32_t z; + + z = get_right(cc, y1); + if (z == ADDR_NULL) { + *al = y2; + return y1; + } + y2 = y1 + TREE_RIGHT_OFF; + y1 = z; + } + } + y1 = get_right(cc, x); + if (y1 != ADDR_NULL) { + y2 = x + TREE_RIGHT_OFF; + for (;;) { + uint32_t z; + + z = get_left(cc, y1); + if (z == ADDR_NULL) { + *al = y2; + return y1; + } + y2 = y1 + TREE_LEFT_OFF; + y1 = z; + } + } + *al = ADDR_NULL; + return ADDR_NULL; +} + +/* + * Set the link at address 'alx' to point to node 'x'. If 'alx' is + * ADDR_NULL, then this sets the tree root to 'x'. + */ +static inline void +set_link(br_ssl_session_cache_lru *cc, uint32_t alx, uint32_t x) +{ + if (alx == ADDR_NULL) { + cc->root = x; + } else { + br_enc32be(cc->store + alx, x); + } +} + +/* + * Remove node 'x' from the tree. This function shall not be called if + * node 'x' is not part of the tree. + */ +static void +remove_node(br_ssl_session_cache_lru *cc, uint32_t x) +{ + uint32_t alx, y, aly; + + /* + * Removal algorithm: + * ------------------ + * + * - If we remove the root, then the tree becomes empty. + * + * - If the removed node has no child, then we can simply remove + * it, with nothing else to do. + * + * - Otherwise, the removed node must be replaced by either its + * rightmost left-descendent, or its leftmost right-descendent. + * The replacement node itself must be removed from its current + * place. By definition, that replacement node has either no + * child, or at most a single child that will replace it in the + * tree. + */ + + /* + * Find node back and its ancestor link. If the node was the + * root, then alx is set to ADDR_NULL. + */ + find_node(cc, cc->store + x + SESSION_ID_OFF, &alx); + + /* + * Find replacement node 'y', and 'aly' is set to the address of + * the link to that replacement node. If the removed node has no + * child, then both 'y' and 'aly' are set to ADDR_NULL. + */ + y = find_replacement_node(cc, x, &aly); + + if (y != ADDR_NULL) { + uint32_t z; + + /* + * The unlinked replacement node may have one child (but + * not two) that takes its place. + */ + z = get_left(cc, y); + if (z == ADDR_NULL) { + z = get_right(cc, y); + } + set_link(cc, aly, z); + + /* + * Link the replacement node in its new place, overwriting + * the current link to the node 'x' (which removes 'x'). + */ + set_link(cc, alx, y); + + /* + * The replacement node adopts the left and right children + * of the removed node. Note that this also works even if + * the replacement node was a direct descendent of the + * removed node, since we unlinked it previously. + */ + set_left(cc, y, get_left(cc, x)); + set_right(cc, y, get_right(cc, x)); + } else { + /* + * No replacement, we simply unlink the node 'x'. + */ + set_link(cc, alx, ADDR_NULL); + } +} + +static void +lru_save(const br_ssl_session_cache_class **ctx, + br_ssl_server_context *server_ctx, + const br_ssl_session_parameters *params) +{ + br_ssl_session_cache_lru *cc; + unsigned char id[SESSION_ID_LEN]; + uint32_t x, alx; + + cc = (br_ssl_session_cache_lru *)ctx; + + /* + * If the buffer is too small, we don't record anything. This + * test avoids problems in subsequent code. + */ + if (cc->store_len < LRU_ENTRY_LEN) { + return; + } + + /* + * Upon the first save in a session cache instance, we obtain + * a random key for our indexing. + */ + if (!cc->init_done) { + br_hmac_drbg_generate(&server_ctx->eng.rng, + cc->index_key, sizeof cc->index_key); + cc->hash = br_hmac_drbg_get_hash(&server_ctx->eng.rng); + cc->init_done = 1; + } + mask_id(cc, params->session_id, id); + + /* + * Look for the node in the tree. If the same ID is already used, + * then reject it. This is a collision event, which should be + * exceedingly rare. + * Note: we do NOT record the emplacement here, because the + * removal of an entry may change the tree topology. + */ + if (find_node(cc, id, NULL) != ADDR_NULL) { + return; + } + + /* + * Find some room for the new parameters. If the cache is not + * full yet, add it to the end of the area and bump the pointer up. + * Otherwise, evict the list tail entry. Note that we already + * filtered out the case of a ridiculously small buffer that + * cannot hold any entry at all; thus, if there is no room for an + * extra entry, then the cache cannot be empty. + */ + if (cc->store_ptr > (cc->store_len - LRU_ENTRY_LEN)) { + /* + * Evict tail. If the buffer has room for a single entry, + * then this may also be the head. + */ + x = cc->tail; + cc->tail = get_prev(cc, x); + if (cc->tail == ADDR_NULL) { + cc->head = ADDR_NULL; + } else { + set_next(cc, cc->tail, ADDR_NULL); + } + + /* + * Remove the node from the tree. + */ + remove_node(cc, x); + } else { + /* + * Allocate room for new node. + */ + x = cc->store_ptr; + cc->store_ptr += LRU_ENTRY_LEN; + } + + /* + * Find the emplacement for the new node, and link it. + */ + find_node(cc, id, &alx); + set_link(cc, alx, x); + set_left(cc, x, ADDR_NULL); + set_right(cc, x, ADDR_NULL); + + /* + * New entry becomes new list head. It may also become the list + * tail if the cache was empty at that point. + */ + if (cc->head == ADDR_NULL) { + cc->tail = x; + } else { + set_prev(cc, cc->head, x); + } + set_prev(cc, x, ADDR_NULL); + set_next(cc, x, cc->head); + cc->head = x; + + /* + * Fill data in the entry. + */ + memcpy(cc->store + x + SESSION_ID_OFF, id, SESSION_ID_LEN); + memcpy(cc->store + x + MASTER_SECRET_OFF, + params->master_secret, MASTER_SECRET_LEN); + br_enc16be(cc->store + x + VERSION_OFF, params->version); + br_enc16be(cc->store + x + CIPHER_SUITE_OFF, params->cipher_suite); +} + +static int +lru_load(const br_ssl_session_cache_class **ctx, + br_ssl_server_context *server_ctx, + br_ssl_session_parameters *params) +{ + br_ssl_session_cache_lru *cc; + unsigned char id[SESSION_ID_LEN]; + uint32_t x; + + (void)server_ctx; + cc = (br_ssl_session_cache_lru *)ctx; + if (!cc->init_done) { + return 0; + } + mask_id(cc, params->session_id, id); + x = find_node(cc, id, NULL); + if (x != ADDR_NULL) { + unsigned version; + + version = br_dec16be(cc->store + x + VERSION_OFF); + if (version == 0) { + /* + * Entry is disabled, we pretend we did not find it. + * Notably, we don't move it to the front of the + * LRU list. + */ + return 0; + } + params->version = version; + params->cipher_suite = br_dec16be( + cc->store + x + CIPHER_SUITE_OFF); + memcpy(params->master_secret, + cc->store + x + MASTER_SECRET_OFF, + MASTER_SECRET_LEN); + if (x != cc->head) { + /* + * Found node is not at list head, so move + * it to the head. + */ + uint32_t p, n; + + p = get_prev(cc, x); + n = get_next(cc, x); + set_next(cc, p, n); + if (n == ADDR_NULL) { + cc->tail = p; + } else { + set_prev(cc, n, p); + } + set_prev(cc, cc->head, x); + set_next(cc, x, cc->head); + set_prev(cc, x, ADDR_NULL); + cc->head = x; + } + return 1; + } + return 0; +} + +static const br_ssl_session_cache_class lru_class = { + sizeof(br_ssl_session_cache_lru), + &lru_save, + &lru_load +}; + +/* see inner.h */ +void +br_ssl_session_cache_lru_init(br_ssl_session_cache_lru *cc, + unsigned char *store, size_t store_len) +{ + cc->vtable = &lru_class; + cc->store = store; + cc->store_len = store_len; + cc->store_ptr = 0; + cc->init_done = 0; + cc->head = ADDR_NULL; + cc->tail = ADDR_NULL; + cc->root = ADDR_NULL; +} + +/* see bearssl_ssl.h */ +void br_ssl_session_cache_lru_forget( + br_ssl_session_cache_lru *cc, const unsigned char *id) +{ + unsigned char mid[SESSION_ID_LEN]; + uint32_t addr; + + /* + * If the cache is not initialised yet, then it is empty, and + * there is nothing to forget. + */ + if (!cc->init_done) { + return; + } + + /* + * Look for the node in the tree. If found, the entry is marked + * as "disabled"; it will be reused in due course, as it ages + * through the list. + * + * We do not go through the complex moves of actually releasing + * the entry right away because explicitly forgetting sessions + * should be a rare event, meant mostly for testing purposes, + * so this is not worth the extra code size. + */ + mask_id(cc, id, mid); + addr = find_node(cc, mid, NULL); + if (addr != ADDR_NULL) { + br_enc16be(cc->store + addr + VERSION_OFF, 0); + } +} |