details: http://hg.nginx.org/nginx/rev/497d0cff8ace
branches:
changeset: 6351:497d0cff8ace
user: Roman Arutyunyan <arut@nginx.com>
date: Tue Jan 26 16:46:59 2016 +0300
description:
Resolver: fixed use-after-free memory accesses with CNAME.

When several requests were waiting for a response, then after getting
a CNAME response only the last request's context had the name updated.
Contexts of other requests had the wrong name. This name was used by
ngx_resolve_name_done() to find the node to remove the request context
from. When the name was wrong, the request could not be properly
cancelled, its context was freed but stayed linked to the node's waiting
list. This happened e.g. when the first request was aborted or timed
out before the resolving completed. When it completed, this triggered
a use-after-free memory access by calling ctx->handler of already freed
request context. The bug manifests itself by
"could not cancel <name> resolving" alerts in error_log.

When a request was responded with a CNAME, the request context kept
the pointer to the original node's rn->u.cname. If the original node
expired before the resolving timed out or completed with an error,
this would trigger a use-after-free memory access via ctx->name in
ctx->handler().

The fix is to keep ctx->name unmodified. The name from context
is no longer used by ngx_resolve_name_done(). Instead, we now keep
the pointer to resolver node to which this request is linked.
Keeping the original name intact also improves logging.

diffstat:

src/core/ngx_resolver.c | 80 ++++++++++++++++++++++--------------------------
src/core/ngx_resolver.h | 2 +
2 files changed, 39 insertions(+), 43 deletions(-)

diffs (219 lines):

diff --git a/src/core/ngx_resolver.c b/src/core/ngx_resolver.c
--- a/src/core/ngx_resolver.c
+++ b/src/core/ngx_resolver.c
@@ -59,7 +59,7 @@ ngx_int_t ngx_udp_connect(ngx_udp_connec
static void ngx_resolver_cleanup(void *data);
static void ngx_resolver_cleanup_tree(ngx_resolver_t *r, ngx_rbtree_t *tree);
static ngx_int_t ngx_resolve_name_locked(ngx_resolver_t *r,
- ngx_resolver_ctx_t *ctx);
+ ngx_resolver_ctx_t *ctx, ngx_str_t *name);
static void ngx_resolver_expire(ngx_resolver_t *r, ngx_rbtree_t *tree,
ngx_queue_t *queue);
static ngx_int_t ngx_resolver_send_query(ngx_resolver_t *r,
@@ -376,7 +376,7 @@ ngx_resolve_name(ngx_resolver_ctx_t *ctx

/* lock name mutex */

- rc = ngx_resolve_name_locked(r, ctx);
+ rc = ngx_resolve_name_locked(r, ctx, &ctx->name);

if (rc == NGX_OK) {
return NGX_OK;
@@ -403,7 +403,6 @@ ngx_resolve_name(ngx_resolver_ctx_t *ctx
void
ngx_resolve_name_done(ngx_resolver_ctx_t *ctx)
{
- uint32_t hash;
ngx_resolver_t *r;
ngx_resolver_ctx_t *w, **p;
ngx_resolver_node_t *rn;
@@ -425,9 +424,7 @@ ngx_resolve_name_done(ngx_resolver_ctx_t

if (ctx->state == NGX_AGAIN || ctx->state == NGX_RESOLVE_TIMEDOUT) {

- hash = ngx_crc32_short(ctx->name.data, ctx->name.len);
-
- rn = ngx_resolver_lookup_name(r, &ctx->name, hash);
+ rn = ctx->node;

if (rn) {
p = &rn->waiting;
@@ -472,20 +469,22 @@ done:


static ngx_int_t
-ngx_resolve_name_locked(ngx_resolver_t *r, ngx_resolver_ctx_t *ctx)
+ngx_resolve_name_locked(ngx_resolver_t *r, ngx_resolver_ctx_t *ctx,
+ ngx_str_t *name)
{
uint32_t hash;
ngx_int_t rc;
+ ngx_str_t cname;
ngx_uint_t naddrs;
ngx_addr_t *addrs;
ngx_resolver_ctx_t *next, *last;
ngx_resolver_node_t *rn;

- ngx_strlow(ctx->name.data, ctx->name.data, ctx->name.len);
-
- hash = ngx_crc32_short(ctx->name.data, ctx->name.len);
-
- rn = ngx_resolver_lookup_name(r, &ctx->name, hash);
+ ngx_strlow(name->data, name->data, name->len);
+
+ hash = ngx_crc32_short(name->data, name->len);
+
+ rn = ngx_resolver_lookup_name(r, name, hash);

if (rn) {

@@ -559,10 +558,10 @@ ngx_resolve_name_locked(ngx_resolver_t *

if (ctx->recursion++ < NGX_RESOLVER_MAX_RECURSION) {

- ctx->name.len = rn->cnlen;
- ctx->name.data = rn->u.cname;
-
- return ngx_resolve_name_locked(r, ctx);
+ cname.len = rn->cnlen;
+ cname.data = rn->u.cname;
+
+ return ngx_resolve_name_locked(r, ctx, &cname);
}

last->next = rn->waiting;
@@ -602,6 +601,11 @@ ngx_resolve_name_locked(ngx_resolver_t *
rn->waiting = ctx;
ctx->state = NGX_AGAIN;

+ do {
+ ctx->node = rn;
+ ctx = ctx->next;
+ } while (ctx);
+
return NGX_AGAIN;
}

@@ -640,14 +644,14 @@ ngx_resolve_name_locked(ngx_resolver_t *
return NGX_ERROR;
}

- rn->name = ngx_resolver_dup(r, ctx->name.data, ctx->name.len);
+ rn->name = ngx_resolver_dup(r, name->data, name->len);
if (rn->name == NULL) {
ngx_resolver_free(r, rn);
return NGX_ERROR;
}

rn->node.key = hash;
- rn->nlen = (u_short) ctx->name.len;
+ rn->nlen = (u_short) name->len;
rn->query = NULL;
#if (NGX_HAVE_INET6)
rn->query6 = NULL;
@@ -656,7 +660,7 @@ ngx_resolve_name_locked(ngx_resolver_t *
ngx_rbtree_insert(&r->name_rbtree, &rn->node);
}

- rc = ngx_resolver_create_name_query(r, rn, &ctx->name);
+ rc = ngx_resolver_create_name_query(r, rn, name);

if (rc == NGX_ERROR) {
goto failed;
@@ -720,6 +724,11 @@ ngx_resolve_name_locked(ngx_resolver_t *

ctx->state = NGX_AGAIN;

+ do {
+ ctx->node = rn;
+ ctx = ctx->next;
+ } while (ctx);
+
return NGX_AGAIN;

failed:
@@ -842,6 +851,7 @@ ngx_resolve_addr(ngx_resolver_ctx_t *ctx
ctx->next = rn->waiting;
rn->waiting = ctx;
ctx->state = NGX_AGAIN;
+ ctx->node = rn;

/* unlock addr mutex */

@@ -927,6 +937,7 @@ ngx_resolve_addr(ngx_resolver_ctx_t *ctx
/* unlock addr mutex */

ctx->state = NGX_AGAIN;
+ ctx->node = rn;

return NGX_OK;

@@ -957,17 +968,11 @@ failed:
void
ngx_resolve_addr_done(ngx_resolver_ctx_t *ctx)
{
- in_addr_t addr;
ngx_queue_t *expire_queue;
ngx_rbtree_t *tree;
ngx_resolver_t *r;
ngx_resolver_ctx_t *w, **p;
- struct sockaddr_in *sin;
ngx_resolver_node_t *rn;
-#if (NGX_HAVE_INET6)
- uint32_t hash;
- struct sockaddr_in6 *sin6;
-#endif

r = ctx->resolver;

@@ -996,21 +1001,7 @@ ngx_resolve_addr_done(ngx_resolver_ctx_t

if (ctx->state == NGX_AGAIN || ctx->state == NGX_RESOLVE_TIMEDOUT) {

- switch (ctx->addr.sockaddr->sa_family) {
-
-#if (NGX_HAVE_INET6)
- case AF_INET6:
- sin6 = (struct sockaddr_in6 *) ctx->addr.sockaddr;
- hash = ngx_crc32_short(sin6->sin6_addr.s6_addr, 16);
- rn = ngx_resolver_lookup_addr6(r, &sin6->sin6_addr, hash);
- break;
-#endif
-
- default: /* AF_INET */
- sin = (struct sockaddr_in *) ctx->addr.sockaddr;
- addr = ntohl(sin->sin_addr.s_addr);
- rn = ngx_resolver_lookup_addr(r, addr);
- }
+ rn = ctx->node;

if (rn) {
p = &rn->waiting;
@@ -2014,9 +2005,12 @@ ngx_resolver_process_a(ngx_resolver_t *r
rn->waiting = NULL;

if (ctx) {
- ctx->name = name;
-
- (void) ngx_resolve_name_locked(r, ctx);
+
+ for (next = ctx; next; next = next->next) {
+ next->node = NULL;
+ }
+
+ (void) ngx_resolve_name_locked(r, ctx, &name);
}

ngx_resolver_free(r, rn->query);
diff --git a/src/core/ngx_resolver.h b/src/core/ngx_resolver.h
--- a/src/core/ngx_resolver.h
+++ b/src/core/ngx_resolver.h
@@ -161,6 +161,8 @@ struct ngx_resolver_ctx_s {
ngx_uint_t quick; /* unsigned quick:1; */
ngx_uint_t recursion;
ngx_event_t *event;
+
+ ngx_resolver_node_t *node;
};



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