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.

src/core/ngx_resolver.c

@@ -59,7 +59,7 @@ ngx_int_t ngx_udp_connect(ngx_udp_connection_t *uc);
 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 *ctx)
 
     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 @@ ngx_resolve_name_done(ngx_resolver_ctx_t *ctx)
 
 
 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);
+    ngx_strlow(name->data, name->data, name->len);
 
-    hash = ngx_crc32_short(ctx->name.data, ctx->name.len);
+    hash = ngx_crc32_short(name->data, name->len);
 
-    rn = ngx_resolver_lookup_name(r, &ctx->name, hash);
+    rn = ngx_resolver_lookup_name(r, name, hash);
 
     if (rn) {
 
@@ -559,10 +558,10 @@ ngx_resolve_name_locked(ngx_resolver_t *r, ngx_resolver_ctx_t *ctx)
 
             if (ctx->recursion++ < NGX_RESOLVER_MAX_RECURSION) {
 
-                ctx->name.len = rn->cnlen;
-                ctx->name.data = rn->u.cname;
+                cname.len = rn->cnlen;
+                cname.data = rn->u.cname;
 
-                return ngx_resolve_name_locked(r, ctx);
+                return ngx_resolve_name_locked(r, ctx, &cname);
             }
 
             last->next = rn->waiting;
@@ -602,6 +601,11 @@ ngx_resolve_name_locked(ngx_resolver_t *r, ngx_resolver_ctx_t *ctx)
             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 *r, ngx_resolver_ctx_t *ctx)
             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 *r, ngx_resolver_ctx_t *ctx)
         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 *r, ngx_resolver_ctx_t *ctx)
 
     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 @@ ngx_resolve_addr(ngx_resolver_ctx_t *ctx)
 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 *ctx)
 
     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, u_char *buf, size_t last,
         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);

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;
 };