From: Ruslan Ermilov <ru@nginx.com>
Date: Tue, 26 Jan 2016 16:46:31 +0300
Subject: Resolver: fixed crashes in timeout handler.

If one or more requests were waiting for a response, then after
getting a CNAME response, the timeout event on the first request
remained active, pointing to the wrong node with an empty
rn->waiting list, and that could cause either null pointer
dereference or use-after-free memory access if this timeout
expired.

If several requests were waiting for a response, and the first
request terminated (e.g., due to client closing a connection),
other requests were left without a timeout and could potentially
wait indefinitely.

This is fixed by introducing per-request independent timeouts.
This change also reverts 954867a2f0a6 and 5004210e8c78.
---
 src/core/ngx_resolver.c | 50 +++++++++++++++++++++++++++++++++----------------
 1 file changed, 34 insertions(+), 16 deletions(-)

diff --git a/src/core/ngx_resolver.c b/src/core/ngx_resolver.c
index 726de9b..8ebef6f 100644
--- a/src/core/ngx_resolver.c
+++ b/src/core/ngx_resolver.c
@@ -417,7 +417,7 @@ ngx_resolve_name_done(ngx_resolver_ctx_t *ctx)
 
     /* lock name mutex */
 
-    if (ctx->state == NGX_AGAIN) {
+    if (ctx->state == NGX_AGAIN || ctx->state == NGX_RESOLVE_TIMEDOUT) {
 
         hash = ngx_crc32_short(ctx->name.data, ctx->name.len);
 
@@ -571,6 +571,20 @@ ngx_resolve_name_locked(ngx_resolver_t *r, ngx_resolver_ctx_t *ctx)
 
         if (rn->waiting) {
 
+            if (ctx->event == NULL) {
+                ctx->event = ngx_resolver_calloc(r, sizeof(ngx_event_t));
+                if (ctx->event == NULL) {
+                    return NGX_ERROR;
+                }
+
+                ctx->event->handler = ngx_resolver_timeout_handler;
+                ctx->event->data = ctx;
+                ctx->event->log = r->log;
+                ctx->ident = -1;
+
+                ngx_add_timer(ctx->event, ctx->timeout);
+            }
+
             ctx->next = rn->waiting;
             rn->waiting = ctx;
             ctx->state = NGX_AGAIN;
@@ -664,7 +678,7 @@ ngx_resolve_name_locked(ngx_resolver_t *r, ngx_resolver_ctx_t *ctx)
         }
 
         ctx->event->handler = ngx_resolver_timeout_handler;
-        ctx->event->data = rn;
+        ctx->event->data = ctx;
         ctx->event->log = r->log;
         ctx->ident = -1;
 
@@ -794,6 +808,18 @@ ngx_resolve_addr(ngx_resolver_ctx_t *ctx)
 
         if (rn->waiting) {
 
+            ctx->event = ngx_resolver_calloc(r, sizeof(ngx_event_t));
+            if (ctx->event == NULL) {
+                return NGX_ERROR;
+            }
+
+            ctx->event->handler = ngx_resolver_timeout_handler;
+            ctx->event->data = ctx;
+            ctx->event->log = r->log;
+            ctx->ident = -1;
+
+            ngx_add_timer(ctx->event, ctx->timeout);
+
             ctx->next = rn->waiting;
             rn->waiting = ctx;
             ctx->state = NGX_AGAIN;
@@ -857,7 +883,7 @@ ngx_resolve_addr(ngx_resolver_ctx_t *ctx)
     }
 
     ctx->event->handler = ngx_resolver_timeout_handler;
-    ctx->event->data = rn;
+    ctx->event->data = ctx;
     ctx->event->log = r->log;
     ctx->ident = -1;
 
@@ -949,7 +975,7 @@ ngx_resolve_addr_done(ngx_resolver_ctx_t *ctx)
 
     /* lock addr mutex */
 
-    if (ctx->state == NGX_AGAIN) {
+    if (ctx->state == NGX_AGAIN || ctx->state == NGX_RESOLVE_TIMEDOUT) {
 
         switch (ctx->addr.sockaddr->sa_family) {
 
@@ -2791,21 +2817,13 @@ done:
 static void
 ngx_resolver_timeout_handler(ngx_event_t *ev)
 {
-    ngx_resolver_ctx_t   *ctx, *next;
-    ngx_resolver_node_t  *rn;
+    ngx_resolver_ctx_t  *ctx;
 
-    rn = ev->data;
-    ctx = rn->waiting;
-    rn->waiting = NULL;
+    ctx = ev->data;
 
-    do {
-        ctx->state = NGX_RESOLVE_TIMEDOUT;
-        next = ctx->next;
-
-        ctx->handler(ctx);
+    ctx->state = NGX_RESOLVE_TIMEDOUT;
 
-        ctx = next;
-    } while (ctx);
+    ctx->handler(ctx);
 }