Fix an integer underflow in build_read_multi_rsp

When p_buf->len is mtu - 1 and p_cmd->multi_req.variable_len
evaluates to true, integer underflow is triggered
in the following line, resulting OOB access.

```
 len = p_rsp->attr_value.len - (total_len - mtu);
```

Bug: 273874525
Test: manual
Ignore-AOSP-First: security
Tag: #security
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:85f4d53c7bf90b806639a3a302f0007ffb3b9f23)
Merged-In: Ia60dd829ff9152c083de1f4c1265bb3ad595dcc4
Change-Id: Ia60dd829ff9152c083de1f4c1265bb3ad595dcc4

system/stack/gatt/gatt_sr.cc

@@ -21,6 +21,7 @@
  *  this file contains the GATT server functions
  *
  ******************************************************************************/
+#include <algorithm>
 #include <string.h>
 
 #include "bt_target.h"
@@ -179,37 +180,38 @@ static void build_read_multi_rsp(tGATT_SR_CMD* p_cmd, uint16_t mtu) {
     }
 
     if (p_rsp != NULL) {
-      total_len = (p_buf->len + p_rsp->attr_value.len);
+      total_len = p_buf->len;
       if (p_cmd->multi_req.variable_len) {
         total_len += 2;
       }
 
       if (total_len > mtu) {
-        /* just send the partial response for the overflow case */
-        len = p_rsp->attr_value.len - (total_len - mtu);
+        VLOG(1) << "Buffer space not enough for this data item, skipping";
+        break;
+      }
+
+      len = std::min((size_t) p_rsp->attr_value.len, mtu - total_len);
+
+      if (len == 0) {
+        VLOG(1) << "Buffer space not enough for this data item, skipping";
+        break;
+      }
+
+      if (len < p_rsp->attr_value.len) {
         is_overflow = true;
         VLOG(1) << StringPrintf(
             "multi read overflow available len=%zu val_len=%d", len,
             p_rsp->attr_value.len);
-      } else {
-        len = p_rsp->attr_value.len;
       }
 
       if (p_cmd->multi_req.variable_len) {
-        UINT16_TO_STREAM(p, len);
+        UINT16_TO_STREAM(p, (uint16_t) len);
         p_buf->len += 2;
       }
 
       if (p_rsp->attr_value.handle == p_cmd->multi_req.handles[ii]) {
-        // check for possible integer overflow
-        if (p_buf->len + len <= UINT16_MAX) {
-          memcpy(p, p_rsp->attr_value.value, len);
-          if (!is_overflow) p += len;
-          p_buf->len += len;
-        } else {
-          p_cmd->status = GATT_NOT_FOUND;
-          break;
-        }
+        ARRAY_TO_STREAM(p, p_rsp->attr_value.value, (uint16_t) len);
+        p_buf->len += (uint16_t) len;
       } else {
         p_cmd->status = GATT_NOT_FOUND;
         break;