Handle locked pages more robustly (Fixes issue #1462)

Memory locks do not stack, that is, pages which have been locked several times by calls to mlock()
will be unlocked by a single call to munlock(). This can result in keying material ending up in swap when
those functions are used naively. In this commit a class "LockedPageManager" is added
that simulates stacking memory locks by keeping a counter per page.

1 files changed, 150 insertions, 15 deletions

diff --git a/src/allocators.h b/src/allocators.h
index ddeabc48c5..99afa10c25 100644
--- a/src/allocators.h
+++ b/src/allocators.h
@@ -7,6 +7,8 @@
 
 #include <string.h>
 #include <string>
+#include <boost/thread/mutex.hpp>
+#include <map>
 
 #ifdef WIN32
 #ifdef _WIN32_WINNT
@@ -22,23 +24,156 @@
 // Note that VirtualLock does not provide this as a guarantee on Windows,
 // but, in practice, memory that has been VirtualLock'd almost never gets written to
 // the pagefile except in rare circumstances where memory is extremely low.
-#define mlock(p, n) VirtualLock((p), (n));
-#define munlock(p, n) VirtualUnlock((p), (n));
 #else
 #include <sys/mman.h>
-#include <limits.h>
-/* This comes from limits.h if it's not defined there set a sane default */
-#ifndef PAGESIZE
-#include <unistd.h>
-#define PAGESIZE sysconf(_SC_PAGESIZE)
+#include <limits.h> // for PAGESIZE
+#include <unistd.h> // for sysconf
 #endif
-#define mlock(a,b) \
-  mlock(((void *)(((size_t)(a)) & (~((PAGESIZE)-1)))),\
-  (((((size_t)(a)) + (b) - 1) | ((PAGESIZE) - 1)) + 1) - (((size_t)(a)) & (~((PAGESIZE) - 1))))
-#define munlock(a,b) \
-  munlock(((void *)(((size_t)(a)) & (~((PAGESIZE)-1)))),\
-  (((((size_t)(a)) + (b) - 1) | ((PAGESIZE) - 1)) + 1) - (((size_t)(a)) & (~((PAGESIZE) - 1))))
+
+/**
+ * Thread-safe class to keep track of locked (ie, non-swappable) memory pages.
+ *
+ * Memory locks do not stack, that is, pages which have been locked several times by calls to mlock()
+ * will be unlocked by a single call to munlock(). This can result in keying material ending up in swap when
+ * those functions are used naively. This class simulates stacking memory locks by keeping a counter per page.
+ *
+ * @note By using a map from each page base address to lock count, this class is optimized for
+ * small objects that span up to a few pages, mostly smaller than a page. To support large allocations,
+ * something like an interval tree would be the preferred data structure.
+ */
+template <class Locker> class LockedPageManagerBase
+{
+public:
+    LockedPageManagerBase(size_t page_size):
+        page_size(page_size)
+    {
+        // Determine bitmask for extracting page from address
+        assert(!(page_size & (page_size-1))); // size must be power of two
+        page_mask = ~(page_size - 1);
+    }
+
+    // For all pages in affected range, increase lock count
+    void LockRange(void *p, size_t size)
+    {
+        boost::mutex::scoped_lock lock(mutex);
+        if(!size) return;
+        const size_t base_addr = reinterpret_cast<size_t>(p);
+        const size_t start_page = base_addr & page_mask;
+        const size_t end_page = (base_addr + size - 1) & page_mask;
+        for(size_t page = start_page; page <= end_page; page += page_size)
+        {
+            Histogram::iterator it = histogram.find(page);
+            if(it == histogram.end()) // Newly locked page
+            {
+                locker.Lock(reinterpret_cast<void*>(page), page_size);
+                histogram.insert(std::make_pair(page, 1));
+            }
+            else // Page was already locked; increase counter
+            {
+                it->second += 1;
+            }
+        }
+    }
+
+    // For all pages in affected range, decrease lock count
+    void UnlockRange(void *p, size_t size)
+    {
+        boost::mutex::scoped_lock lock(mutex);
+        if(!size) return;
+        const size_t base_addr = reinterpret_cast<size_t>(p);
+        const size_t start_page = base_addr & page_mask;
+        const size_t end_page = (base_addr + size - 1) & page_mask;
+        for(size_t page = start_page; page <= end_page; page += page_size)
+        {
+            Histogram::iterator it = histogram.find(page);
+            assert(it != histogram.end()); // Cannot unlock an area that was not locked
+            // Decrease counter for page, when it is zero, the page will be unlocked
+            it->second -= 1;
+            if(it->second == 0) // Nothing on the page anymore that keeps it locked
+            {
+                // Unlock page and remove the count from histogram
+                locker.Unlock(reinterpret_cast<void*>(page), page_size);
+                histogram.erase(it);
+            }
+        }
+    }
+
+    // Get number of locked pages for diagnostics
+    int GetLockedPageCount()
+    {
+        boost::mutex::scoped_lock lock(mutex);
+        return histogram.size();
+    }
+
+private:
+    Locker locker;
+    boost::mutex mutex;
+    size_t page_size, page_mask;
+    // map of page base address to lock count
+    typedef std::map<size_t,int> Histogram;
+    Histogram histogram;
+};
+
+/** Determine system page size in bytes */
+static inline size_t GetSystemPageSize()
+{
+    size_t page_size;
+#if defined(WIN32)
+    SYSTEM_INFO sSysInfo;
+    GetSystemInfo(&sSysInfo);
+    page_size = sSysInfo.dwPageSize;
+#elif defined(PAGESIZE) // defined in limits.h
+    page_size = PAGESIZE;
+#else // assume some POSIX OS
+    page_size = sysconf(_SC_PAGESIZE);
+#endif
+    return page_size;
+}
+
+/**
+ * OS-dependent memory page locking/unlocking.
+ * Defined as policy class to make stubbing for test possible.
+ */
+class MemoryPageLocker
+{
+public:
+    /** Lock memory pages.
+     * addr and len must be a multiple of the system page size
+     */
+    bool Lock(const void *addr, size_t len)
+    {
+#ifdef WIN32
+        return VirtualLock(const_cast<void*>(addr), len);
+#else
+        return mlock(addr, len) == 0;
+#endif
+    }
+    /** Unlock memory pages.
+     * addr and len must be a multiple of the system page size
+     */
+    bool Unlock(const void *addr, size_t len)
+    {
+#ifdef WIN32
+        return VirtualUnlock(const_cast<void*>(addr), len);
+#else
+        return munlock(addr, len) == 0;
 #endif
+    }
+};
+
+/**
+ * Singleton class to keep track of locked (ie, non-swappable) memory pages, for use in
+ * std::allocator templates.
+ */
+class LockedPageManager: public LockedPageManagerBase<MemoryPageLocker>
+{
+public:
+    static LockedPageManager instance; // instantiated in util.cpp
+private:
+    LockedPageManager():
+        LockedPageManagerBase<MemoryPageLocker>(GetSystemPageSize())
+    {}
+};
 
 //
 // Allocator that locks its contents from being paged
@@ -69,7 +204,7 @@ struct secure_allocator : public std::allocator<T>
         T *p;
         p = std::allocator<T>::allocate(n, hint);
         if (p != NULL)
-            mlock(p, sizeof(T) * n);
+            LockedPageManager::instance.LockRange(p, sizeof(T) * n);
         return p;
     }
 
@@ -78,7 +213,7 @@ struct secure_allocator : public std::allocator<T>
         if (p != NULL)
         {
             memset(p, 0, sizeof(T) * n);
-            munlock(p, sizeof(T) * n);
+            LockedPageManager::instance.UnlockRange(p, sizeof(T) * n);
         }
         std::allocator<T>::deallocate(p, n);
     }