Fix overflow in CommandAllocator

It was essentially checking "currentPtr + commandSize <= endPtr" and
commandSize could make currentPtr overflow, making the comparison
succeed when it shouldn't have. This was caught through flakiness of the
LargeCommands allocator test.

Added a test provoking an overflow in Allocate and checking nullptr is
returned.

BUG=

Change-Id: I8ae4dad5b33c9d2005027c4d45b110ee0c65dd9a
Reviewed-on: https://dawn-review.googlesource.com/c/2841
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Stephen White <senorblanco@chromium.org>

src/dawn_native/CommandAllocator.cpp

@@ -177,34 +177,57 @@ namespace dawn_native {
 
         // It should always be possible to allocate one id, for EndOfBlock tagging,
         ASSERT(IsPtrAligned(mCurrentPtr, alignof(uint32_t)));
-        ASSERT(mCurrentPtr + sizeof(uint32_t) <= mEndPtr);
+        ASSERT(mEndPtr >= mCurrentPtr);
+        ASSERT(static_cast<size_t>(mEndPtr - mCurrentPtr) >= sizeof(uint32_t));
+
+        // The memory after the ID will contain the following:
+        //   - the current ID
+        //   - padding to align the command, maximum kMaxSupportedAlignment
+        //   - the command of size commandSize
+        //   - padding to align the next ID, maximum alignof(uint32_t)
+        //   - the next ID of size sizeof(uint32_t)
+        //
+        // To avoid checking for overflows at every step of the computations we compute an upper
+        // bound of the space that will be needed in addition to the command data.
+        static constexpr size_t kWorstCaseAdditionalSize =
+            sizeof(uint32_t) + kMaxSupportedAlignment + alignof(uint32_t) + sizeof(uint32_t);
+
+        // This can't overflow because by construction mCurrentPtr always has space for the next ID.
+        size_t remainingSize = static_cast<size_t>(mEndPtr - mCurrentPtr);
+
+        // The good case were we have enough space for the command data and upper bound of the
+        // extra required space.
+        if ((remainingSize >= kWorstCaseAdditionalSize) &&
+            (remainingSize - kWorstCaseAdditionalSize >= commandSize)) {
             uint32_t* idAlloc = reinterpret_cast<uint32_t*>(mCurrentPtr);
+            *idAlloc = commandId;
 
             uint8_t* commandAlloc = AlignPtr(mCurrentPtr + sizeof(uint32_t), commandAlignment);
-        uint8_t* nextPtr = AlignPtr(commandAlloc + commandSize, alignof(uint32_t));
+            mCurrentPtr = AlignPtr(commandAlloc + commandSize, alignof(uint32_t));
 
-        // When there is not enough space, we signal the EndOfBlock, so that the iterator nows to
+            return commandAlloc;
+        }
+
+        // When there is not enough space, we signal the EndOfBlock, so that the iterator knows to
         // move to the next one. EndOfBlock on the last block means the end of the commands.
-        if (nextPtr + sizeof(uint32_t) > mEndPtr) {
+        uint32_t* idAlloc = reinterpret_cast<uint32_t*>(mCurrentPtr);
-            // Even if we are not able to get another block, the list of commands will be
-            // well-formed and iterable as this block will be that last one.
         *idAlloc = EndOfBlock;
 
-            // Make sure we have space for current allocation, plus end of block and alignment
+        // We'll request a block that can contain at least the command ID, the command and an
-            // padding for the first id.
+        // additional ID to contain the EndOfBlock tag.
-            ASSERT(nextPtr > mCurrentPtr);
+        size_t requestedBlockSize = commandSize + kWorstCaseAdditionalSize;
-            if (!GetNewBlock(static_cast<size_t>(nextPtr - mCurrentPtr) + sizeof(uint32_t) +
+
-                             alignof(uint32_t))) {
+        // The computation of the request could overflow.
+        if (DAWN_UNLIKELY(requestedBlockSize <= commandSize)) {
+            return nullptr;
+        }
+
+        if (DAWN_UNLIKELY(!GetNewBlock(requestedBlockSize))) {
             return nullptr;
         }
         return Allocate(commandId, commandSize, commandAlignment);
     }
 
-        *idAlloc = commandId;
-        mCurrentPtr = nextPtr;
-        return commandAlloc;
-    }
-
     uint8_t* CommandAllocator::AllocateData(size_t commandSize, size_t commandAlignment) {
         return Allocate(AdditionalData, commandSize, commandAlignment);
     }
@@ -215,7 +238,7 @@ namespace dawn_native {
             std::max(minimumSize, std::min(mLastAllocationSize * 2, size_t(16384)));
 
         uint8_t* block = reinterpret_cast<uint8_t*>(malloc(mLastAllocationSize));
-        if (block == nullptr) {
+        if (DAWN_UNLIKELY(block == nullptr)) {
             return false;
         }
 

src/dawn_native/CommandAllocator.h

@@ -34,9 +34,8 @@ namespace dawn_native {
     //
     //     CommandIterator commands(allocator);
     //     CommandType type;
-    //     void* command;
+    //     while(commands.NextCommandId(&type)) {
-    //     while(commands.NextCommandId(&e)) {
+    //         switch(type) {
-    //         switch(e) {
     //              case CommandType::Draw:
     //                  DrawCommand* draw = commands.NextCommand<DrawCommand>();
     //                  // Do the draw
@@ -113,16 +112,22 @@ namespace dawn_native {
         T* Allocate(E commandId) {
             static_assert(sizeof(E) == sizeof(uint32_t), "");
             static_assert(alignof(E) == alignof(uint32_t), "");
+            static_assert(alignof(T) <= kMaxSupportedAlignment, "");
             return reinterpret_cast<T*>(
                 Allocate(static_cast<uint32_t>(commandId), sizeof(T), alignof(T)));
         }
 
         template <typename T>
         T* AllocateData(size_t count) {
+            static_assert(alignof(T) <= kMaxSupportedAlignment, "");
             return reinterpret_cast<T*>(AllocateData(sizeof(T) * count, alignof(T)));
         }
 
       private:
+        // This is used for some internal computations and can be any power of two as long as code
+        // using the CommandAllocator passes the static_asserts.
+        static constexpr size_t kMaxSupportedAlignment = 8;
+
         friend CommandIterator;
         CommandBlocks&& AcquireBlocks();
 
@@ -134,7 +139,7 @@ namespace dawn_native {
         size_t mLastAllocationSize = 2048;
 
         // Pointers to the current range of allocation in the block. Guaranteed to allow for at
-        // least one uint32_t is not nullptr, so that the special EndOfBlock command id can always
+        // least one uint32_t if not nullptr, so that the special EndOfBlock command id can always
         // be written. Nullptr iff the blocks were moved out.
         uint8_t* mCurrentPtr = nullptr;
         uint8_t* mEndPtr = nullptr;

src/tests/unittests/CommandAllocatorTests.cpp

@@ -16,6 +16,8 @@
 
 #include "dawn_native/CommandAllocator.h"
 
+#include <limits>
+
 using namespace dawn_native;
 
 // Definition of the command types used in the tests
@@ -362,3 +364,40 @@ TEST(CommandAllocator, EmptyIterator) {
         iterator2.DataWasDestroyed();
     }
 }
+
+template <size_t A>
+struct alignas(A) AlignedStruct {
+    char dummy;
+};
+
+// Test for overflows in Allocate's computations, size 1 variant
+TEST(CommandAllocator, AllocationOverflow_1) {
+    CommandAllocator allocator;
+    AlignedStruct<1>* data =
+        allocator.AllocateData<AlignedStruct<1>>(std::numeric_limits<size_t>::max() / 1);
+    ASSERT_EQ(data, nullptr);
+}
+
+// Test for overflows in Allocate's computations, size 2 variant
+TEST(CommandAllocator, AllocationOverflow_2) {
+    CommandAllocator allocator;
+    AlignedStruct<2>* data =
+        allocator.AllocateData<AlignedStruct<2>>(std::numeric_limits<size_t>::max() / 2);
+    ASSERT_EQ(data, nullptr);
+}
+
+// Test for overflows in Allocate's computations, size 4 variant
+TEST(CommandAllocator, AllocationOverflow_4) {
+    CommandAllocator allocator;
+    AlignedStruct<4>* data =
+        allocator.AllocateData<AlignedStruct<4>>(std::numeric_limits<size_t>::max() / 4);
+    ASSERT_EQ(data, nullptr);
+}
+
+// Test for overflows in Allocate's computations, size 8 variant
+TEST(CommandAllocator, AllocationOverflow_8) {
+    CommandAllocator allocator;
+    AlignedStruct<8>* data =
+        allocator.AllocateData<AlignedStruct<8>>(std::numeric_limits<size_t>::max() / 8);
+    ASSERT_EQ(data, nullptr);
+}