security: ByteBuffer: fix heap buffer overflow on slice realloc

Motivation:

ByteBuffer had a very bad (exploitable!) security vulnerability if the
following conditions are all true:

- user writes to a ByteBuffer which is a slice (slice.lowerBound != 0)
- the slice is uniquely referenced (ie. the buffer that it was sliced
  from is gone)
- the write triggers a re-allocation

Then the slice is actually _larger_ than the overall available capacity
so another write to said ByteBuffer could end up out of bounds.

Modifications:

- fixed slice reallocation

Result:

- fixed security vulnerability

Sources/NIO/ByteBuffer-core.swift

@@ -218,12 +218,13 @@ public struct ByteBuffer {
             return new
         }
 
-        public func reallocStorage(capacity: Capacity) {
-            let ptr = self.allocator.realloc(self.bytes, Int(capacity))
+        public func reallocStorage(capacity minimumNeededCapacity: Capacity) {
+            let newCapacity = minimumNeededCapacity.nextPowerOf2ClampedToMax()
+            let ptr = self.allocator.realloc(self.bytes, Int(newCapacity))
             /* bind the memory so we can assume it elsewhere to be bound to UInt8 */
-            ptr.bindMemory(to: UInt8.self, capacity: Int(capacity))
+            ptr.bindMemory(to: UInt8.self, capacity: Int(newCapacity))
             self.bytes = ptr
-            self.capacity = capacity
+            self.capacity = newCapacity
             self.fullSlice = 0..<self.capacity
         }
 
@@ -268,23 +269,28 @@ public struct ByteBuffer {
     private mutating func ensureAvailableCapacity(_ capacity: Capacity, at index: Index) {
         assert(isKnownUniquelyReferenced(&self._storage))
 
-        if self._slice.lowerBound + index + capacity > self._slice.upperBound {
-            // double the capacity, we may want to use different strategies depending on the actual current capacity later on.
-            var newCapacity = max(1, toCapacity(self.capacity))
-
-            // double the capacity until the requested capacity can be full-filled
-            repeat {
-                precondition(newCapacity != Capacity.max, "cannot make ByteBuffers larger than \(newCapacity)")
-                if newCapacity < (Capacity.max >> 1) {
-                    newCapacity = newCapacity << 1
+        let totalNeededCapacityWhenKeepingSlice = self._slice.lowerBound + index + capacity
+        if totalNeededCapacityWhenKeepingSlice > self._slice.upperBound {
+            // we need to at least adjust the slice's upper bound which we can do as we're the unique owner of the storage,
+            // let's see if adjusting the slice's upper bound buys us enough storage
+            if totalNeededCapacityWhenKeepingSlice > self._storage.capacity {
+                let newStorageMinCapacity = index + capacity
+                // nope, we need to actually re-allocate again. If our slice does not start at 0, let's also rebase
+                if self._slice.lowerBound == 0 {
+                    self._storage.reallocStorage(capacity: newStorageMinCapacity)
                 } else {
-                    newCapacity = Capacity.max
+                    self._storage = self._storage.reallocSlice(self._slice.lowerBound ..< self._slice.upperBound,
+                                                               capacity: newStorageMinCapacity)
                 }
-            } while newCapacity < index || newCapacity - index < capacity
-
-            self._storage.reallocStorage(capacity: newCapacity)
-            self._slice = _slice.lowerBound..<_slice.lowerBound + newCapacity
+                self._slice = self._storage.fullSlice
+            } else {
+                // yes, let's just extend the slice until the end of the buffer
+                self._slice = _slice.lowerBound ..< self._storage.capacity
+            }
         }
+        assert(self._slice.lowerBound + index + capacity <= self._slice.upperBound)
+        assert(self._slice.lowerBound >= 0, "illegal slice: negative lower bound: \(self._slice.lowerBound)")
+        assert(self._slice.upperBound <= self._storage.capacity, "illegal slice: upper bound (\(self._slice.upperBound)) exceeds capacity: \(self._storage.capacity)")
     }
 
     // MARK: Internal API

Tests/NIOTests/ByteBufferTest+XCTest.swift

@@ -30,6 +30,8 @@ extension ByteBufferTest {
                 ("testEqualsComparesReadBuffersOnly", testEqualsComparesReadBuffersOnly),
                 ("testSimpleReadTest", testSimpleReadTest),
                 ("testSimpleWrites", testSimpleWrites),
+                ("testMakeSureUniquelyOwnedSliceDoesNotGetReallocatedOnWrite", testMakeSureUniquelyOwnedSliceDoesNotGetReallocatedOnWrite),
+                ("testWriteToUniquelyOwnedSliceWhichTriggersAReallocation", testWriteToUniquelyOwnedSliceWhichTriggersAReallocation),
                 ("testReadWrite", testReadWrite),
                 ("testStaticStringReadTests", testStaticStringReadTests),
                 ("testString", testString),

Tests/NIOTests/ByteBufferTest.swift

@@ -94,6 +94,39 @@ class ByteBufferTest: XCTestCase {
         XCTAssertEqual(6, buf.readableBytes)
     }
 
+    func makeSliceToBufferWhichIsDeallocated() -> ByteBuffer {
+        var buf = self.allocator.buffer(capacity: 16)
+        let oldCapacity = buf.capacity
+        buf.write(bytes: 0..<16)
+        XCTAssertEqual(oldCapacity, buf.capacity)
+        return buf.getSlice(at: 15, length: 1)!
+    }
+
+    func testMakeSureUniquelyOwnedSliceDoesNotGetReallocatedOnWrite() {
+        var slice = self.makeSliceToBufferWhichIsDeallocated()
+        XCTAssertEqual(1, slice.capacity)
+        let oldStorageBegin = slice.withUnsafeReadableBytes { ptr in
+            return UInt(bitPattern: ptr.baseAddress!)
+        }
+        slice.set(integer: 1 as UInt8, at: 0)
+        let newStorageBegin = slice.withUnsafeReadableBytes { ptr in
+            return UInt(bitPattern: ptr.baseAddress!)
+        }
+        XCTAssertEqual(oldStorageBegin, newStorageBegin)
+    }
+
+    func testWriteToUniquelyOwnedSliceWhichTriggersAReallocation() {
+        var slice = self.makeSliceToBufferWhichIsDeallocated()
+        XCTAssertEqual(1, slice.capacity)
+        // this will cause a re-allocation, the whole buffer should be 32 bytes then, the slice having 17 of that.
+        // this fills 16 bytes so will still fit
+        slice.write(bytes: Array(16..<32))
+        XCTAssertEqual(Array(15..<32), slice.readBytes(length: slice.readableBytes)!)
+
+        // and this will need another re-allocation
+        slice.write(bytes: Array(32..<47))
+    }
+
     func testReadWrite() {
         buf.write(string: "X")
         buf.write(string: "Y")