Source: https://code.google.com/p/google-security-research/issues/detail?id=664
There is an overflow in the ui::PlatformCursor WebCursor::GetPlatformCursor method. In src/content/common/cursors/webcursor_aurax11.cc&q=webcursor_aurax11.cc, there is the following code:
bitmap.allocN32Pixels(custom_size_.width(), custom_size_.height());
memcpy(bitmap.getAddr32(0, 0), custom_data_.data(), custom_data_.size());
The bitmap buffer is allocated based on the width and height of the custom_size_, but the memcpy is performed using the size of the custom_data_.
These values are set during WebCursor deserialization in src/content/common/cursors/webcursor.cc in WebCursor::Deserialize.
custom_size_ is setfrom two integers that a deserialized from a message and can be between 0 and 1024. custom_data_ is setfrom a vector that is deserialized, and can be any size, unrelated to the width and height. The custom_data_ is verified not to be smaller than the expected pixel buffer based on the width and height, but can be longer.
GetPlatformCursor is called indirectly by RenderWidgetHostImpl::OnSetCursor, which is called in response to aViewHostMsg_SetCursor message from the renderer.
The issue above is in the x11 implementation, but it appears also affect other platform-specific implementations other than the Windows one, which instead reads out of bounds.
I recommend this issue be fixed by changing the check in WebCursor::Deserialize:
if(size_x * size_y * 4 > data_len)return false;
to
if(size_x * size_y * 4 != data_len)return false;
to prevent the issue in all platform-specific implementations.
To reproduce the issue replace WebCursor::Serialize with:
bool WebCursor::Serialize(base::Pickle* pickle) const {if(type_ == WebCursorInfo::TypeCustom){
LOG(WARNING) << "IN SERIALIZE\n";if(!pickle->WriteInt(type_)||!pickle->WriteInt(hotspot_.x())||!pickle->WriteInt(hotspot_.y())||!pickle->WriteInt(2)||!pickle->WriteInt(1)||!pickle->WriteFloat(custom_scale_))return false;}else{if(!pickle->WriteInt(type_)||!pickle->WriteInt(hotspot_.x())||!pickle->WriteInt(hotspot_.y())||!pickle->WriteInt(custom_size_.width())||!pickle->WriteInt(custom_size_.height())||!pickle->WriteFloat(custom_scale_))return false;}
const char* data = NULL;if(!custom_data_.empty())data = &custom_data_[0];if(!pickle->WriteData(data, custom_data_.size()))return false;return SerializePlatformData(pickle);}
and visit the attached html page, with the attached image in the same directory.
Proof of Concept:
https://gitlab.com/exploit-database/exploitdb-bin-sploits/-/raw/main/bin-sploits/39039.zip
