[Spice-devel] [PATCH 1/2] Ensure that plugins cannot bypass version check
Frediano Ziglio
fziglio at redhat.com
Fri Mar 23 15:49:14 UTC 2018
>
> From: Christophe de Dinechin <dinechin at redhat.com>
>
> This change addresses three issues related to plugin version checking:
>
> 1. It is possible for plugins to bypass version checking or do it wrong
> (as a matter of fact, the mjpeg fallback sets a bad example)
>
> 2. The current plugin version check violates the C++ ODR, i.e.
> it relies on undefined behaviors when the header used to compile
> the plugin and to compile the agent are not identical,
>
> 3. A major.minor numbering scheme is not ideal for ABI checks.
> In particular, it makes it difficult to react to an incompatibility
> that was detected post release.
>
> [More info]
>
> 1. Make it impossible to bypass version checking
>
> The current code depends on the plugin implementing the version check
> correctly by calling PluginVersionIsCompatible. To make things worse,
> the only publicly available example gets this wrong and uses an
> ad-hoc version check, so anybody copy-pasting this code will get it
> wrong.
>
> It is more robust to do the version check in the agent before calling
> any method in the plugin. It ensures that version checking cannot be
> bypassed, is done consistently and generates consistent error messages.
>
> As an indication that the aproach is robust, the new check correctly
> refuses to load older plugins that use the old version checking method:
>
> spice-streaming-agent[5167]:
> error loading plugin [...].so: no version information
>
This just proves that the ABI changed, not that the check is robust,
I'd remove this comment.
Is more robust as the init function is not called before checking the
version but you already said this above.
OT: Technically to call dlsym you already loaded the shared object into
memory so you could in theory already execute some code form old plugin
but if at this stage you are not able to unload it is a bug in the plugin.
> 2. ODR-related problems
>
> The C++ One Definition Rule (ODR) states that all translation units
> must see the same definitions. In the current code, when we call
> Agent::PluginVersionIsCompatible from the plugin, it is an ODR
> violation as soon as we have made any change in the Agent class
> compared to what the plugin was compiled against.
>
> The current code therefore relies on implementation dependent knowlege
> of how virtual functions are laid out in the vtable, and puts
> unnecessary constraints on the changes allowed in the classes
> (e.g. it's not allowed to put anything before some member functions)
>
How in the new schema are you updating the interface?
Is not clear how this patch is going to solve this, can you provide
an example?
> 3. Major.minor numbering scheme
>
> The major.minor numbering scheme initially selected makes it harder
> to fixes cases where an incompatibility was detected after release.
>
> For example, the major.minor version checking assumes that agent 1.21
> is compatible with plugins 1.21, 1.13 or 1.03. If later testing
> shows that 1.13 actually introduced an incompatiliy, you have to
> special-case 1.13 in the compatibiliy check.
>
typos: incompatiliy, compatibiliy
> An approach that does not have this problem is to rely on incremented
> version numbers, with a "current" and "oldest compatible" version
> number. This is used for example by libtools [1].
>
> Since the change required for #1 and #2 introduces an ABI break,
> it is as good a time as any to also change the numbering scheme,
> since changing it later would introduce another unnecessary ABI break.
>
> [1]
> https://www.gnu.org/software/libtool/manual/html_node/Libtool-versioning.html
>
> Signed-off-by: Christophe de Dinechin <dinechin at redhat.com>
> ---
> include/spice-streaming-agent/plugin.hpp | 50
> +++++++++++++++++---------------
> src/concrete-agent.cpp | 35 +++++++++++-----------
> src/concrete-agent.hpp | 4 ---
> src/mjpeg-fallback.cpp | 3 --
> 4 files changed, 45 insertions(+), 47 deletions(-)
>
> diff --git a/include/spice-streaming-agent/plugin.hpp
> b/include/spice-streaming-agent/plugin.hpp
> index e08e3a6..0ec5040 100644
> --- a/include/spice-streaming-agent/plugin.hpp
> +++ b/include/spice-streaming-agent/plugin.hpp
> @@ -23,11 +23,22 @@ namespace streaming_agent {
> class FrameCapture;
>
> /*!
> - * Plugin version, only using few bits, schema is 0xMMmm
> - * where MM is major and mm is the minor, can be easily expanded
> - * using more bits in the future
> + * Plugins use a versioning system similar to that implemented by libtool
> + *
> http://www.gnu.org/software/libtool/manual/html_node/Libtool-versioning.html
> + * Update the version information as follows:
> + * [ANY CHANGE] If any interfaces have been added, removed, or changed since
> the last update,
> + * increment PluginInterfaceVersion.
> + * [COMPATIBLE CHANGE] If interfaces have only been added since the last
> public release,
> + * leave PluginInterfaceOldestCompatibleVersion identical.
> + * [INCOMPATIBLE CHANGE] If any interfaces have been removed or changed
> since the last release,
> + * set PluginInterfaceOldestCompatibleVersion to PluginInterfaceVersion.
> + * [DETECTED INCOMPATIBILITY]: If an incompatibility is detected after a
> release,
> + * set PluginInterfaceOldestCompatibleVersion to the last known compatible
> version.
It seems that changing PluginInterfaceOldestCompatibleVersion is like
bumping the major but I think I'll get the reply when I got an explanation
how the interface is updated.
> */
> -enum Constants : unsigned { PluginVersion = 0x100u };
> +enum Constants : unsigned {
> + PluginInterfaceVersion = 1,
> + PluginInterfaceOldestCompatibleVersion = 1
Does the plugin needs to know PluginInterfaceOldestCompatibleVersion?
> +};
>
> enum Ranks : unsigned {
> /// this plugin should not be used
> @@ -103,20 +114,6 @@ class Agent
> {
> public:
> /*!
> - * Get agent version.
> - * Plugin should check the version for compatibility before doing
> - * everything.
> - * \return version specified like PluginVersion
> - */
> - virtual unsigned Version() const = 0;
> -
> - /*!
> - * Check if a given plugin version is compatible with this agent
> - * \return true is compatible
> - */
> - virtual bool PluginVersionIsCompatible(unsigned pluginVersion) const =
> 0;
> -
> - /*!
> * Register a plugin in the system.
> */
> virtual void Register(Plugin& plugin) = 0;
> @@ -136,18 +133,25 @@ typedef bool
> PluginInitFunc(spice::streaming_agent::Agent* agent);
>
> #ifndef SPICE_STREAMING_AGENT_PROGRAM
> /*!
> + * Plugin interface version
> + * Each plugin should define this variable and set it to
> PluginInterfaceVersion
> + * That version will be checked by the agent before executing any plugin
> code
> + */
> +extern "C" unsigned spice_streaming_agent_plugin_interface_version;
> +
> +/*!
> * Plugin main entry point.
> - * Plugins should check if the version of the agent is compatible.
> - * If is compatible should register itself to the agent and return
> - * true.
> - * If is not compatible can decide to stay in memory or not returning
> - * true (do not unload) or false (safe to unload). This is necessary
> + * This entry point is only called if the version check passed.
> + * It should return true if it loaded and initialized successfully.
> + * If the plugin does not initialize and does not want to be unloaded,
> + * it may still return true on failure. This is necessary
> * if the plugin uses some library which are not safe to be unloaded.
> * This public interface is also designed to avoid exporting data from
> * the plugin which could be a problem in some systems.
> * \return true if plugin should stay loaded, false otherwise
> */
> extern "C" spice::streaming_agent::PluginInitFunc
> spice_streaming_agent_plugin_init;
> +
> #endif
>
> #endif // SPICE_STREAMING_AGENT_PLUGIN_HPP
> diff --git a/src/concrete-agent.cpp b/src/concrete-agent.cpp
> index 4cf70e7..eb4f333 100644
> --- a/src/concrete-agent.cpp
> +++ b/src/concrete-agent.cpp
> @@ -15,28 +15,11 @@
>
> using namespace spice::streaming_agent;
>
> -static inline unsigned MajorVersion(unsigned version)
> -{
> - return version >> 8;
> -}
> -
> -static inline unsigned MinorVersion(unsigned version)
> -{
> - return version & 0xffu;
> -}
> -
> ConcreteAgent::ConcreteAgent()
> {
> options.push_back(ConcreteConfigureOption(nullptr, nullptr));
> }
>
> -bool ConcreteAgent::PluginVersionIsCompatible(unsigned pluginVersion) const
> -{
> - unsigned version = Version();
> - return MajorVersion(version) == MajorVersion(pluginVersion) &&
> - MinorVersion(version) >= MinorVersion(pluginVersion);
> -}
> -
> void ConcreteAgent::Register(Plugin& plugin)
> {
> plugins.push_back(std::shared_ptr<Plugin>(&plugin));
> @@ -83,6 +66,24 @@ void ConcreteAgent::LoadPlugin(const std::string
> &plugin_filename)
> return;
> }
>
> + unsigned *version =
> + (unsigned *) dlsym(dl,
> "spice_streaming_agent_plugin_interface_version");
> + if (!version) {
> + syslog(LOG_ERR, "error loading plugin %s: no version information",
> + plugin_filename.c_str());
> + return;
> + }
> + if (*version < PluginInterfaceOldestCompatibleVersion ||
> + *version > PluginInterfaceVersion) {
> + syslog(LOG_ERR,
> + "error loading plugin %s: plugin interface version %u, "
> + "agent accepts version %u...%u",
> + plugin_filename.c_str(), *version,
> + PluginInterfaceOldestCompatibleVersion,
> + PluginInterfaceVersion);
> + return;
> + }
> +
> try {
> PluginInitFunc* init_func =
> (PluginInitFunc *) dlsym(dl,
> "spice_streaming_agent_plugin_init");
> diff --git a/src/concrete-agent.hpp b/src/concrete-agent.hpp
> index 5bca23b..c631916 100644
> --- a/src/concrete-agent.hpp
> +++ b/src/concrete-agent.hpp
> @@ -27,16 +27,12 @@ class ConcreteAgent final : public Agent
> {
> public:
> ConcreteAgent();
> - unsigned Version() const override {
> - return PluginVersion;
> - }
> void Register(Plugin& plugin) override;
> const ConfigureOption* Options() const override;
> void LoadPlugins(const std::string &directory);
> // pointer must remain valid
> void AddOption(const char *name, const char *value);
> FrameCapture *GetBestFrameCapture(const std::set<SpiceVideoCodecType>&
> codecs);
> - bool PluginVersionIsCompatible(unsigned pluginVersion) const override;
> private:
> void LoadPlugin(const std::string &plugin_filename);
> std::vector<std::shared_ptr<Plugin>> plugins;
> diff --git a/src/mjpeg-fallback.cpp b/src/mjpeg-fallback.cpp
> index 68c282f..605a4b3 100644
> --- a/src/mjpeg-fallback.cpp
> +++ b/src/mjpeg-fallback.cpp
> @@ -180,9 +180,6 @@ SpiceVideoCodecType MjpegPlugin::VideoCodecType() const {
>
> bool MjpegPlugin::Register(Agent* agent)
> {
> - if (agent->Version() != PluginVersion)
> - return false;
> -
> std::unique_ptr<MjpegPlugin> plugin(new MjpegPlugin());
>
> try {
Frediano
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