Re: [PATCH v7 03/11] task_isolation: support PR_TASK_ISOLATION_STRICT mode

On Mon, Sep 28, 2015 at 2:54 PM, Chris Metcalf [off-list ref] wrote:

On 09/28/2015 04:51 PM, Andy Lutomirski wrote:

quoted

On Mon, Sep 28, 2015 at 11:17 AM, Chris Metcalf [off-list ref]

quoted

@@ -35,8 +36,12 @@ static inline enum ctx_state exception_enter(void)
                 return 0;

         prev_ctx = this_cpu_read(context_tracking.state);
-       if (prev_ctx != CONTEXT_KERNEL)
-               context_tracking_exit(prev_ctx);
+       if (prev_ctx != CONTEXT_KERNEL) {
+               if (context_tracking_exit(prev_ctx)) {
+                       if (task_isolation_strict())
+                               task_isolation_exception();
+               }
+       }

         return prev_ctx;
  }

x86 does not promise to call this function.  In fact, x86 is rather
likely to stop ever calling this function in the reasonably near
future.

Yes, in which case we'd have to do it the same way we are doing
it for arm64 (see patch 09/11), by calling task_isolation_exception()
explicitly from within the relevant exception handlers.  If we start
doing that, it's probably worth wrapping up the logic into a single
inline function to keep the added code short and sweet.

If in fact this might happen in the short term, it might be a good
idea to hook the individual exception handlers in x86 now, and not
hook the exception_enter() mechanism at all.

It's already like that in Linus' tree.

FWIW, most of those exception handlers send signals, so it might pay
to do it in notify_die or die instead.

quoted

quoted

--- a/kernel/context_tracking.c
+++ b/kernel/context_tracking.c

@@ -144,15 +144,16 @@ NOKPROBE_SYMBOL(context_tracking_user_enter);
   * This call supports re-entrancy. This way it can be called from any

exception
   * handler without needing to know if we came from userspace or not.
   */
-void context_tracking_exit(enum ctx_state state)
+bool context_tracking_exit(enum ctx_state state)

This needs clear documentation of what the return value means.

Added:

 * Return: if called with state == CONTEXT_USER, the function returns
 * true if we were in fact previously in user mode.

This should note that it only returns true if context tracking is on.

quoted

quoted

+void task_isolation_exception(void)
+{
+       pr_warn("%s/%d: task_isolation strict mode violated by
exception\n",
+               current->comm, current->pid);
+       kill_task_isolation_strict_task();
+}

Should this say what exception?

I could modify it to take a string argument (and then use it for
the arm64 case at least).  For the exception_enter() caller, we actually
don't have the information available to pass down, and it would
be hard to get it.

For x86, the relevant info might be the actual hw error number
(error_code, which makes it into die) or the signal.  If we send a
death signal, then reporting the error number the usual way might make
sense.

--Andy