x86/speculation/mds: Improve CPU buffer clear documentation

On x86_64, all returns to usermode go through
prepare_exit_to_usermode(), with the sole exception of do_nmi().
This even includes machine checks -- this was added several years
ago to support MCE recovery.  Update the documentation.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@suse.de>
Cc: Frederic Weisbecker <frederic@kernel.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jon Masters <jcm@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: stable@vger.kernel.org
Fixes: 04dcbdb805 ("x86/speculation/mds: Clear CPU buffers on exit to user")
Link: http://lkml.kernel.org/r/999fa9e126ba6a48e9d214d2f18dbde5c62ac55c.1557865329.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

Documentation/x86/mds.rst

@@ -142,38 +142,13 @@ Mitigation points
    mds_user_clear.
 
    The mitigation is invoked in prepare_exit_to_usermode() which covers
-   most of the kernel to user space transitions. There are a few exceptions
-   which are not invoking prepare_exit_to_usermode() on return to user
-   space. These exceptions use the paranoid exit code.
+   all but one of the kernel to user space transitions.  The exception
+   is when we return from a Non Maskable Interrupt (NMI), which is
+   handled directly in do_nmi().
 
-   - Non Maskable Interrupt (NMI):
-
-     Access to sensible data like keys, credentials in the NMI context is
-     mostly theoretical: The CPU can do prefetching or execute a
-     misspeculated code path and thereby fetching data which might end up
-     leaking through a buffer.
-
-     But for mounting other attacks the kernel stack address of the task is
-     already valuable information. So in full mitigation mode, the NMI is
-     mitigated on the return from do_nmi() to provide almost complete
-     coverage.
-
-   - Machine Check Exception (#MC):
-
-     Another corner case is a #MC which hits between the CPU buffer clear
-     invocation and the actual return to user. As this still is in kernel
-     space it takes the paranoid exit path which does not clear the CPU
-     buffers. So the #MC handler repopulates the buffers to some
-     extent. Machine checks are not reliably controllable and the window is
-     extremly small so mitigation would just tick a checkbox that this
-     theoretical corner case is covered. To keep the amount of special
-     cases small, ignore #MC.
-
-   - Debug Exception (#DB):
-
-     This takes the paranoid exit path only when the INT1 breakpoint is in
-     kernel space. #DB on a user space address takes the regular exit path,
-     so no extra mitigation required.
+   (The reason that NMI is special is that prepare_exit_to_usermode() can
+    enable IRQs.  In NMI context, NMIs are blocked, and we don't want to
+    enable IRQs with NMIs blocked.)
 
 
 2. C-State transition