x86/speculation/mds: Clear CPU buffers on exit to user

Add a static key which controls the invocation of the CPU buffer clear
mechanism on exit to user space and add the call into
prepare_exit_to_usermode() and do_nmi() right before actually returning.

Add documentation which kernel to user space transition this covers and
explain why some corner cases are not mitigated.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Reviewed-by: Jon Masters <jcm@redhat.com>
Tested-by: Jon Masters <jcm@redhat.com>

1 files changed, 3 insertions, 0 deletions

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 01874d54f4fd..dbb45014de1b 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -63,6 +63,9 @@ DEFINE_STATIC_KEY_FALSE(switch_mm_cond_ibpb);
 /* Control unconditional IBPB in switch_mm() */
 DEFINE_STATIC_KEY_FALSE(switch_mm_always_ibpb);
 
+/* Control MDS CPU buffer clear before returning to user space */
+DEFINE_STATIC_KEY_FALSE(mds_user_clear);
+
 void __init check_bugs(void)
 {
 	identify_boot_cpu();