[PATCH 18/23] x86/mm: Allow temporary mms when IRQs are on

[Andy Lutomirski <luto@kernel.org>]

EFI runtime services should use temporary mms, but EFI runtime services
want IRQs on.  Preemption must still be disabled in a temporary mm context.

At some point, the entirely temporary mm mechanism should be moved out of
arch code.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
---
 arch/x86/mm/tlb.c | 19 ++++++++++++-------
 1 file changed, 12 insertions(+), 7 deletions(-)

diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
index 4e371f30e2ab..36ce9dffb963 100644
--- a/arch/x86/mm/tlb.c
+++ b/arch/x86/mm/tlb.c
@@ -708,18 +708,23 @@ void enter_lazy_tlb(struct mm_struct *mm, struct task_struct *tsk)
  * that override the kernel memory protections (e.g., W^X), without exposing the
  * temporary page-table mappings that are required for these write operations to
  * other CPUs. Using a temporary mm also allows to avoid TLB shootdowns when the
- * mapping is torn down.
+ * mapping is torn down.  Temporary mms can also be used for EFI runtime service
+ * calls or similar functionality.
  *
- * Context: The temporary mm needs to be used exclusively by a single core. To
- *          harden security IRQs must be disabled while the temporary mm is
- *          loaded, thereby preventing interrupt handler bugs from overriding
- *          the kernel memory protection.
+ * It is illegal to schedule while using a temporary mm -- the context switch
+ * code is unaware of the temporary mm and does not know how to context switch.
+ * Use a real (non-temporary) mm in a kernel thread if you need to sleep.
+ *
+ * Note: For sensitive memory writes, the temporary mm needs to be used
+ *       exclusively by a single core, and IRQs should be disabled while the
+ *       temporary mm is loaded, thereby preventing interrupt handler bugs from
+ *       overriding the kernel memory protection.
  */
 temp_mm_state_t use_temporary_mm(struct mm_struct *mm)
 {
 	temp_mm_state_t temp_state;
 
-	lockdep_assert_irqs_disabled();
+	lockdep_assert_preemption_disabled();
 
 	/*
 	 * Make sure not to be in TLB lazy mode, as otherwise we'll end up
@@ -751,7 +756,7 @@ temp_mm_state_t use_temporary_mm(struct mm_struct *mm)
 
 void unuse_temporary_mm(temp_mm_state_t prev_state)
 {
-	lockdep_assert_irqs_disabled();
+	lockdep_assert_preemption_disabled();
 	switch_mm_irqs_off(NULL, prev_state.mm, current);
 
 	/*
-- 
2.33.1