arm64: Move BP hardening to check_and_switch_context

We call arm64_apply_bp_hardening() from post_ttbr_update_workaround,
which has the unexpected consequence of being triggered on every
exception return to userspace when ARM64_SW_TTBR0_PAN is selected,
even if no context switch actually occured.

This is a bit suboptimal, and it would be more logical to only
invalidate the branch predictor when we actually switch to
a different mm.

In order to solve this, move the call to arm64_apply_bp_hardening()
into check_and_switch_context(), where we're guaranteed to pick
a different mm context.

Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

1 files changed, 3 insertions, 2 deletions

diff --git a/arch/arm64/mm/context.c b/arch/arm64/mm/context.c
index ff99a880a730..301417ae2ba8 100644
--- a/arch/arm64/mm/context.c
+++ b/arch/arm64/mm/context.c
@@ -234,6 +234,9 @@ void check_and_switch_context(struct mm_struct *mm, unsigned int cpu)
         raw_spin_unlock_irqrestore(&cpu_asid_lock, flags);
 
 switch_mm_fastpath:
+
+        arm64_apply_bp_hardening();
+
         /*
          * Defer TTBR0_EL1 setting for user threads to uaccess_enable() when
          * emulating PAN.
@@ -249,8 +252,6 @@ asmlinkage void post_ttbr_update_workaround(void)
                         "ic iallu; dsb nsh; isb",
                         ARM64_WORKAROUND_CAVIUM_27456,
                         CONFIG_CAVIUM_ERRATUM_27456));
-
-        arm64_apply_bp_hardening();
 }
 
 static int asids_init(void)