[PATCH 5.5 27/80] arm64: ssbs: Fix context-switch when SSBS is present on all CPUs

[Greg Kroah-Hartman]

From: Will Deacon <will@xxxxxxxxxx>

commit fca3d33d8ad61eb53eca3ee4cac476d1e31b9008 upstream.

When all CPUs in the system implement the SSBS extension, the SSBS field
in PSTATE is the definitive indication of the mitigation state. Further,
when the CPUs implement the SSBS manipulation instructions (advertised
to userspace via an HWCAP), EL0 can toggle the SSBS field directly and
so we cannot rely on any shadow state such as TIF_SSBD at all.

Avoid forcing the SSBS field in context-switch on such a system, and
simply rely on the PSTATE register instead.

Cc: <stable@xxxxxxxxxxxxxxx>
Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
Cc: Srinivas Ramana <sramana@xxxxxxxxxxxxxx>
Fixes: cbdf8a189a66 ("arm64: Force SSBS on context switch")
Reviewed-by: Marc Zyngier <maz@xxxxxxxxxx>
Signed-off-by: Will Deacon <will@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 arch/arm64/kernel/process.c |    7 +++++++
 1 file changed, 7 insertions(+)

--- a/arch/arm64/kernel/process.c
+++ b/arch/arm64/kernel/process.c
@@ -466,6 +466,13 @@ static void ssbs_thread_switch(struct ta
 	if (unlikely(next->flags & PF_KTHREAD))
 		return;
 
+	/*
+	 * If all CPUs implement the SSBS extension, then we just need to
+	 * context-switch the PSTATE field.
+	 */
+	if (cpu_have_feature(cpu_feature(SSBS)))
+		return;
+
 	/* If the mitigation is enabled, then we leave SSBS clear. */
 	if ((arm64_get_ssbd_state() == ARM64_SSBD_FORCE_ENABLE) ||
 	    test_tsk_thread_flag(next, TIF_SSBD))