release/5.2.37/arm64-ssbs-Fix-context-switch-when-SSBS-is-present-o.patch

From bdb4fe359ed037674bec973980d462e65e37ffbc Mon Sep 17 00:00:00 2001
From: Will Deacon <will@kernel.org>
Date: Thu, 6 Feb 2020 10:42:58 +0000
Subject: [PATCH] arm64: ssbs: Fix context-switch when SSBS is present on all
 CPUs

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@vger.kernel.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Srinivas Ramana <sramana@codeaurora.org>
Fixes: cbdf8a189a66 ("arm64: Force SSBS on context switch")
Reviewed-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Will Deacon <will@kernel.org>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>

diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
index 398601b29627..c6d7a2912e30 100644
--- a/arch/arm64/kernel/process.c
+++ b/arch/arm64/kernel/process.c
@@ -455,6 +455,13 @@ static void ssbs_thread_switch(struct task_struct *next)
 	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))
-- 
2.7.4