From:   Will Deacon <will@kernel.org>
To:     linux-arm-kernel@lists.infradead.org
Cc:     mark.rutland@arm.com, kernel-team@android.com,
        Will Deacon <will@kernel.org>, stable@vger.kernel.org,
        Marc Zyngier <maz@kernel.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Srinivas Ramana <sramana@codeaurora.org>
Subject: [PATCH] arm64: ssbs: Fix context-switch when SSBS instructions are present
Date:   Thu,  6 Feb 2020 11:34:10 +0000
Message-Id: <20200206113410.18301-1-will@kernel.org>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Sender: stable-owner@vger.kernel.org
Precedence: bulk

When all CPUs in the system implement the SSBS instructions, we
advertise this via an HWCAP and allow EL0 to toggle the SSBS field
in PSTATE directly. Consequently, the state of the mitigation is not
accurately tracked by the TIF_SSBD thread flag and the PSTATE value
is authoritative.

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: Marc Zyngier <maz@kernel.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Srinivas Ramana <sramana@codeaurora.org>
Fixes: cbdf8a189a66 ("arm64: Force SSBS on context switch")
Signed-off-by: Will Deacon <will@kernel.org>
---
 arch/arm64/kernel/process.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
index d54586d5b031..45e867f40a7a 100644
--- a/arch/arm64/kernel/process.c
+++ b/arch/arm64/kernel/process.c
@@ -466,6 +466,13 @@ static void ssbs_thread_switch(struct task_struct *next)
 	if (unlikely(next->flags & PF_KTHREAD))
 		return;
 
+	/*
+	 * If all CPUs implement the SSBS instructions, 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))
-- 
2.25.0.341.g760bfbb309-goog