From: Will Deacon Date: Thu, 6 Feb 2020 10:42:58 +0000 (+0000) Subject: arm64: ssbs: Fix context-switch when SSBS is present on all CPUs X-Git-Tag: v5.5.5~54 X-Git-Url: https://www.infradead.org/git/?a=commitdiff_plain;h=ab3584014c6c31290520460ae503e27e738c6ed1;p=users%2Fdwmw2%2Flinux.git 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: Cc: Catalin Marinas Cc: Srinivas Ramana Fixes: cbdf8a189a66 ("arm64: Force SSBS on context switch") Reviewed-by: Marc Zyngier Signed-off-by: Will Deacon Signed-off-by: Greg Kroah-Hartman --- diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c index d54586d5b031f..fab013c5ee8c9 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 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))