[PATCH 5.4 083/244] x86/microcode: Check for offline CPUs before requesting new microcode

From: Otavio Pontes otavio.pontes@intel.com

[ Upstream commit 7189b3c11903667808029ec9766a6e96de5012a5 ]

Currently, the late microcode loading mechanism checks whether any CPUs are offlined, and, in such a case, aborts the load attempt.

However, this must be done before the kernel caches new microcode from the filesystem. Otherwise, when offlined CPUs are onlined later, those cores are going to be updated through the CPU hotplug notifier callback with the new microcode, while CPUs previously onine will continue to run with the older microcode.

For example:

Turn off one core (2 threads):

echo 0 > /sys/devices/system/cpu/cpu3/online echo 0 > /sys/devices/system/cpu/cpu1/online

Install the ucode fails because a primary SMT thread is offline:

cp intel-ucode/06-8e-09 /lib/firmware/intel-ucode/ echo 1 > /sys/devices/system/cpu/microcode/reload bash: echo: write error: Invalid argument

Turn the core back on

echo 1 > /sys/devices/system/cpu/cpu3/online echo 1 > /sys/devices/system/cpu/cpu1/online cat /proc/cpuinfo |grep microcode microcode : 0x30 microcode : 0xde microcode : 0x30 microcode : 0xde

The rationale for why the update is aborted when at least one primary thread is offline is because even if that thread is soft-offlined and idle, it will still have to participate in broadcasted MCE's synchronization dance or enter SMM, and in both examples it will execute instructions so it better have the same microcode revision as the other cores.

[ bp: Heavily edit and extend commit message with the reasoning behind all this. ]

Fixes: 30ec26da9967 ("x86/microcode: Do not upload microcode if CPUs are offline") Signed-off-by: Otavio Pontes otavio.pontes@intel.com Signed-off-by: Borislav Petkov bp@suse.de Reviewed-by: Tony Luck tony.luck@intel.com Acked-by: Ashok Raj ashok.raj@intel.com Link: https://lkml.kernel.org/r/20210319165515.9240-2-otavio.pontes@intel.com Signed-off-by: Sasha Levin sashal@kernel.org --- arch/x86/kernel/cpu/microcode/core.c | 8 ++++---- 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kernel/cpu/microcode/core.c b/arch/x86/kernel/cpu/microcode/core.c index cb0fdcaf1415..4a4198b806b4 100644 --- a/arch/x86/kernel/cpu/microcode/core.c +++ b/arch/x86/kernel/cpu/microcode/core.c @@ -626,16 +626,16 @@ static ssize_t reload_store(struct device *dev, if (val != 1) return size;

- tmp_ret = microcode_ops->request_microcode_fw(bsp, &microcode_pdev->dev, true); - if (tmp_ret != UCODE_NEW) - return size; - get_online_cpus();

ret = check_online_cpus(); if (ret) goto put;

+ tmp_ret = microcode_ops->request_microcode_fw(bsp, &microcode_pdev->dev, true); + if (tmp_ret != UCODE_NEW) + goto put; + mutex_lock(&microcode_mutex); ret = microcode_reload_late(); mutex_unlock(&microcode_mutex);