This is a note to let you know that I've just added the patch titled
x86/microcode: Default-disable late loading
to the 5.15-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
x86-microcode-default-disable-late-loading.patch
and it can be found in the queue-5.15 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
commit 589bcef52498f37e2cc3adfe305afe7b66d0e81a
Author: Borislav Petkov <bp@xxxxxxx>
Date: Wed May 25 18:12:30 2022 +0200
x86/microcode: Default-disable late loading
[ Upstream commit a77a94f86273ce42a39cb479217dd8d68acfe0ff ]
It is dangerous and it should not be used anyway - there's a nice early
loading already.
Requested-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Signed-off-by: Borislav Petkov <bp@xxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Link: https://lore.kernel.org/r/20220525161232.14924-3-bp@xxxxxxxxx
Stable-dep-of: c0dd9245aa9e ("x86/microcode: Check CPU capabilities after late microcode update correctly")
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index da87b3bfc9132..a08ce6360382a 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -1320,6 +1320,17 @@ config MICROCODE_AMD
If you select this option, microcode patch loading support for AMD
processors will be enabled.
+config MICROCODE_LATE_LOADING
+ bool "Late microcode loading (DANGEROUS)"
+ default n
+ depends on MICROCODE
+ help
+ Loading microcode late, when the system is up and executing instructions
+ is a tricky business and should be avoided if possible. Just the sequence
+ of synchronizing all cores and SMT threads is one fragile dance which does
+ not guarantee that cores might not softlock after the loading. Therefore,
+ use this at your own risk. Late loading taints the kernel too.
+
config X86_MSR
tristate "/dev/cpu/*/msr - Model-specific register support"
help
diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 1698470dbea5f..6b71f40cd52d6 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -2149,6 +2149,7 @@ void cpu_init_secondary(void)
}
#endif
+#ifdef CONFIG_MICROCODE_LATE_LOADING
/*
* The microcode loader calls this upon late microcode load to recheck features,
* only when microcode has been updated. Caller holds microcode_mutex and CPU
@@ -2178,6 +2179,7 @@ void microcode_check(void)
pr_warn("x86/CPU: CPU features have changed after loading microcode, but might not take effect.\n");
pr_warn("x86/CPU: Please consider either early loading through initrd/built-in or a potential BIOS update.\n");
}
+#endif
/*
* Invoked from core CPU hotplug code after hotplug operations
diff --git a/arch/x86/kernel/cpu/microcode/core.c b/arch/x86/kernel/cpu/microcode/core.c
index 951677121c77d..dc346bbb06777 100644
--- a/arch/x86/kernel/cpu/microcode/core.c
+++ b/arch/x86/kernel/cpu/microcode/core.c
@@ -393,6 +393,7 @@ static int apply_microcode_on_target(int cpu)
/* fake device for request_firmware */
static struct platform_device *microcode_pdev;
+#ifdef CONFIG_MICROCODE_LATE_LOADING
/*
* Late loading dance. Why the heavy-handed stomp_machine effort?
*
@@ -560,6 +561,9 @@ static ssize_t reload_store(struct device *dev,
return ret;
}
+static DEVICE_ATTR_WO(reload);
+#endif
+
static ssize_t version_show(struct device *dev,
struct device_attribute *attr, char *buf)
{
@@ -576,7 +580,6 @@ static ssize_t pf_show(struct device *dev,
return sprintf(buf, "0x%x\n", uci->cpu_sig.pf);
}
-static DEVICE_ATTR_WO(reload);
static DEVICE_ATTR(version, 0444, version_show, NULL);
static DEVICE_ATTR(processor_flags, 0444, pf_show, NULL);
@@ -729,7 +732,9 @@ static int mc_cpu_down_prep(unsigned int cpu)
}
static struct attribute *cpu_root_microcode_attrs[] = {
+#ifdef CONFIG_MICROCODE_LATE_LOADING
&dev_attr_reload.attr,
+#endif
NULL
};
