x86/microcode: Taint and warn on late loading

Warn before it is attempted and taint the kernel. Late loading microcode
can lead to malfunction of the kernel when the microcode update changes
behaviour. There is no way for the kernel to determine whether its safe or
not.

Signed-off-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20220525161232.14924-4-bp@alien8.de

diff --git a/arch/x86/kernel/cpu/microcode/core.c b/arch/x86/kernel/cpu/microcode/core.c
index c717db6b6856d2b05e5e86bca33ac3fa844ca401..801b44ac38514081c38b8684a54e7d94bbe7f72b 100644 (file)
--- a/arch/x86/kernel/cpu/microcode/core.c
+++ b/arch/x86/kernel/cpu/microcode/core.c
@@ -493,6 +493,9 @@ static int microcode_reload_late(void)
 {
        int ret;
 
+       pr_err("Attempting late microcode loading - it is dangerous and taints the kernel.\n");
+       pr_err("You should switch to early loading, if possible.\n");
+
        atomic_set(&late_cpus_in,  0);
        atomic_set(&late_cpus_out, 0);
 
@@ -541,6 +544,8 @@ put:
        if (ret == 0)
                ret = size;
 
+       add_taint(TAINT_CPU_OUT_OF_SPEC, LOCKDEP_STILL_OK);
+
        return ret;
 }