x86/cpu: Warn louder about the {set,clear}cpuid boot parameters

Commit 814165e9fd1f6 ("x86/cpu: Add the 'setcpuid=' boot parameter")
recently expanded the user's ability to break their system horribly by
overriding effective CPU flags. This was reflected with updates to the
documentation to try and make people aware that this is dangerous.

To further reduce the risk of users mistaking this for a "real feature",
and try to help them figure out why their kernel is tainted if they do
use it:

- Upgrade the existing printk to pr_warn, to help ensure kernel logs
  reflect what changes are in effect.

- Print an extra warning that tries to be as dramatic as possible, while
  also highlighting the fact that it tainted the kernel.

Suggested-by: Ingo Molnar <mingo@redhat.com>
Signed-off-by: Brendan Jackman <jackmanb@google.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20250303-setcpuid-taint-louder-v1-2-8d255032cb4c@google.com

diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index b5fdaa6fd4c4ff7701009a0443ff57332503bc77..c1ced31f976d970efd24d6c9e4ac77cbff3371b9 100644 (file)
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -1479,12 +1479,12 @@ static void detect_nopl(void)
 #endif
 }
 
-static inline void parse_set_clear_cpuid(char *arg, bool set)
+static inline bool parse_set_clear_cpuid(char *arg, bool set)
 {
        char *opt;
        int taint = 0;
 
-       pr_info("%s CPUID bits:", set ? "Force-enabling" : "Clearing");
+       pr_warn("%s CPUID bits:", set ? "Force-enabling" : "Clearing");
 
        while (arg) {
                bool found __maybe_unused = false;
@@ -1547,10 +1547,9 @@ static inline void parse_set_clear_cpuid(char *arg, bool set)
                        pr_cont(" (unknown: %s)", opt);
        }
 
-       if (taint)
-               add_taint(TAINT_CPU_OUT_OF_SPEC, LOCKDEP_STILL_OK);
-
        pr_cont("\n");
+
+       return taint;
 }
 
 
@@ -1560,6 +1559,7 @@ static inline void parse_set_clear_cpuid(char *arg, bool set)
  */
 static void __init cpu_parse_early_param(void)
 {
+       bool cpuid_taint = false;
        char arg[128];
        int arglen;
 
@@ -1594,11 +1594,16 @@ static void __init cpu_parse_early_param(void)
 
        arglen = cmdline_find_option(boot_command_line, "clearcpuid", arg, sizeof(arg));
        if (arglen > 0)
-               parse_set_clear_cpuid(arg, false);
+               cpuid_taint |= parse_set_clear_cpuid(arg, false);
 
        arglen = cmdline_find_option(boot_command_line, "setcpuid", arg, sizeof(arg));
        if (arglen > 0)
-               parse_set_clear_cpuid(arg, true);
+               cpuid_taint |= parse_set_clear_cpuid(arg, true);
+
+       if (cpuid_taint) {
+               pr_warn("!!! setcpuid=/clearcpuid= in use, this is for TESTING ONLY, may break things horribly. Tainting kernel.\n");
+               add_taint(TAINT_CPU_OUT_OF_SPEC, LOCKDEP_STILL_OK);
+       }
 }
 
 /*