x86/speculation: Warn about Spectre v2 LFENCE mitigation

stable inclusion
from stable-v4.19.234
commit 8bfdba77595aee5c3e83ed1c9994c35d6d409605
category: bugfix
bugzilla: 186453, https://gitee.com/src-openeuler/kernel/issues/I50WBM
CVE: CVE-2022-0001

--------------------------------

commit eafd987d upstream.

With:

  f8a66d60 ("x86,bugs: Unconditionally allow spectre_v2=retpoline,amd")

it became possible to enable the LFENCE "retpoline" on Intel. However,
Intel doesn't recommend it, as it has some weaknesses compared to
retpoline.

Now AMD doesn't recommend it either.

It can still be left available as a cmdline option. It's faster than
retpoline but is weaker in certain scenarios -- particularly SMT, but
even non-SMT may be vulnerable in some cases.

So just unconditionally warn if the user requests it on the cmdline.

  [ bp: Massage commit message. ]
Signed-off-by: NJosh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: NBorislav Petkov <bp@suse.de>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: NChen Jiahao <chenjiahao16@huawei.com>
Reviewed-by: NHanjun Guo <guohanjun@huawei.com>
Reviewed-by: NLiao Chang <liaochang1@huawei.com>
Signed-off-by: NYongqiang Liu <liuyongqiang13@huawei.com>

arch/x86/kernel/cpu/bugs.c

@@ -608,6 +608,7 @@ static inline const char *spectre_v2_module_string(void)
 static inline const char *spectre_v2_module_string(void) { return ""; }
 #endif
 
+#define SPECTRE_V2_LFENCE_MSG "WARNING: LFENCE mitigation is not recommended for this CPU, data leaks possible!\n"
 #define SPECTRE_V2_EIBRS_EBPF_MSG "WARNING: Unprivileged eBPF is enabled with eIBRS on, data leaks possible via Spectre v2 BHB attacks!\n"
 
 #ifdef CONFIG_BPF_SYSCALL
@@ -929,6 +930,7 @@ static void __init spectre_v2_select_mitigation(void)
 		break;
 
 	case SPECTRE_V2_CMD_RETPOLINE_LFENCE:
+		pr_err(SPECTRE_V2_LFENCE_MSG);
 		mode = SPECTRE_V2_LFENCE;
 		break;
 
@@ -1680,6 +1682,9 @@ static char *ibpb_state(void)
 
 static ssize_t spectre_v2_show_state(char *buf)
 {
+	if (spectre_v2_enabled == SPECTRE_V2_LFENCE)
+		return sprintf(buf, "Vulnerable: LFENCE\n");
+
 	if (spectre_v2_enabled == SPECTRE_V2_EIBRS && unprivileged_ebpf_enabled())
 		return sprintf(buf, "Vulnerable: Unprivileged eBPF enabled\n");