[SRU Bionic 4/5] x86/speculation: Warn about Spectre v2 LFENCE mitigation

[Thadeu Lima de Souza Cascardo]

From: Josh Poimboeuf <jpoimboe at redhat.com>

With:

  f8a66d608a3e ("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: Josh Poimboeuf <jpoimboe at redhat.com>
Signed-off-by: Borislav Petkov <bp at suse.de>
(cherry picked from commit eafd987d4a82c7bb5aa12f0e3b4f8f3dea93e678)
CVE-2021-26401
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo at canonical.com>
---
 arch/x86/kernel/cpu/bugs.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 0fd0830229f7..f9ed37aca24b 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/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;
 
@@ -1693,6 +1695,9 @@ 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");
 
-- 
2.32.0