[PATCH 4.9 15/24] x86/speculation: Warn about Spectre v2 LFENCE mitigation

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

From: Josh Poimboeuf <jpoimboe@redhat.com>

commit eafd987d4a82c7bb5aa12f0e3b4f8f3dea93e678 upstream.

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@redhat.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 arch/x86/kernel/cpu/bugs.c |    5 +++++
 1 file changed, 5 insertions(+)

--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -607,6 +607,7 @@ static inline const char *spectre_v2_mod
 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
@@ -928,6 +929,7 @@ static void __init spectre_v2_select_mit
 		break;
 
 	case SPECTRE_V2_CMD_RETPOLINE_LFENCE:
+		pr_err(SPECTRE_V2_LFENCE_MSG);
 		mode = SPECTRE_V2_LFENCE;
 		break;
 
@@ -1694,6 +1696,9 @@ 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");