[tip: x86/bugs] x86/srso: Fix vulnerability reporting for missing microcode

[tip-bot2 for Josh Poimboeuf]

The following commit has been merged into the x86/bugs branch of tip:

Commit-ID:     b3be1397be0340b2c30b2dcd7339dbfaa5563e2b
Gitweb:        https://git.kernel.org/tip/b3be1397be0340b2c30b2dcd7339dbfaa5563e2b
Author:        Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
AuthorDate:    Fri, 25 Aug 2023 00:01:41 -07:00
Committer:     Ingo Molnar <mingo@xxxxxxxxxx>
CommitterDate: Fri, 25 Aug 2023 11:21:59 +02:00

x86/srso: Fix vulnerability reporting for missing microcode

The SRSO default safe-ret mitigation is reported as "mitigated" even if
microcode hasn't been updated.  That's wrong because userspace may still
be vulnerable to SRSO attacks due to IBPB not flushing branch type
predictions.

Report the safe-ret + !microcode case as vulnerable.

Also report the microcode-only case as vulnerable as it leaves the
kernel open to attacks.

Fixes: fb3bd914b3ec ("x86/srso: Add a Speculative RAS Overflow mitigation")
Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
Link: https://lore.kernel.org/r/65556eeb1bf7cb9bd7db8662ef115dd73191db84.1692919072.git.jpoimboe@xxxxxxxxxx
---
 Documentation/admin-guide/hw-vuln/srso.rst | 22 ++++++++++----
 arch/x86/kernel/cpu/bugs.c                 | 34 ++++++++++++---------
 2 files changed, 37 insertions(+), 19 deletions(-)

diff --git a/Documentation/admin-guide/hw-vuln/srso.rst b/Documentation/admin-guide/hw-vuln/srso.rst
index b6cfb51..4516719 100644
--- a/Documentation/admin-guide/hw-vuln/srso.rst
+++ b/Documentation/admin-guide/hw-vuln/srso.rst
@@ -46,12 +46,22 @@ The possible values in this file are:
 
    The processor is not vulnerable
 
- * 'Vulnerable: no microcode':
+* 'Vulnerable':
+
+   The processor is vulnerable and no mitigations have been applied.
+
+ * 'Vulnerable: No microcode':
 
    The processor is vulnerable, no microcode extending IBPB
    functionality to address the vulnerability has been applied.
 
- * 'Mitigation: microcode':
+ * 'Vulnerable: Safe RET, no microcode':
+
+   The "Safe Ret" mitigation (see below) has been applied to protect the
+   kernel, but the IBPB-extending microcode has not been applied.  User
+   space tasks may still be vulnerable.
+
+ * 'Vulnerable: Microcode, no safe RET':
 
    Extended IBPB functionality microcode patch has been applied. It does
    not address User->Kernel and Guest->Host transitions protection but it
@@ -72,11 +82,11 @@ The possible values in this file are:
 
    (spec_rstack_overflow=microcode)
 
- * 'Mitigation: safe RET':
+ * 'Mitigation: Safe RET':
 
-   Software-only mitigation. It complements the extended IBPB microcode
-   patch functionality by addressing User->Kernel and Guest->Host
-   transitions protection.
+   Combined microcode/software mitigation. It complements the
+   extended IBPB microcode patch functionality by addressing
+   User->Kernel and Guest->Host transitions protection.
 
    Selected by default or by spec_rstack_overflow=safe-ret
 
diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 6c47f37..d883d1c 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -2353,6 +2353,8 @@ early_param("l1tf", l1tf_cmdline);
 
 enum srso_mitigation {
 	SRSO_MITIGATION_NONE,
+	SRSO_MITIGATION_UCODE_NEEDED,
+	SRSO_MITIGATION_SAFE_RET_UCODE_NEEDED,
 	SRSO_MITIGATION_MICROCODE,
 	SRSO_MITIGATION_SAFE_RET,
 	SRSO_MITIGATION_IBPB,
@@ -2368,11 +2370,13 @@ enum srso_mitigation_cmd {
 };
 
 static const char * const srso_strings[] = {
-	[SRSO_MITIGATION_NONE]           = "Vulnerable",
-	[SRSO_MITIGATION_MICROCODE]      = "Mitigation: microcode",
-	[SRSO_MITIGATION_SAFE_RET]	 = "Mitigation: safe RET",
-	[SRSO_MITIGATION_IBPB]		 = "Mitigation: IBPB",
-	[SRSO_MITIGATION_IBPB_ON_VMEXIT] = "Mitigation: IBPB on VMEXIT only"
+	[SRSO_MITIGATION_NONE]			= "Vulnerable",
+	[SRSO_MITIGATION_UCODE_NEEDED]		= "Vulnerable: No microcode",
+	[SRSO_MITIGATION_SAFE_RET_UCODE_NEEDED]	= "Vulnerable: Safe RET, no microcode",
+	[SRSO_MITIGATION_MICROCODE]		= "Vulnerable: Microcode, no safe RET",
+	[SRSO_MITIGATION_SAFE_RET]		= "Mitigation: Safe RET",
+	[SRSO_MITIGATION_IBPB]			= "Mitigation: IBPB",
+	[SRSO_MITIGATION_IBPB_ON_VMEXIT]	= "Mitigation: IBPB on VMEXIT only"
 };
 
 static enum srso_mitigation srso_mitigation __ro_after_init = SRSO_MITIGATION_NONE;
@@ -2409,10 +2413,7 @@ static void __init srso_select_mitigation(void)
 	if (!boot_cpu_has_bug(X86_BUG_SRSO) || cpu_mitigations_off())
 		goto pred_cmd;
 
-	if (!has_microcode) {
-		pr_warn("IBPB-extending microcode not applied!\n");
-		pr_warn(SRSO_NOTICE);
-	} else {
+	if (has_microcode) {
 		/*
 		 * Zen1/2 with SMT off aren't vulnerable after the right
 		 * IBPB microcode has been applied.
@@ -2428,6 +2429,12 @@ static void __init srso_select_mitigation(void)
 			srso_mitigation = SRSO_MITIGATION_IBPB;
 			goto out;
 		}
+	} else {
+		pr_warn("IBPB-extending microcode not applied!\n");
+		pr_warn(SRSO_NOTICE);
+
+		/* may be overwritten by SRSO_CMD_SAFE_RET below */
+		srso_mitigation = SRSO_MITIGATION_UCODE_NEEDED;
 	}
 
 	switch (srso_cmd) {
@@ -2457,7 +2464,10 @@ static void __init srso_select_mitigation(void)
 				setup_force_cpu_cap(X86_FEATURE_SRSO);
 				x86_return_thunk = srso_return_thunk;
 			}
-			srso_mitigation = SRSO_MITIGATION_SAFE_RET;
+			if (has_microcode)
+				srso_mitigation = SRSO_MITIGATION_SAFE_RET;
+			else
+				srso_mitigation = SRSO_MITIGATION_SAFE_RET_UCODE_NEEDED;
 		} else {
 			pr_err("WARNING: kernel not compiled with CPU_SRSO.\n");
 		}
@@ -2701,9 +2711,7 @@ static ssize_t srso_show_state(char *buf)
 	if (boot_cpu_has(X86_FEATURE_SRSO_NO))
 		return sysfs_emit(buf, "Mitigation: SMT disabled\n");
 
-	return sysfs_emit(buf, "%s%s\n",
-			  srso_strings[srso_mitigation],
-			  boot_cpu_has(X86_FEATURE_IBPB_BRTYPE) ? "" : ", no microcode");
+	return sysfs_emit(buf, "%s\n", srso_strings[srso_mitigation]);
 }
 
 static ssize_t gds_show_state(char *buf)