x86/bugs: Add retbleed=ibpb

jmp2ret mitigates the easy-to-attack case at relatively low overhead.
It mitigates the long speculation windows after a mispredicted RET, but
it does not mitigate the short speculation window from arbitrary
instruction boundaries.

On Zen2, there is a chicken bit which needs setting, which mitigates
"arbitrary instruction boundaries" down to just "basic block boundaries".

But there is no fix for the short speculation window on basic block
boundaries, other than to flush the entire BTB to evict all attacker
predictions.

On the spectrum of "fast & blurry" -> "safe", there is (on top of STIBP
or no-SMT):

  1) Nothing		System wide open
  2) jmp2ret		May stop a script kiddy
  3) jmp2ret+chickenbit  Raises the bar rather further
  4) IBPB		Only thing which can count as "safe".

Tentative numbers put IBPB-on-entry at a 2.5x hit on Zen2, and a 10x hit
on Zen1 according to lmbench.

  [ bp: Fixup feature bit comments, document option, 32-bit build fix. ]
Suggested-by: Andrew Cooper <Andrew.Cooper3@citrix.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Borislav Petkov <bp@suse.de>

Documentation/admin-guide/kernel-parameters.txt

@@ -5207,6 +5207,9 @@
 				       disabling SMT if necessary for
 				       the full mitigation (only on Zen1
 				       and older without STIBP).
+			ibpb	     - mitigate short speculation windows on
+				       basic block boundaries too. Safe, highest
+				       perf impact.
 			unret        - force enable untrained return thunks,
 				       only effective on AMD f15h-f17h
 				       based systems.

arch/x86/entry/Makefile

@@ -11,7 +11,7 @@ CFLAGS_REMOVE_common.o		= $(CC_FLAGS_FTRACE)
 
 CFLAGS_common.o			+= -fno-stack-protector
 
-obj-y				:= entry_$(BITS).o thunk_$(BITS).o syscall_$(BITS).o
+obj-y				:= entry.o entry_$(BITS).o thunk_$(BITS).o syscall_$(BITS).o
 obj-y				+= common.o
 
 obj-y				+= vdso/

arch/x86/entry/entry.S

+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * Common place for both 32- and 64-bit entry routines.
+ */
+
+#include <linux/linkage.h>
+#include <asm/export.h>
+#include <asm/msr-index.h>
+
+.pushsection .noinstr.text, "ax"
+
+SYM_FUNC_START(entry_ibpb)
+	movl	$MSR_IA32_PRED_CMD, %ecx
+	movl	$PRED_CMD_IBPB, %eax
+	xorl	%edx, %edx
+	wrmsr
+	RET
+SYM_FUNC_END(entry_ibpb)
+/* For KVM */
+EXPORT_SYMBOL_GPL(entry_ibpb);
+
+.popsection

arch/x86/include/asm/cpufeatures.h

@@ -296,7 +296,7 @@
 #define X86_FEATURE_PER_THREAD_MBA	(11*32+ 7) /* "" Per-thread Memory Bandwidth Allocation */
 #define X86_FEATURE_SGX1		(11*32+ 8) /* "" Basic SGX */
 #define X86_FEATURE_SGX2		(11*32+ 9) /* "" SGX Enclave Dynamic Memory Management (EDMM) */
-/* FREE!				(11*32+10) */
+#define X86_FEATURE_ENTRY_IBPB		(11*32+10) /* "" Issue an IBPB on kernel entry */
 /* FREE!				(11*32+11) */
 #define X86_FEATURE_RETPOLINE		(11*32+12) /* "" Generic Retpoline mitigation for Spectre variant 2 */
 #define X86_FEATURE_RETPOLINE_LFENCE	(11*32+13) /* "" Use LFENCE for Spectre variant 2 */

arch/x86/include/asm/nospec-branch.h

@@ -123,14 +123,17 @@
  * return thunk isn't mapped into the userspace tables (then again, AMD
  * typically has NO_MELTDOWN).
  *
- * Doesn't clobber any registers but does require a stable stack.
+ * While zen_untrain_ret() doesn't clobber anything but requires stack,
+ * entry_ibpb() will clobber AX, CX, DX.
  *
  * As such, this must be placed after every *SWITCH_TO_KERNEL_CR3 at a point
  * where we have a stack but before any RET instruction.
  */
 .macro UNTRAIN_RET
 #ifdef CONFIG_RETPOLINE
-	ALTERNATIVE "", "call zen_untrain_ret", X86_FEATURE_UNRET
+	ALTERNATIVE_2 "",						\
+	              "call zen_untrain_ret", X86_FEATURE_UNRET,	\
+		      "call entry_ibpb", X86_FEATURE_ENTRY_IBPB
 #endif
 .endm
 
@@ -147,6 +150,7 @@ extern retpoline_thunk_t __x86_indirect_thunk_array[];
 
 extern void __x86_return_thunk(void);
 extern void zen_untrain_ret(void);
+extern void entry_ibpb(void);
 
 #ifdef CONFIG_RETPOLINE
 

arch/x86/kernel/cpu/bugs.c

@@ -805,6 +805,7 @@ static enum spectre_v2_mitigation spectre_v2_enabled __ro_after_init =
 enum retbleed_mitigation {
 	RETBLEED_MITIGATION_NONE,
 	RETBLEED_MITIGATION_UNRET,
+	RETBLEED_MITIGATION_IBPB,
 	RETBLEED_MITIGATION_IBRS,
 	RETBLEED_MITIGATION_EIBRS,
 };
@@ -813,11 +814,13 @@ enum retbleed_mitigation_cmd {
 	RETBLEED_CMD_OFF,
 	RETBLEED_CMD_AUTO,
 	RETBLEED_CMD_UNRET,
+	RETBLEED_CMD_IBPB,
 };
 
 const char * const retbleed_strings[] = {
 	[RETBLEED_MITIGATION_NONE]	= "Vulnerable",
 	[RETBLEED_MITIGATION_UNRET]	= "Mitigation: untrained return thunk",
+	[RETBLEED_MITIGATION_IBPB]	= "Mitigation: IBPB",
 	[RETBLEED_MITIGATION_IBRS]	= "Mitigation: IBRS",
 	[RETBLEED_MITIGATION_EIBRS]	= "Mitigation: Enhanced IBRS",
 };
@@ -847,6 +850,8 @@ static int __init retbleed_parse_cmdline(char *str)
 			retbleed_cmd = RETBLEED_CMD_AUTO;
 		} else if (!strcmp(str, "unret")) {
 			retbleed_cmd = RETBLEED_CMD_UNRET;
+		} else if (!strcmp(str, "ibpb")) {
+			retbleed_cmd = RETBLEED_CMD_IBPB;
 		} else if (!strcmp(str, "nosmt")) {
 			retbleed_nosmt = true;
 		} else {
@@ -861,11 +866,13 @@ static int __init retbleed_parse_cmdline(char *str)
 early_param("retbleed", retbleed_parse_cmdline);
 
 #define RETBLEED_UNTRAIN_MSG "WARNING: BTB untrained return thunk mitigation is only effective on AMD/Hygon!\n"
-#define RETBLEED_COMPILER_MSG "WARNING: kernel not compiled with RETPOLINE or -mfunction-return capable compiler!\n"
+#define RETBLEED_COMPILER_MSG "WARNING: kernel not compiled with RETPOLINE or -mfunction-return capable compiler; falling back to IBPB!\n"
 #define RETBLEED_INTEL_MSG "WARNING: Spectre v2 mitigation leaves CPU vulnerable to RETBleed attacks, data leaks possible!\n"
 
 static void __init retbleed_select_mitigation(void)
 {
+	bool mitigate_smt = false;
+
 	if (!boot_cpu_has_bug(X86_BUG_RETBLEED) || cpu_mitigations_off())
 		return;
 
@@ -877,11 +884,21 @@ static void __init retbleed_select_mitigation(void)
 		retbleed_mitigation = RETBLEED_MITIGATION_UNRET;
 		break;
 
+	case RETBLEED_CMD_IBPB:
+		retbleed_mitigation = RETBLEED_MITIGATION_IBPB;
+		break;
+
 	case RETBLEED_CMD_AUTO:
 	default:
 		if (boot_cpu_data.x86_vendor == X86_VENDOR_AMD ||
-		    boot_cpu_data.x86_vendor == X86_VENDOR_HYGON)
-			retbleed_mitigation = RETBLEED_MITIGATION_UNRET;
+		    boot_cpu_data.x86_vendor == X86_VENDOR_HYGON) {
+
+			if (IS_ENABLED(CONFIG_RETPOLINE) &&
+			    IS_ENABLED(CONFIG_CC_HAS_RETURN_THUNK))
+				retbleed_mitigation = RETBLEED_MITIGATION_UNRET;
+			else
+				retbleed_mitigation = RETBLEED_MITIGATION_IBPB;
+		}
 
 		/*
 		 * The Intel mitigation (IBRS) was already selected in
@@ -897,26 +914,34 @@ static void __init retbleed_select_mitigation(void)
 		if (!IS_ENABLED(CONFIG_RETPOLINE) ||
 		    !IS_ENABLED(CONFIG_CC_HAS_RETURN_THUNK)) {
 			pr_err(RETBLEED_COMPILER_MSG);
-			retbleed_mitigation = RETBLEED_MITIGATION_NONE;
-			break;
+			retbleed_mitigation = RETBLEED_MITIGATION_IBPB;
+			goto retbleed_force_ibpb;
 		}
 
 		setup_force_cpu_cap(X86_FEATURE_RETHUNK);
 		setup_force_cpu_cap(X86_FEATURE_UNRET);
 
-		if (!boot_cpu_has(X86_FEATURE_STIBP) &&
-		    (retbleed_nosmt || cpu_mitigations_auto_nosmt()))
-			cpu_smt_disable(false);
-
 		if (boot_cpu_data.x86_vendor != X86_VENDOR_AMD &&
 		    boot_cpu_data.x86_vendor != X86_VENDOR_HYGON)
 			pr_err(RETBLEED_UNTRAIN_MSG);
+
+		mitigate_smt = true;
+		break;
+
+	case RETBLEED_MITIGATION_IBPB:
+retbleed_force_ibpb:
+		setup_force_cpu_cap(X86_FEATURE_ENTRY_IBPB);
+		mitigate_smt = true;
 		break;
 
 	default:
 		break;
 	}
 
+	if (mitigate_smt && !boot_cpu_has(X86_FEATURE_STIBP) &&
+	    (retbleed_nosmt || cpu_mitigations_auto_nosmt()))
+		cpu_smt_disable(false);
+
 	/*
 	 * Let IBRS trump all on Intel without affecting the effects of the
 	 * retbleed= cmdline option.