Merge tag 'x86_core_for_v6.2' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull x86 core updates from Borislav Petkov:

 - Add the call depth tracking mitigation for Retbleed which has been
   long in the making. It is a lighterweight software-only fix for
   Skylake-based cores where enabling IBRS is a big hammer and causes a
   significant performance impact.

   What it basically does is, it aligns all kernel functions to 16 bytes
   boundary and adds a 16-byte padding before the function, objtool
   collects all functions' locations and when the mitigation gets
   applied, it patches a call accounting thunk which is used to track
   the call depth of the stack at any time.

   When that call depth reaches a magical, microarchitecture-specific
   value for the Return Stack Buffer, the code stuffs that RSB and
   avoids its underflow which could otherwise lead to the Intel variant
   of Retbleed.

   This software-only solution brings a lot of the lost performance
   back, as benchmarks suggest:

       https://lore.kernel.org/all/20220915111039.092790446@infradead.org/

   That page above also contains a lot more detailed explanation of the
   whole mechanism

 - Implement a new control flow integrity scheme called FineIBT which is
   based on the software kCFI implementation and uses hardware IBT
   support where present to annotate and track indirect branches using a
   hash to validate them

 - Other misc fixes and cleanups

* tag 'x86_core_for_v6.2' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip: (80 commits)
  x86/paravirt: Use common macro for creating simple asm paravirt functions
  x86/paravirt: Remove clobber bitmask from .parainstructions
  x86/debug: Include percpu.h in debugreg.h to get DECLARE_PER_CPU() et al
  x86/cpufeatures: Move X86_FEATURE_CALL_DEPTH from bit 18 to bit 19 of word 11, to leave space for WIP X86_FEATURE_SGX_EDECCSSA bit
  x86/Kconfig: Enable kernel IBT by default
  x86,pm: Force out-of-line memcpy()
  objtool: Fix weak hole vs prefix symbol
  objtool: Optimize elf_dirty_reloc_sym()
  x86/cfi: Add boot time hash randomization
  x86/cfi: Boot time selection of CFI scheme
  x86/ibt: Implement FineIBT
  objtool: Add --cfi to generate the .cfi_sites section
  x86: Add prefix symbols for function padding
  objtool: Add option to generate prefix symbols
  objtool: Avoid O(bloody terrible) behaviour -- an ode to libelf
  objtool: Slice up elf_create_section_symbol()
  kallsyms: Revert "Take callthunks into account"
  x86: Unconfuse CONFIG_ and X86_FEATURE_ namespaces
  x86/retpoline: Fix crash printing warning
  x86/paravirt: Fix a !PARAVIRT build warning
  ...

1 files changed, 32 insertions, 2 deletions

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index a8a5033a804d..d970ddb0cc65 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -787,6 +787,7 @@ enum retbleed_mitigation {
 	RETBLEED_MITIGATION_IBPB,
 	RETBLEED_MITIGATION_IBRS,
 	RETBLEED_MITIGATION_EIBRS,
+	RETBLEED_MITIGATION_STUFF,
 };
 
 enum retbleed_mitigation_cmd {
@@ -794,6 +795,7 @@ enum retbleed_mitigation_cmd {
 	RETBLEED_CMD_AUTO,
 	RETBLEED_CMD_UNRET,
 	RETBLEED_CMD_IBPB,
+	RETBLEED_CMD_STUFF,
 };
 
 static const char * const retbleed_strings[] = {
@@ -802,6 +804,7 @@ static const char * const retbleed_strings[] = {
 	[RETBLEED_MITIGATION_IBPB]	= "Mitigation: IBPB",
 	[RETBLEED_MITIGATION_IBRS]	= "Mitigation: IBRS",
 	[RETBLEED_MITIGATION_EIBRS]	= "Mitigation: Enhanced IBRS",
+	[RETBLEED_MITIGATION_STUFF]	= "Mitigation: Stuffing",
 };
 
 static enum retbleed_mitigation retbleed_mitigation __ro_after_init =
@@ -831,8 +834,12 @@ static int __init retbleed_parse_cmdline(char *str)
 			retbleed_cmd = RETBLEED_CMD_UNRET;
 		} else if (!strcmp(str, "ibpb")) {
 			retbleed_cmd = RETBLEED_CMD_IBPB;
+		} else if (!strcmp(str, "stuff")) {
+			retbleed_cmd = RETBLEED_CMD_STUFF;
 		} else if (!strcmp(str, "nosmt")) {
 			retbleed_nosmt = true;
+		} else if (!strcmp(str, "force")) {
+			setup_force_cpu_bug(X86_BUG_RETBLEED);
 		} else {
 			pr_err("Ignoring unknown retbleed option (%s).", str);
 		}
@@ -879,6 +886,21 @@ static void __init retbleed_select_mitigation(void)
 		}
 		break;
 
+	case RETBLEED_CMD_STUFF:
+		if (IS_ENABLED(CONFIG_CALL_DEPTH_TRACKING) &&
+		    spectre_v2_enabled == SPECTRE_V2_RETPOLINE) {
+			retbleed_mitigation = RETBLEED_MITIGATION_STUFF;
+
+		} else {
+			if (IS_ENABLED(CONFIG_CALL_DEPTH_TRACKING))
+				pr_err("WARNING: retbleed=stuff depends on spectre_v2=retpoline\n");
+			else
+				pr_err("WARNING: kernel not compiled with CALL_DEPTH_TRACKING.\n");
+
+			goto do_cmd_auto;
+		}
+		break;
+
 do_cmd_auto:
 	case RETBLEED_CMD_AUTO:
 	default:
@@ -916,6 +938,12 @@ do_cmd_auto:
 		mitigate_smt = true;
 		break;
 
+	case RETBLEED_MITIGATION_STUFF:
+		setup_force_cpu_cap(X86_FEATURE_RETHUNK);
+		setup_force_cpu_cap(X86_FEATURE_CALL_DEPTH);
+		x86_set_skl_return_thunk();
+		break;
+
 	default:
 		break;
 	}
@@ -926,7 +954,7 @@ do_cmd_auto:
 
 	/*
 	 * Let IBRS trump all on Intel without affecting the effects of the
-	 * retbleed= cmdline option.
+	 * retbleed= cmdline option except for call depth based stuffing
 	 */
 	if (boot_cpu_data.x86_vendor == X86_VENDOR_INTEL) {
 		switch (spectre_v2_enabled) {
@@ -939,7 +967,8 @@ do_cmd_auto:
 			retbleed_mitigation = RETBLEED_MITIGATION_EIBRS;
 			break;
 		default:
-			pr_err(RETBLEED_INTEL_MSG);
+			if (retbleed_mitigation != RETBLEED_MITIGATION_STUFF)
+				pr_err(RETBLEED_INTEL_MSG);
 		}
 	}
 
@@ -1413,6 +1442,7 @@ static void __init spectre_v2_select_mitigation(void)
 		if (IS_ENABLED(CONFIG_CPU_IBRS_ENTRY) &&
 		    boot_cpu_has_bug(X86_BUG_RETBLEED) &&
 		    retbleed_cmd != RETBLEED_CMD_OFF &&
+		    retbleed_cmd != RETBLEED_CMD_STUFF &&
 		    boot_cpu_has(X86_FEATURE_IBRS) &&
 		    boot_cpu_data.x86_vendor == X86_VENDOR_INTEL) {
 			mode = SPECTRE_V2_IBRS;