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, 74 insertions, 1 deletions

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 9f8c03b347f9..61b4c7bcd4d9 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -292,6 +292,8 @@ config X86
 	select X86_FEATURE_NAMES		if PROC_FS
 	select PROC_PID_ARCH_STATUS		if PROC_FS
 	select HAVE_ARCH_NODE_DEV_GROUP		if X86_SGX
+	select FUNCTION_ALIGNMENT_16B		if X86_64 || X86_ALIGNMENT_16
+	select FUNCTION_ALIGNMENT_4B
 	imply IMA_SECURE_AND_OR_TRUSTED_BOOT    if EFI
 	select HAVE_DYNAMIC_FTRACE_NO_PATCHABLE
 
@@ -1855,7 +1857,7 @@ config CC_HAS_IBT
 
 config X86_KERNEL_IBT
 	prompt "Indirect Branch Tracking"
-	bool
+	def_bool y
 	depends on X86_64 && CC_HAS_IBT && HAVE_OBJTOOL
 	# https://github.com/llvm/llvm-project/commit/9d7001eba9c4cb311e03cd8cdc231f9e579f2d0f
 	depends on !LD_IS_LLD || LLD_VERSION >= 140000
@@ -2492,6 +2494,46 @@ config CC_HAS_SLS
 config CC_HAS_RETURN_THUNK
 	def_bool $(cc-option,-mfunction-return=thunk-extern)
 
+config CC_HAS_ENTRY_PADDING
+	def_bool $(cc-option,-fpatchable-function-entry=16,16)
+
+config FUNCTION_PADDING_CFI
+	int
+	default 59 if FUNCTION_ALIGNMENT_64B
+	default 27 if FUNCTION_ALIGNMENT_32B
+	default 11 if FUNCTION_ALIGNMENT_16B
+	default  3 if FUNCTION_ALIGNMENT_8B
+	default  0
+
+# Basically: FUNCTION_ALIGNMENT - 5*CFI_CLANG
+# except Kconfig can't do arithmetic :/
+config FUNCTION_PADDING_BYTES
+	int
+	default FUNCTION_PADDING_CFI if CFI_CLANG
+	default FUNCTION_ALIGNMENT
+
+config CALL_PADDING
+	def_bool n
+	depends on CC_HAS_ENTRY_PADDING && OBJTOOL
+	select FUNCTION_ALIGNMENT_16B
+
+config FINEIBT
+	def_bool y
+	depends on X86_KERNEL_IBT && CFI_CLANG && RETPOLINE
+	select CALL_PADDING
+
+config HAVE_CALL_THUNKS
+	def_bool y
+	depends on CC_HAS_ENTRY_PADDING && RETHUNK && OBJTOOL
+
+config CALL_THUNKS
+	def_bool n
+	select CALL_PADDING
+
+config PREFIX_SYMBOLS
+	def_bool y
+	depends on CALL_PADDING && !CFI_CLANG
+
 menuconfig SPECULATION_MITIGATIONS
 	bool "Mitigations for speculative execution vulnerabilities"
 	default y
@@ -2543,6 +2585,37 @@ config CPU_UNRET_ENTRY
 	help
 	  Compile the kernel with support for the retbleed=unret mitigation.
 
+config CALL_DEPTH_TRACKING
+	bool "Mitigate RSB underflow with call depth tracking"
+	depends on CPU_SUP_INTEL && HAVE_CALL_THUNKS
+	select HAVE_DYNAMIC_FTRACE_NO_PATCHABLE
+	select CALL_THUNKS
+	default y
+	help
+	  Compile the kernel with call depth tracking to mitigate the Intel
+	  SKL Return-Speculation-Buffer (RSB) underflow issue. The
+	  mitigation is off by default and needs to be enabled on the
+	  kernel command line via the retbleed=stuff option. For
+	  non-affected systems the overhead of this option is marginal as
+	  the call depth tracking is using run-time generated call thunks
+	  in a compiler generated padding area and call patching. This
+	  increases text size by ~5%. For non affected systems this space
+	  is unused. On affected SKL systems this results in a significant
+	  performance gain over the IBRS mitigation.
+
+config CALL_THUNKS_DEBUG
+	bool "Enable call thunks and call depth tracking debugging"
+	depends on CALL_DEPTH_TRACKING
+	select FUNCTION_ALIGNMENT_32B
+	default n
+	help
+	  Enable call/ret counters for imbalance detection and build in
+	  a noisy dmesg about callthunks generation and call patching for
+	  trouble shooting. The debug prints need to be enabled on the
+	  kernel command line with 'debug-callthunks'.
+	  Only enable this, when you are debugging call thunks as this
+	  creates a noticable runtime overhead. If unsure say N.
+
 config CPU_IBPB_ENTRY
 	bool "Enable IBPB on kernel entry"
 	depends on CPU_SUP_AMD && X86_64