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, 170 insertions, 6 deletions

diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
index dfdb103ae4f6..771b0a2b7a34 100644
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -12,8 +12,104 @@
 #include <asm/msr-index.h>
 #include <asm/unwind_hints.h>
 #include <asm/percpu.h>
+#include <asm/current.h>
 
-#define RETPOLINE_THUNK_SIZE	32
+/*
+ * Call depth tracking for Intel SKL CPUs to address the RSB underflow
+ * issue in software.
+ *
+ * The tracking does not use a counter. It uses uses arithmetic shift
+ * right on call entry and logical shift left on return.
+ *
+ * The depth tracking variable is initialized to 0x8000.... when the call
+ * depth is zero. The arithmetic shift right sign extends the MSB and
+ * saturates after the 12th call. The shift count is 5 for both directions
+ * so the tracking covers 12 nested calls.
+ *
+ *  Call
+ *  0: 0x8000000000000000	0x0000000000000000
+ *  1: 0xfc00000000000000	0xf000000000000000
+ * ...
+ * 11: 0xfffffffffffffff8	0xfffffffffffffc00
+ * 12: 0xffffffffffffffff	0xffffffffffffffe0
+ *
+ * After a return buffer fill the depth is credited 12 calls before the
+ * next stuffing has to take place.
+ *
+ * There is a inaccuracy for situations like this:
+ *
+ *  10 calls
+ *   5 returns
+ *   3 calls
+ *   4 returns
+ *   3 calls
+ *   ....
+ *
+ * The shift count might cause this to be off by one in either direction,
+ * but there is still a cushion vs. the RSB depth. The algorithm does not
+ * claim to be perfect and it can be speculated around by the CPU, but it
+ * is considered that it obfuscates the problem enough to make exploitation
+ * extremly difficult.
+ */
+#define RET_DEPTH_SHIFT			5
+#define RSB_RET_STUFF_LOOPS		16
+#define RET_DEPTH_INIT			0x8000000000000000ULL
+#define RET_DEPTH_INIT_FROM_CALL	0xfc00000000000000ULL
+#define RET_DEPTH_CREDIT		0xffffffffffffffffULL
+
+#ifdef CONFIG_CALL_THUNKS_DEBUG
+# define CALL_THUNKS_DEBUG_INC_CALLS				\
+	incq	%gs:__x86_call_count;
+# define CALL_THUNKS_DEBUG_INC_RETS				\
+	incq	%gs:__x86_ret_count;
+# define CALL_THUNKS_DEBUG_INC_STUFFS				\
+	incq	%gs:__x86_stuffs_count;
+# define CALL_THUNKS_DEBUG_INC_CTXSW				\
+	incq	%gs:__x86_ctxsw_count;
+#else
+# define CALL_THUNKS_DEBUG_INC_CALLS
+# define CALL_THUNKS_DEBUG_INC_RETS
+# define CALL_THUNKS_DEBUG_INC_STUFFS
+# define CALL_THUNKS_DEBUG_INC_CTXSW
+#endif
+
+#if defined(CONFIG_CALL_DEPTH_TRACKING) && !defined(COMPILE_OFFSETS)
+
+#include <asm/asm-offsets.h>
+
+#define CREDIT_CALL_DEPTH					\
+	movq	$-1, PER_CPU_VAR(pcpu_hot + X86_call_depth);
+
+#define ASM_CREDIT_CALL_DEPTH					\
+	movq	$-1, PER_CPU_VAR(pcpu_hot + X86_call_depth);
+
+#define RESET_CALL_DEPTH					\
+	mov	$0x80, %rax;					\
+	shl	$56, %rax;					\
+	movq	%rax, PER_CPU_VAR(pcpu_hot + X86_call_depth);
+
+#define RESET_CALL_DEPTH_FROM_CALL				\
+	mov	$0xfc, %rax;					\
+	shl	$56, %rax;					\
+	movq	%rax, PER_CPU_VAR(pcpu_hot + X86_call_depth);	\
+	CALL_THUNKS_DEBUG_INC_CALLS
+
+#define INCREMENT_CALL_DEPTH					\
+	sarq	$5, %gs:pcpu_hot + X86_call_depth;		\
+	CALL_THUNKS_DEBUG_INC_CALLS
+
+#define ASM_INCREMENT_CALL_DEPTH				\
+	sarq	$5, PER_CPU_VAR(pcpu_hot + X86_call_depth);	\
+	CALL_THUNKS_DEBUG_INC_CALLS
+
+#else
+#define CREDIT_CALL_DEPTH
+#define ASM_CREDIT_CALL_DEPTH
+#define RESET_CALL_DEPTH
+#define INCREMENT_CALL_DEPTH
+#define ASM_INCREMENT_CALL_DEPTH
+#define RESET_CALL_DEPTH_FROM_CALL
+#endif
 
 /*
  * Fill the CPU return stack buffer.
@@ -32,6 +128,7 @@
  * from C via asm(".include <asm/nospec-branch.h>") but let's not go there.
  */
 
+#define RETPOLINE_THUNK_SIZE	32
 #define RSB_CLEAR_LOOPS		32	/* To forcibly overwrite all entries */
 
 /*
@@ -60,7 +157,9 @@
 	dec	reg;					\
 	jnz	771b;					\
 	/* barrier for jnz misprediction */		\
-	lfence;
+	lfence;						\
+	ASM_CREDIT_CALL_DEPTH				\
+	CALL_THUNKS_DEBUG_INC_CTXSW
 #else
 /*
  * i386 doesn't unconditionally have LFENCE, as such it can't
@@ -185,11 +284,32 @@
  * where we have a stack but before any RET instruction.
  */
 .macro UNTRAIN_RET
-#if defined(CONFIG_CPU_UNRET_ENTRY) || defined(CONFIG_CPU_IBPB_ENTRY)
+#if defined(CONFIG_CPU_UNRET_ENTRY) || defined(CONFIG_CPU_IBPB_ENTRY) || \
+	defined(CONFIG_CALL_DEPTH_TRACKING)
 	ANNOTATE_UNRET_END
-	ALTERNATIVE_2 "",						\
-	              CALL_ZEN_UNTRAIN_RET, X86_FEATURE_UNRET,		\
-		      "call entry_ibpb", X86_FEATURE_ENTRY_IBPB
+	ALTERNATIVE_3 "",						\
+		      CALL_ZEN_UNTRAIN_RET, X86_FEATURE_UNRET,		\
+		      "call entry_ibpb", X86_FEATURE_ENTRY_IBPB,	\
+		      __stringify(RESET_CALL_DEPTH), X86_FEATURE_CALL_DEPTH
+#endif
+.endm
+
+.macro UNTRAIN_RET_FROM_CALL
+#if defined(CONFIG_CPU_UNRET_ENTRY) || defined(CONFIG_CPU_IBPB_ENTRY) || \
+	defined(CONFIG_CALL_DEPTH_TRACKING)
+	ANNOTATE_UNRET_END
+	ALTERNATIVE_3 "",						\
+		      CALL_ZEN_UNTRAIN_RET, X86_FEATURE_UNRET,		\
+		      "call entry_ibpb", X86_FEATURE_ENTRY_IBPB,	\
+		      __stringify(RESET_CALL_DEPTH_FROM_CALL), X86_FEATURE_CALL_DEPTH
+#endif
+.endm
+
+
+.macro CALL_DEPTH_ACCOUNT
+#ifdef CONFIG_CALL_DEPTH_TRACKING
+	ALTERNATIVE "",							\
+		    __stringify(ASM_INCREMENT_CALL_DEPTH), X86_FEATURE_CALL_DEPTH
 #endif
 .endm
 
@@ -203,11 +323,45 @@
 
 typedef u8 retpoline_thunk_t[RETPOLINE_THUNK_SIZE];
 extern retpoline_thunk_t __x86_indirect_thunk_array[];
+extern retpoline_thunk_t __x86_indirect_call_thunk_array[];
+extern retpoline_thunk_t __x86_indirect_jump_thunk_array[];
 
 extern void __x86_return_thunk(void);
 extern void zen_untrain_ret(void);
 extern void entry_ibpb(void);
 
+#ifdef CONFIG_CALL_THUNKS
+extern void (*x86_return_thunk)(void);
+#else
+#define x86_return_thunk	(&__x86_return_thunk)
+#endif
+
+#ifdef CONFIG_CALL_DEPTH_TRACKING
+extern void __x86_return_skl(void);
+
+static inline void x86_set_skl_return_thunk(void)
+{
+	x86_return_thunk = &__x86_return_skl;
+}
+
+#define CALL_DEPTH_ACCOUNT					\
+	ALTERNATIVE("",						\
+		    __stringify(INCREMENT_CALL_DEPTH),		\
+		    X86_FEATURE_CALL_DEPTH)
+
+#ifdef CONFIG_CALL_THUNKS_DEBUG
+DECLARE_PER_CPU(u64, __x86_call_count);
+DECLARE_PER_CPU(u64, __x86_ret_count);
+DECLARE_PER_CPU(u64, __x86_stuffs_count);
+DECLARE_PER_CPU(u64, __x86_ctxsw_count);
+#endif
+#else
+static inline void x86_set_skl_return_thunk(void) {}
+
+#define CALL_DEPTH_ACCOUNT ""
+
+#endif
+
 #ifdef CONFIG_RETPOLINE
 
 #define GEN(reg) \
@@ -215,6 +369,16 @@ extern void entry_ibpb(void);
 #include <asm/GEN-for-each-reg.h>
 #undef GEN
 
+#define GEN(reg)						\
+	extern retpoline_thunk_t __x86_indirect_call_thunk_ ## reg;
+#include <asm/GEN-for-each-reg.h>
+#undef GEN
+
+#define GEN(reg)						\
+	extern retpoline_thunk_t __x86_indirect_jump_thunk_ ## reg;
+#include <asm/GEN-for-each-reg.h>
+#undef GEN
+
 #ifdef CONFIG_X86_64
 
 /*