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hc
2024-02-20 102a0743326a03cd1a1202ceda21e175b7d3575c 
 kernel/arch/x86/lib/retpoline.S
....@@ -4,18 +4,39 @@
44 #include <linux/linkage.h>
55 #include <asm/dwarf2.h>
66 #include <asm/cpufeatures.h>
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-#include <asm/alternative-asm.h>
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+#include <asm/alternative.h>
88 #include <asm/export.h>
99 #include <asm/nospec-branch.h>
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+#include <asm/unwind_hints.h>
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+#include <asm/frame.h>
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+#include <asm/nops.h>
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+
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+	.section .text..__x86.indirect_thunk
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+
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+.macro RETPOLINE reg
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+	ANNOTATE_INTRA_FUNCTION_CALL
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+	call    .Ldo_rop_\@
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+.Lspec_trap_\@:
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+	UNWIND_HINT_EMPTY
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+	pause
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+	lfence
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+	jmp .Lspec_trap_\@
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+.Ldo_rop_\@:
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+	mov     %\reg, (%_ASM_SP)
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+	UNWIND_HINT_FUNC
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+	RET
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+.endm
1029 
1130 .macro THUNK reg
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-	.section .text.__x86.indirect_thunk
1331 
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-ENTRY(__x86_indirect_thunk_\reg)
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-	CFI_STARTPROC
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-	JMP_NOSPEC %\reg
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-	CFI_ENDPROC
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-ENDPROC(__x86_indirect_thunk_\reg)
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+	.align RETPOLINE_THUNK_SIZE
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+SYM_INNER_LABEL(__x86_indirect_thunk_\reg, SYM_L_GLOBAL)
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+	UNWIND_HINT_EMPTY
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+
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+	ALTERNATIVE_2 __stringify(RETPOLINE \reg), \
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+		      __stringify(lfence; ANNOTATE_RETPOLINE_SAFE; jmp *%\reg; int3), X86_FEATURE_RETPOLINE_LFENCE, \
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+		      __stringify(ANNOTATE_RETPOLINE_SAFE; jmp *%\reg), ALT_NOT(X86_FEATURE_RETPOLINE)
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+
1940 .endm
2041 
2142 /*
....@@ -24,25 +45,223 @@
2445  * only see one instance of "__x86_indirect_thunk_\reg" rather
2546  * than one per register with the correct names. So we do it
2647  * the simple and nasty way...
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+ *
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+ * Worse, you can only have a single EXPORT_SYMBOL per line,
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+ * and CPP can't insert newlines, so we have to repeat everything
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+ * at least twice.
2752  */
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-#define __EXPORT_THUNK(sym) _ASM_NOKPROBE(sym); EXPORT_SYMBOL(sym)
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-#define EXPORT_THUNK(reg) __EXPORT_THUNK(__x86_indirect_thunk_ ## reg)
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-#define GENERATE_THUNK(reg) THUNK reg ; EXPORT_THUNK(reg)
3153 
32
-GENERATE_THUNK(_ASM_AX)
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-GENERATE_THUNK(_ASM_BX)
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-GENERATE_THUNK(_ASM_CX)
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-GENERATE_THUNK(_ASM_DX)
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-GENERATE_THUNK(_ASM_SI)
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-GENERATE_THUNK(_ASM_DI)
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-GENERATE_THUNK(_ASM_BP)
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-#ifdef CONFIG_64BIT
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-GENERATE_THUNK(r8)
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-GENERATE_THUNK(r9)
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-GENERATE_THUNK(r10)
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-GENERATE_THUNK(r11)
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-GENERATE_THUNK(r12)
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-GENERATE_THUNK(r13)
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-GENERATE_THUNK(r14)
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-GENERATE_THUNK(r15)
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+#define __EXPORT_THUNK(sym)	_ASM_NOKPROBE(sym); EXPORT_SYMBOL(sym)
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+#define EXPORT_THUNK(reg)	__EXPORT_THUNK(__x86_indirect_thunk_ ## reg)
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+
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+	.align RETPOLINE_THUNK_SIZE
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+SYM_CODE_START(__x86_indirect_thunk_array)
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+
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+#define GEN(reg) THUNK reg
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+#include <asm/GEN-for-each-reg.h>
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+#undef GEN
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+
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+	.align RETPOLINE_THUNK_SIZE
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+SYM_CODE_END(__x86_indirect_thunk_array)
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+
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+#define GEN(reg) EXPORT_THUNK(reg)
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+#include <asm/GEN-for-each-reg.h>
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+#undef GEN
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+
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+/*
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+ * This function name is magical and is used by -mfunction-return=thunk-extern
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+ * for the compiler to generate JMPs to it.
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+ */
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+#ifdef CONFIG_RETHUNK
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+
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+/*
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+ * srso_alias_untrain_ret() and srso_alias_safe_ret() are placed at
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+ * special addresses:
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+ *
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+ * - srso_alias_untrain_ret() is 2M aligned
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+ * - srso_alias_safe_ret() is also in the same 2M page but bits 2, 8, 14
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+ * and 20 in its virtual address are set (while those bits in the
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+ * srso_alias_untrain_ret() function are cleared).
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+ *
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+ * This guarantees that those two addresses will alias in the branch
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+ * target buffer of Zen3/4 generations, leading to any potential
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+ * poisoned entries at that BTB slot to get evicted.
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+ *
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+ * As a result, srso_alias_safe_ret() becomes a safe return.
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+ */
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+#ifdef CONFIG_CPU_SRSO
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+	.section .text..__x86.rethunk_untrain
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+
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+SYM_START(srso_alias_untrain_ret, SYM_L_GLOBAL, SYM_A_NONE)
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+	UNWIND_HINT_FUNC
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+	ASM_NOP2
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+	lfence
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+	jmp srso_alias_return_thunk
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+SYM_FUNC_END(srso_alias_untrain_ret)
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+__EXPORT_THUNK(srso_alias_untrain_ret)
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+
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+	.section .text..__x86.rethunk_safe
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+#else
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+/* dummy definition for alternatives */
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+SYM_START(srso_alias_untrain_ret, SYM_L_GLOBAL, SYM_A_NONE)
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+	ANNOTATE_UNRET_SAFE
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+	ret
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+	int3
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+SYM_FUNC_END(srso_alias_untrain_ret)
48111 #endif
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+
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+SYM_START(srso_alias_safe_ret, SYM_L_GLOBAL, SYM_A_NONE)
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+	lea 8(%_ASM_SP), %_ASM_SP
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+	UNWIND_HINT_FUNC
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+	ANNOTATE_UNRET_SAFE
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+	ret
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+	int3
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+SYM_FUNC_END(srso_alias_safe_ret)
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+
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+	.section .text..__x86.return_thunk
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+
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+SYM_CODE_START(srso_alias_return_thunk)
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+	UNWIND_HINT_FUNC
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+	ANNOTATE_NOENDBR
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+	call srso_alias_safe_ret
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+	ud2
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+SYM_CODE_END(srso_alias_return_thunk)
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+
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+/*
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+ * Some generic notes on the untraining sequences:
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+ *
133
+ * They are interchangeable when it comes to flushing potentially wrong
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+ * RET predictions from the BTB.
135
+ *
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+ * The SRSO Zen1/2 (MOVABS) untraining sequence is longer than the
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+ * Retbleed sequence because the return sequence done there
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+ * (srso_safe_ret()) is longer and the return sequence must fully nest
139
+ * (end before) the untraining sequence. Therefore, the untraining
140
+ * sequence must fully overlap the return sequence.
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+ *
142
+ * Regarding alignment - the instructions which need to be untrained,
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+ * must all start at a cacheline boundary for Zen1/2 generations. That
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+ * is, instruction sequences starting at srso_safe_ret() and
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+ * the respective instruction sequences at retbleed_return_thunk()
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+ * must start at a cacheline boundary.
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+ */
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+
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+/*
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+ * Safety details here pertain to the AMD Zen{1,2} microarchitecture:
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+ * 1) The RET at retbleed_return_thunk must be on a 64 byte boundary, for
152
+ *    alignment within the BTB.
153
+ * 2) The instruction at retbleed_untrain_ret must contain, and not
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+ *    end with, the 0xc3 byte of the RET.
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+ * 3) STIBP must be enabled, or SMT disabled, to prevent the sibling thread
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+ *    from re-poisioning the BTB prediction.
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+ */
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+	.align 64
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+	.skip 64 - (retbleed_return_thunk - retbleed_untrain_ret), 0xcc
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+SYM_FUNC_START_NOALIGN(retbleed_untrain_ret);
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+
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+	/*
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+	 * As executed from retbleed_untrain_ret, this is:
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+	 *
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+	 *   TEST $0xcc, %bl
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+	 *   LFENCE
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+	 *   JMP retbleed_return_thunk
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+	 *
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+	 * Executing the TEST instruction has a side effect of evicting any BTB
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+	 * prediction (potentially attacker controlled) attached to the RET, as
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+	 * retbleed_return_thunk + 1 isn't an instruction boundary at the moment.
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+	 */
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+	.byte	0xf6
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+
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+	/*
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+	 * As executed from retbleed_return_thunk, this is a plain RET.
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+	 *
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+	 * As part of the TEST above, RET is the ModRM byte, and INT3 the imm8.
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+	 *
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+	 * We subsequently jump backwards and architecturally execute the RET.
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+	 * This creates a correct BTB prediction (type=ret), but in the
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+	 * meantime we suffer Straight Line Speculation (because the type was
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+	 * no branch) which is halted by the INT3.
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+	 *
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+	 * With SMT enabled and STIBP active, a sibling thread cannot poison
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+	 * RET's prediction to a type of its choice, but can evict the
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+	 * prediction due to competitive sharing. If the prediction is
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+	 * evicted, retbleed_return_thunk will suffer Straight Line Speculation
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+	 * which will be contained safely by the INT3.
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+	 */
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+SYM_INNER_LABEL(retbleed_return_thunk, SYM_L_GLOBAL)
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+	ret
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+	int3
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+SYM_CODE_END(retbleed_return_thunk)
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+
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+	/*
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+	 * Ensure the TEST decoding / BTB invalidation is complete.
198
+	 */
199
+	lfence
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+
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+	/*
202
+	 * Jump back and execute the RET in the middle of the TEST instruction.
203
+	 * INT3 is for SLS protection.
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+	 */
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+	jmp retbleed_return_thunk
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+	int3
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+SYM_FUNC_END(retbleed_untrain_ret)
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+__EXPORT_THUNK(retbleed_untrain_ret)
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+
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+/*
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+ * SRSO untraining sequence for Zen1/2, similar to retbleed_untrain_ret()
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+ * above. On kernel entry, srso_untrain_ret() is executed which is a
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+ *
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+ * movabs $0xccccc30824648d48,%rax
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+ *
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+ * and when the return thunk executes the inner label srso_safe_ret()
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+ * later, it is a stack manipulation and a RET which is mispredicted and
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+ * thus a "safe" one to use.
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+ */
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+	.align 64
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+	.skip 64 - (srso_safe_ret - srso_untrain_ret), 0xcc
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+SYM_START(srso_untrain_ret, SYM_L_GLOBAL, SYM_A_NONE)
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+	.byte 0x48, 0xb8
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+
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+/*
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+ * This forces the function return instruction to speculate into a trap
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+ * (UD2 in srso_return_thunk() below).  This RET will then mispredict
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+ * and execution will continue at the return site read from the top of
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+ * the stack.
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+ */
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+SYM_INNER_LABEL(srso_safe_ret, SYM_L_GLOBAL)
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+	lea 8(%_ASM_SP), %_ASM_SP
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+	ret
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+	int3
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+	int3
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+	/* end of movabs */
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+	lfence
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+	call srso_safe_ret
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+	ud2
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+SYM_CODE_END(srso_safe_ret)
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+SYM_FUNC_END(srso_untrain_ret)
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+__EXPORT_THUNK(srso_untrain_ret)
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+
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+SYM_CODE_START(srso_return_thunk)
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+	UNWIND_HINT_FUNC
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+	ANNOTATE_NOENDBR
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+	call srso_safe_ret
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+	ud2
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+SYM_CODE_END(srso_return_thunk)
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+
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+SYM_FUNC_START(entry_untrain_ret)
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+	ALTERNATIVE_2 "jmp retbleed_untrain_ret", \
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+		      "jmp srso_untrain_ret", X86_FEATURE_SRSO, \
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+		      "jmp srso_alias_untrain_ret", X86_FEATURE_SRSO_ALIAS
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+SYM_FUNC_END(entry_untrain_ret)
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+__EXPORT_THUNK(entry_untrain_ret)
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+
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+SYM_CODE_START(__x86_return_thunk)
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+	UNWIND_HINT_FUNC
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+	ANNOTATE_NOENDBR
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+	ANNOTATE_UNRET_SAFE
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+	ret
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+	int3
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+SYM_CODE_END(__x86_return_thunk)
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+EXPORT_SYMBOL(__x86_return_thunk)
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+
267
+#endif /* CONFIG_RETHUNK */