diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 4cc0125..15739a2 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -288,6 +288,7 @@ SYM_FUNC_END(__switch_to_asm)
 SYM_CODE_START_NOALIGN(ret_from_fork)
 	UNWIND_HINT_EMPTY
 	ANNOTATE_NOENDBR // copy_thread
+	CALL_DEPTH_ACCOUNT
 	movq	%rax, %rdi
 	call	schedule_tail			/* rdi: 'prev' task parameter */
 
@@ -332,7 +333,7 @@ SYM_CODE_START(xen_error_entry)
 	UNWIND_HINT_FUNC
 	PUSH_AND_CLEAR_REGS save_ret=1
 	ENCODE_FRAME_POINTER 8
-	UNTRAIN_RET
+	UNTRAIN_RET_FROM_CALL
 	RET
 SYM_CODE_END(xen_error_entry)
 
@@ -977,7 +978,7 @@ SYM_CODE_START(paranoid_entry)
 	 * CR3 above, keep the old value in a callee saved register.
 	 */
 	IBRS_ENTER save_reg=%r15
-	UNTRAIN_RET
+	UNTRAIN_RET_FROM_CALL
 
 	RET
 SYM_CODE_END(paranoid_entry)
@@ -1062,7 +1063,7 @@ SYM_CODE_START(error_entry)
 	/* We have user CR3.  Change to kernel CR3. */
 	SWITCH_TO_KERNEL_CR3 scratch_reg=%rax
 	IBRS_ENTER
-	UNTRAIN_RET
+	UNTRAIN_RET_FROM_CALL
 
 	leaq	8(%rsp), %rdi			/* arg0 = pt_regs pointer */
 	/* Put us onto the real thread stack. */
@@ -1097,6 +1098,7 @@ SYM_CODE_START(error_entry)
 	 */
 .Lerror_entry_done_lfence:
 	FENCE_SWAPGS_KERNEL_ENTRY
+	CALL_DEPTH_ACCOUNT
 	leaq	8(%rsp), %rax			/* return pt_regs pointer */
 	ANNOTATE_UNRET_END
 	RET
@@ -1115,7 +1117,7 @@ SYM_CODE_START(error_entry)
 	FENCE_SWAPGS_USER_ENTRY
 	SWITCH_TO_KERNEL_CR3 scratch_reg=%rax
 	IBRS_ENTER
-	UNTRAIN_RET
+	UNTRAIN_RET_FROM_CALL
 
 	/*
 	 * Pretend that the exception came from user mode: set up pt_regs
diff --git a/arch/x86/include/asm/current.h b/arch/x86/include/asm/current.h
index b89aba0..a1168e7 100644
--- a/arch/x86/include/asm/current.h
+++ b/arch/x86/include/asm/current.h
@@ -17,6 +17,9 @@ struct pcpu_hot {
 			struct task_struct	*current_task;
 			int			preempt_count;
 			int			cpu_number;
+#ifdef CONFIG_CALL_DEPTH_TRACKING
+			u64			call_depth;
+#endif
 			unsigned long		top_of_stack;
 			void			*hardirq_stack_ptr;
 			u16			softirq_pending;
diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
index f10ca33..d4be826 100644
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -12,8 +12,83 @@
 #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
+
+#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);
+
+#define INCREMENT_CALL_DEPTH					\
+	sarq	$5, %gs:pcpu_hot + X86_call_depth;
+
+#define ASM_INCREMENT_CALL_DEPTH				\
+	sarq	$5, PER_CPU_VAR(pcpu_hot + X86_call_depth);
+
+#else
+#define CREDIT_CALL_DEPTH
+#define RESET_CALL_DEPTH
+#define INCREMENT_CALL_DEPTH
+#define RESET_CALL_DEPTH_FROM_CALL
+#endif
 
 /*
  * Fill the CPU return stack buffer.
@@ -32,6 +107,7 @@
  * 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 +136,8 @@
 	dec	reg;					\
 	jnz	771b;					\
 	/* barrier for jnz misprediction */		\
-	lfence;
+	lfence;						\
+	ASM_CREDIT_CALL_DEPTH
 #else
 /*
  * i386 doesn't unconditionally have LFENCE, as such it can't
@@ -185,11 +262,32 @@
  * 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_X86_FEATURE_CALL_DEPTH)
 	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_X86_FEATURE_CALL_DEPTH)
+	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
 
@@ -214,6 +312,17 @@ extern void (*x86_return_thunk)(void);
 #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;
+}
+#else
+static inline void x86_set_skl_return_thunk(void) {}
+#endif
+
 #ifdef CONFIG_RETPOLINE
 
 #define GEN(reg) \
diff --git a/arch/x86/kernel/asm-offsets.c b/arch/x86/kernel/asm-offsets.c
index a982431..13afdbb 100644
--- a/arch/x86/kernel/asm-offsets.c
+++ b/arch/x86/kernel/asm-offsets.c
@@ -110,6 +110,9 @@ static void __used common(void)
 	OFFSET(TSS_sp2, tss_struct, x86_tss.sp2);
 
 	OFFSET(X86_top_of_stack, pcpu_hot, top_of_stack);
+#ifdef CONFIG_CALL_DEPTH_TRACKING
+	OFFSET(X86_call_depth, pcpu_hot, call_depth);
+#endif
 
 	if (IS_ENABLED(CONFIG_KVM_INTEL)) {
 		BLANK();
diff --git a/arch/x86/kvm/svm/vmenter.S b/arch/x86/kvm/svm/vmenter.S
index 723f853..09eacf1 100644
--- a/arch/x86/kvm/svm/vmenter.S
+++ b/arch/x86/kvm/svm/vmenter.S
@@ -1,6 +1,7 @@
 /* SPDX-License-Identifier: GPL-2.0 */
 #include <linux/linkage.h>
 #include <asm/asm.h>
+#include <asm/asm-offsets.h>
 #include <asm/bitsperlong.h>
 #include <asm/kvm_vcpu_regs.h>
 #include <asm/nospec-branch.h>
diff --git a/arch/x86/lib/retpoline.S b/arch/x86/lib/retpoline.S
index 073289a..1e79ecc 100644
--- a/arch/x86/lib/retpoline.S
+++ b/arch/x86/lib/retpoline.S
@@ -5,9 +5,11 @@
 #include <asm/dwarf2.h>
 #include <asm/cpufeatures.h>
 #include <asm/alternative.h>
+#include <asm/asm-offsets.h>
 #include <asm/export.h>
 #include <asm/nospec-branch.h>
 #include <asm/unwind_hints.h>
+#include <asm/percpu.h>
 #include <asm/frame.h>
 
 	.section .text.__x86.indirect_thunk
@@ -140,3 +142,32 @@ __EXPORT_THUNK(zen_untrain_ret)
 EXPORT_SYMBOL(__x86_return_thunk)
 
 #endif /* CONFIG_RETHUNK */
+
+#ifdef CONFIG_CALL_DEPTH_TRACKING
+
+	.align 64
+SYM_FUNC_START(__x86_return_skl)
+	ANNOTATE_NOENDBR
+	/* Keep the hotpath in a 16byte I-fetch */
+	shlq	$5, PER_CPU_VAR(pcpu_hot + X86_call_depth)
+	jz	1f
+	ANNOTATE_UNRET_SAFE
+	ret
+	int3
+1:
+	.rept	16
+	ANNOTATE_INTRA_FUNCTION_CALL
+	call	2f
+	int3
+2:
+	.endr
+	add	$(8*16), %rsp
+
+	CREDIT_CALL_DEPTH
+
+	ANNOTATE_UNRET_SAFE
+	ret
+	int3
+SYM_FUNC_END(__x86_return_skl)
+
+#endif /* CONFIG_CALL_DEPTH_TRACKING */
