Revert "x86/entry: Stuff RSB for entry to kernel for non-SMEP platform"

CVE-2017-5753 (revert embargoed)
CVE-2017-5715 (revert embargoed)

This reverts commit 47a07600.
Signed-off-by: Andy Whitcroft <apw@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

arch/x86/entry/entry_64.S

@@ -175,7 +175,6 @@ GLOBAL(entry_SYSCALL_64_after_swapgs)
 	sub	$(6*8), %rsp			/* pt_regs->bp, bx, r12-15 not saved */
 
 	ENABLE_IBRS
-	STUFF_RSB
 
 	testl	$_TIF_WORK_SYSCALL_ENTRY, ASM_THREAD_INFO(TI_flags, %rsp, SIZEOF_PTREGS)
 	jnz	tracesys
@@ -536,12 +535,6 @@ END(irq_entries_start)
 	ALLOC_PT_GPREGS_ON_STACK
 	SAVE_C_REGS
 	SAVE_EXTRA_REGS
-	/*
-	 * Have to do stuffing before encoding frame pointer.
-	 * Could add some unnecessary RSB clearing if coming
-	 * from kernel for non-SMEP platform.
-	 */
-	STUFF_RSB
 
 	testb	$3, CS(%rsp)
 	jz	1f
@@ -1054,10 +1047,6 @@ ENTRY(paranoid_entry)
 	cld
 	SAVE_C_REGS 8
 	SAVE_EXTRA_REGS 8
-	/*
-	 * Do the stuffing unconditionally from user/kernel to be safe
-	 */
-	STUFF_RSB
 	movl	$1, %ebx
 	movl	$MSR_GS_BASE, %ecx
 	rdmsr
@@ -1141,7 +1130,6 @@ ENTRY(error_entry)
 	 * the kernel CR3 here.
 	 */
 	SWITCH_KERNEL_CR3
-	STUFF_RSB
 	xorl	%ebx, %ebx
 	testb	$3, CS+8(%rsp)
 	jz	.Lerror_kernelspace

arch/x86/entry/entry_64_compat.S

@@ -101,7 +101,6 @@ ENTRY(entry_SYSENTER_compat)
 	cld
 
 	ENABLE_IBRS
-	STUFF_RSB
 
 	/*
 	 * Sysenter doesn't filter flags, so we need to clear NT
@@ -202,7 +201,6 @@ ENTRY(entry_SYSCALL_compat)
 	pushq   %r8                     /* pt_regs->r15 = 0 */
 
 	ENABLE_IBRS
-	STUFF_RSB
 
 	/*
 	 * User mode is traced as though IRQs are on, and SYSENTER
@@ -314,7 +312,6 @@ ENTRY(entry_INT80_compat)
 	cld
 
 	ENABLE_IBRS
-	STUFF_RSB
 
 	/*
 	 * User mode is traced as though IRQs are on, and the interrupt

arch/x86/include/asm/spec_ctrl.h

@@ -35,73 +35,6 @@
 	popq %rdx;				\
 	popq %rcx;				\
 	popq %rax
-#define __ASM_STUFF_RSB				\
-	call	1f;				\
-	pause;					\
-1:	call	2f;				\
-	pause;					\
-2:	call	3f;				\
-	pause;					\
-3:	call	4f;				\
-	pause;					\
-4:	call	5f;				\
-	pause;					\
-5:	call	6f;				\
-	pause;					\
-6:	call	7f;				\
-	pause;					\
-7:	call	8f;				\
-	pause;					\
-8:	call	9f;				\
-	pause;					\
-9:	call	10f;				\
-	pause;					\
-10:	call	11f;				\
-	pause;					\
-11:	call	12f;				\
-	pause;					\
-12:	call	13f;				\
-	pause;					\
-13:	call	14f;				\
-	pause;					\
-14:	call	15f;				\
-	pause;					\
-15:	call	16f;				\
-	pause;					\
-16:	call	17f;				\
-	pause;					\
-17:	call	18f;				\
-	pause;					\
-18:	call	19f;				\
-	pause;					\
-19:	call	20f;				\
-	pause;					\
-20:	call	21f;				\
-	pause;					\
-21:	call	22f;				\
-	pause;					\
-22:	call	23f;				\
-	pause;					\
-23:	call	24f;				\
-	pause;					\
-24:	call	25f;				\
-	pause;					\
-25:	call	26f;				\
-	pause;					\
-26:	call	27f;				\
-	pause;					\
-27:	call	28f;				\
-	pause;					\
-28:	call	29f;				\
-	pause;					\
-29:	call	30f;				\
-	pause;					\
-30:	call	31f;				\
-	pause;					\
-31:	call	32f;				\
-	pause;					\
-32:						\
-	add $(32*8), %rsp;
 
 .macro ENABLE_IBRS
 ALTERNATIVE "", __stringify(__ASM_ENABLE_IBRS), X86_FEATURE_SPEC_CTRL
@@ -115,9 +48,5 @@ ALTERNATIVE "", __stringify(__ASM_ENABLE_IBRS_CLOBBER), X86_FEATURE_SPEC_CTRL
 ALTERNATIVE "", __stringify(__ASM_DISABLE_IBRS), X86_FEATURE_SPEC_CTRL
 .endm
 
-.macro STUFF_RSB
-ALTERNATIVE __stringify(__ASM_STUFF_RSB), "", X86_FEATURE_SMEP
-.endm
-
 #endif /* __ASSEMBLY__ */
 #endif /* _ASM_X86_SPEC_CTRL_H */