Revert "x86/syscall: Clear unused extra registers on 32-bit compatible syscall entrance"

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

This reverts commit 70df98e2.
Signed-off-by: Andy Whitcroft <apw@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

arch/x86/entry/calling.h

@@ -195,17 +195,6 @@ For 32-bit we have the following conventions - kernel is built with
 	subq $-(15*8+\addskip), %rsp
 	.endm
 
-	.macro CLEAR_R8_TO_R15
-	xorq %r15, %r15
-	xorq %r14, %r14
-	xorq %r13, %r13
-	xorq %r12, %r12
-	xorq %r11, %r11
-	xorq %r10, %r10
-	xorq %r9, %r9
-	xorq %r8, %r8
-	.endm
-
 	.macro CLEAR_EXTRA_REGS
 	xorq %r15, %r15
 	xorq %r14, %r14

arch/x86/entry/entry_64_compat.S

@@ -103,8 +103,6 @@ ENTRY(entry_SYSENTER_compat)
 	ENABLE_IBRS
 	STUFF_RSB
 
-	CLEAR_R8_TO_R15
-
 	/*
 	 * Sysenter doesn't filter flags, so we need to clear NT
 	 * ourselves.  To save a few cycles, we can check whether
@@ -198,12 +196,10 @@ ENTRY(entry_SYSCALL_compat)
 	pushq   %r8                     /* pt_regs->r11 = 0 */
 	pushq   %rbx                    /* pt_regs->rbx */
 	pushq   %rbp                    /* pt_regs->rbp (will be overwritten) */
-	pushq   %r12                    /* pt_regs->r12 */
-	pushq   %r13                    /* pt_regs->r13 */
-	pushq   %r14                    /* pt_regs->r14 */
-	pushq   %r15                    /* pt_regs->r15 */
-
-	CLEAR_R8_TO_R15
+	pushq   %r8                     /* pt_regs->r12 = 0 */
+	pushq   %r8                     /* pt_regs->r13 = 0 */
+	pushq   %r8                     /* pt_regs->r14 = 0 */
+	pushq   %r8                     /* pt_regs->r15 = 0 */
 
 	ENABLE_IBRS
 	STUFF_RSB
@@ -225,11 +221,6 @@ sysret32_from_system_call:
 	TRACE_IRQS_ON			/* User mode traces as IRQs on. */
 	DISABLE_IBRS
 	SWITCH_USER_CR3
-
-	movq    R15(%rsp), %r15         /* pt_regs->r15 */
-	movq    R14(%rsp), %r14         /* pt_regs->r14 */
-	movq    R13(%rsp), %r13         /* pt_regs->r13 */
-	movq    R12(%rsp), %r12         /* pt_regs->r12 */
 	movq	RBX(%rsp), %rbx		/* pt_regs->rbx */
 	movq	RBP(%rsp), %rbp		/* pt_regs->rbp */
 	movq	EFLAGS(%rsp), %r11	/* pt_regs->flags (in r11) */
@@ -325,8 +316,6 @@ ENTRY(entry_INT80_compat)
 	ENABLE_IBRS
 	STUFF_RSB
 
-	CLEAR_R8_TO_R15
-
 	/*
 	 * User mode is traced as though IRQs are on, and the interrupt
 	 * gate turned them off.