linux-kernel - Re: [PATCH v3 2/3] x86/entry: Clear registers for 64bit exceptions/interrupts

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Message-ID: <20180206091727.GB15925@isilmar-4.linta.de>
Date:   Tue, 6 Feb 2018 10:17:27 +0100
From:   Dominik Brodowski <linux@...inikbrodowski.net>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     tglx@...utronix.de, mingo@...nel.org,
        Andi Kleen <ak@...ux.intel.com>, torvalds@...ux-foundation.org,
        linux-kernel@...r.kernel.org, luto@...nel.org
Subject: Re: [PATCH v3 2/3] x86/entry: Clear registers for 64bit
 exceptions/interrupts

On Mon, Feb 05, 2018 at 05:18:11PM -0800, Dan Williams wrote:
> From: Andi Kleen <ak@...ux.intel.com>
> 
> Clear the 'extra' registers on entering the 64bit kernel for exceptions
> and interrupts. The common registers are not cleared since they are
> likely clobbered well before they can be exploited in a speculative
> execution attack.

Could the clever trick from patch 3/3 (interleave xorq with movq) be
used here as well? Something like below (untested)? This includes removing
the (seemingly?) unused SAVE_C_REGS_EXCEPT_* macros, which probably needs to
become a spearate patch.

Thanks,
	Dominik


diff --git a/arch/x86/entry/calling.h b/arch/x86/entry/calling.h
index 3f48f695d5e6..7bdee6d14f0a 100644
--- a/arch/x86/entry/calling.h
+++ b/arch/x86/entry/calling.h
@@ -101,49 +101,36 @@ For 32-bit we have the following conventions - kernel is built with
 	addq	$-(15*8), %rsp
 	.endm
 
-	.macro SAVE_C_REGS_HELPER offset=0 rax=1 rcx=1 r8910=1 r11=1
-	.if \r11
+	.macro SAVE_C_REGS offset=0
+	xorq %r11, %r11				/* nospec r11 */
 	movq %r11, 6*8+\offset(%rsp)
-	.endif
-	.if \r8910
 	movq %r10, 7*8+\offset(%rsp)
+	xorq %r10, %r10				/* nospec r10 */
 	movq %r9,  8*8+\offset(%rsp)
+	xorq %r9, %r9				/* nospec r9 */
 	movq %r8,  9*8+\offset(%rsp)
-	.endif
-	.if \rax
+	xorq %r8, %r8				/* nospec r8 */
 	movq %rax, 10*8+\offset(%rsp)
-	.endif
-	.if \rcx
 	movq %rcx, 11*8+\offset(%rsp)
-	.endif
 	movq %rdx, 12*8+\offset(%rsp)
 	movq %rsi, 13*8+\offset(%rsp)
 	movq %rdi, 14*8+\offset(%rsp)
 	UNWIND_HINT_REGS offset=\offset extra=0
 	.endm
-	.macro SAVE_C_REGS offset=0
-	SAVE_C_REGS_HELPER \offset, 1, 1, 1, 1
-	.endm
-	.macro SAVE_C_REGS_EXCEPT_RAX_RCX offset=0
-	SAVE_C_REGS_HELPER \offset, 0, 0, 1, 1
-	.endm
-	.macro SAVE_C_REGS_EXCEPT_R891011
-	SAVE_C_REGS_HELPER 0, 1, 1, 0, 0
-	.endm
-	.macro SAVE_C_REGS_EXCEPT_RCX_R891011
-	SAVE_C_REGS_HELPER 0, 1, 0, 0, 0
-	.endm
-	.macro SAVE_C_REGS_EXCEPT_RAX_RCX_R11
-	SAVE_C_REGS_HELPER 0, 0, 0, 1, 0
-	.endm
 
 	.macro SAVE_EXTRA_REGS offset=0
 	movq %r15, 0*8+\offset(%rsp)
+	xorq %r15, %r15				/* nospec r15 */
 	movq %r14, 1*8+\offset(%rsp)
+	xorq %r14, %r14				/* nospec r14 */
 	movq %r13, 2*8+\offset(%rsp)
+	xorq %r13, %r13				/* nospec r13 */
 	movq %r12, 3*8+\offset(%rsp)
+	xorq %r12, %r12				/* nospec r12*/
 	movq %rbp, 4*8+\offset(%rsp)
+	xorl %ebp, %ebp				/* nospec rbp */
 	movq %rbx, 5*8+\offset(%rsp)
+	xorl %ebx, %ebx				/* nospec rbx */
 	UNWIND_HINT_REGS offset=\offset
 	.endm
 
diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index c752abe89d80..de19a46f40b2 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -1218,7 +1218,6 @@ ENTRY(error_entry)
 	SAVE_C_REGS 8
 	SAVE_EXTRA_REGS 8
 	ENCODE_FRAME_POINTER 8
-	xorl	%ebx, %ebx
 	testb	$3, CS+8(%rsp)
 	jz	.Lerror_kernelspace
 
@@ -1405,15 +1404,25 @@ ENTRY(nmi)
 	pushq   %rcx		/* pt_regs->cx */
 	pushq   %rax		/* pt_regs->ax */
 	pushq   %r8		/* pt_regs->r8 */
+	xorq    %r8, %r8	/* nospec   r8 */
 	pushq   %r9		/* pt_regs->r9 */
+	xorq    %r9, %r9	/* nospec   r9 */
 	pushq   %r10		/* pt_regs->r10 */
+	xorq    %r10, %r10	/* nospec   r10 */
 	pushq   %r11		/* pt_regs->r11 */
+	xorq    %r11, %r11	/* nospec   r11*/
 	pushq	%rbx		/* pt_regs->rbx */
+	xorl    %ebx, %ebx	/* nospec   rbx*/
 	pushq	%rbp		/* pt_regs->rbp */
+	xorl    %ebp, %ebp	/* nospec   rbp*/
 	pushq	%r12		/* pt_regs->r12 */
+	xorq    %r12, %r12	/* nospec   r12*/
 	pushq	%r13		/* pt_regs->r13 */
+	xorq    %r13, %r13	/* nospec   r13*/
 	pushq	%r14		/* pt_regs->r14 */
+	xorq    %r14, %r14	/* nospec   r14*/
 	pushq	%r15		/* pt_regs->r15 */
+	xorq    %r15, %r15	/* nospec   r15*/
 	UNWIND_HINT_REGS
 	ENCODE_FRAME_POINTER