linux-kernel - [PATCH 4.14 057/167] x86/entry/64: Clear registers for exceptions/interrupts, to reduce speculation attack surface

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Date:   Wed, 21 Feb 2018 13:47:48 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Dan Williams <dan.j.williams@...el.com>,
        Andy Lutomirski <luto@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        Brian Gerst <brgerst@...il.com>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>
Subject: [PATCH 4.14 057/167] x86/entry/64: Clear registers for exceptions/interrupts, to reduce speculation attack surface

4.14-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Dan Williams <dan.j.williams@...el.com>

commit 3ac6d8c787b835b997eb23e43e09aa0895ef7d58 upstream.

Clear the 'extra' registers on entering the 64-bit 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.

Originally-From: Andi Kleen <ak@...ux.intel.com>
Signed-off-by: Dan Williams <dan.j.williams@...el.com>
Cc: <stable@...r.kernel.org>
Cc: Andy Lutomirski <luto@...nel.org>
Cc: Borislav Petkov <bp@...en8.de>
Cc: Brian Gerst <brgerst@...il.com>
Cc: Denys Vlasenko <dvlasenk@...hat.com>
Cc: H. Peter Anvin <hpa@...or.com>
Cc: Josh Poimboeuf <jpoimboe@...hat.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Link: http://lkml.kernel.org/r/151787989146.7847.15749181712358213254.stgit@dwillia2-desk3.amr.corp.intel.com
[ Made small improvements to the changelog and the code comments. ]
Signed-off-by: Ingo Molnar <mingo@...nel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 arch/x86/entry/calling.h  |   19 +++++++++++++++++++
 arch/x86/entry/entry_64.S |    6 +++++-
 2 files changed, 24 insertions(+), 1 deletion(-)

--- a/arch/x86/entry/calling.h
+++ b/arch/x86/entry/calling.h
@@ -147,6 +147,25 @@ For 32-bit we have the following convent
 	UNWIND_HINT_REGS offset=\offset
 	.endm
 
+	/*
+	 * Sanitize registers of values that a speculation attack
+	 * might otherwise want to exploit. The lower registers are
+	 * likely clobbered well before they could be put to use in
+	 * a speculative execution gadget:
+	 */
+	.macro CLEAR_REGS_NOSPEC
+	xorl %ebp, %ebp
+	xorl %ebx, %ebx
+	xorq %r8, %r8
+	xorq %r9, %r9
+	xorq %r10, %r10
+	xorq %r11, %r11
+	xorq %r12, %r12
+	xorq %r13, %r13
+	xorq %r14, %r14
+	xorq %r15, %r15
+	.endm
+
 	.macro POP_EXTRA_REGS
 	popq %r15
 	popq %r14
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -571,6 +571,7 @@ END(irq_entries_start)
 	ALLOC_PT_GPREGS_ON_STACK
 	SAVE_C_REGS
 	SAVE_EXTRA_REGS
+	CLEAR_REGS_NOSPEC
 	ENCODE_FRAME_POINTER
 
 	testb	$3, CS(%rsp)
@@ -1127,6 +1128,7 @@ ENTRY(xen_failsafe_callback)
 	ALLOC_PT_GPREGS_ON_STACK
 	SAVE_C_REGS
 	SAVE_EXTRA_REGS
+	CLEAR_REGS_NOSPEC
 	ENCODE_FRAME_POINTER
 	jmp	error_exit
 END(xen_failsafe_callback)
@@ -1172,6 +1174,7 @@ ENTRY(paranoid_entry)
 	cld
 	SAVE_C_REGS 8
 	SAVE_EXTRA_REGS 8
+	CLEAR_REGS_NOSPEC
 	ENCODE_FRAME_POINTER 8
 	movl	$1, %ebx
 	movl	$MSR_GS_BASE, %ecx
@@ -1224,8 +1227,8 @@ ENTRY(error_entry)
 	cld
 	SAVE_C_REGS 8
 	SAVE_EXTRA_REGS 8
+	CLEAR_REGS_NOSPEC
 	ENCODE_FRAME_POINTER 8
-	xorl	%ebx, %ebx
 	testb	$3, CS+8(%rsp)
 	jz	.Lerror_kernelspace
 
@@ -1422,6 +1425,7 @@ ENTRY(nmi)
 	pushq	%r14		/* pt_regs->r14 */
 	pushq	%r15		/* pt_regs->r15 */
 	UNWIND_HINT_REGS
+	CLEAR_REGS_NOSPEC
 	ENCODE_FRAME_POINTER
 
 	/*