x86/entry/64: Clear registers for exceptions/interrupts, to reduce speculation attack surface

Web:        https://git.kernel.org/torvalds/c/3ac6d8c787b835b997eb23e43e09aa0895ef7d58
Commit:     3ac6d8c787b835b997eb23e43e09aa0895ef7d58
Parent:     8e1eb3fa009aa7c0b944b3c8b26b07de0efb3200
Refname:    refs/heads/master
Author:     Dan Williams <[email protected]>
AuthorDate: Mon Feb 5 17:18:11 2018 -0800
Committer:  Ingo Molnar <[email protected]>
CommitDate: Tue Feb 6 11:47:44 2018 +0100

    x86/entry/64: Clear registers for exceptions/interrupts, to reduce speculation attack surface
    
    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 <[email protected]>
    Signed-off-by: Dan Williams <[email protected]>
    Cc: <[email protected]>
    Cc: Andy Lutomirski <[email protected]>
    Cc: Borislav Petkov <[email protected]>
    Cc: Brian Gerst <[email protected]>
    Cc: Denys Vlasenko <[email protected]>
    Cc: H. Peter Anvin <[email protected]>
    Cc: Josh Poimboeuf <[email protected]>
    Cc: Linus Torvalds <[email protected]>
    Cc: Peter Zijlstra <[email protected]>
    Cc: Thomas Gleixner <[email protected]>
    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 <[email protected]>
---
 arch/x86/entry/calling.h  | 19 +++++++++++++++++++
 arch/x86/entry/entry_64.S |  6 +++++-
 2 files changed, 24 insertions(+), 1 deletion(-)

diff --git a/arch/x86/entry/calling.h b/arch/x86/entry/calling.h
index 3f48f695d5e6..f4b129d4af42 100644
--- a/arch/x86/entry/calling.h
+++ b/arch/x86/entry/calling.h
@@ -147,6 +147,25 @@ For 32-bit we have the following conventions - kernel is built with
 	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
diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 065a71b90808..9e48002b953b 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -575,6 +575,7 @@ 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)
@@ -1133,6 +1134,7 @@ 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)
@@ -1178,6 +1180,7 @@ 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
@@ -1230,8 +1233,8 @@ 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
 
@@ -1428,6 +1431,7 @@ ENTRY(nmi)
 	pushq	%r14		/* pt_regs->r14 */
 	pushq	%r15		/* pt_regs->r15 */
 	UNWIND_HINT_REGS
+	CLEAR_REGS_NOSPEC
 	ENCODE_FRAME_POINTER
 
 	/*
--
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