Commit 16561f27 authored by Dave Hansen's avatar Dave Hansen Committed by Thomas Gleixner
Browse files

x86/entry: Add some paranoid entry/exit CR3 handling comments 



Andi Kleen was just asking me about the NMI CR3 handling and why
we restore it unconditionally.  I was *sure* we had documented it
well.  We did not.

Add some documentation.  We have common entry code where the CR3
value is stashed, but three places in two big code paths where we
restore it.  I put bulk of the comments in this common path and
then refer to it from the other spots.

Signed-off-by: default avatarDave Hansen <dave.hansen@linux.intel.com>
Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
Cc: luto@kernel.org
Cc: bp@alien8.de
Cc: "H. Peter Anvin" <hpa@zytor.come
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Link: https://lkml.kernel.org/r/20181012232118.3EAAE77B@viggo.jf.intel.com
parent b59167ac

arch/x86/entry/entry_64.S

+15 −0
Original line number Diff line number Diff line
@@ -1187,6 +1187,18 @@ ENTRY(paranoid_entry) 
	xorl	%ebx, %ebx



1:

	/*

	 * Always stash CR3 in %r14.  This value will be restored,

	 * verbatim, at exit.  Needed if kernel is interrupted

	 * after switching to the user CR3 value but before

	 * returning to userspace.

	 *

	 * This is also why CS (stashed in the "iret frame" by the

	 * hardware at entry) can not be used: this may be a return

	 * to kernel code, but with a user CR3 value.  The %ebx flag

	 * for SWAPGS is also unusable for CR3 because there is a

	 * window with a user GS and a kernel CR3.

	 */

	SAVE_AND_SWITCH_TO_KERNEL_CR3 scratch_reg=%rax save_reg=%r14



	ret
@@ -1211,11 +1223,13 @@ ENTRY(paranoid_exit) 
	testl	%ebx, %ebx			/* swapgs needed? */

	jnz	.Lparanoid_exit_no_swapgs

	TRACE_IRQS_IRETQ

	/* Always restore stashed CR3 value (see paranoid_entry) */

	RESTORE_CR3	scratch_reg=%rbx save_reg=%r14

	SWAPGS_UNSAFE_STACK

	jmp	.Lparanoid_exit_restore

.Lparanoid_exit_no_swapgs:

	TRACE_IRQS_IRETQ_DEBUG

	/* Always restore stashed CR3 value (see paranoid_entry) */

	RESTORE_CR3	scratch_reg=%rbx save_reg=%r14

.Lparanoid_exit_restore:

	jmp restore_regs_and_return_to_kernel
@@ -1626,6 +1640,7 @@ end_repeat_nmi: 
	movq	$-1, %rsi

	call	do_nmi



	/* Always restore stashed CR3 value (see paranoid_entry) */

	RESTORE_CR3 scratch_reg=%r15 save_reg=%r14



	testl	%ebx, %ebx			/* swapgs needed? */

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