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From: Andy Lutomirski <luto@kernel.org>
To: X86 ML <x86@kernel.org>
Cc: Borislav Petkov <bpetkov@suse.de>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Brian Gerst <brgerst@gmail.com>,
	Dave Hansen <dave.hansen@intel.com>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Andy Lutomirski <luto@kernel.org>,
	Juergen Gross <jgross@suse.com>,
	Boris Ostrovsky <boris.ostrovsky@oracle.com>
Subject: [PATCH v2 11/20] x86/asm/64: De-Xen-ify our NMI code
Date: Thu,  2 Nov 2017 00:59:08 -0700
Message-ID: <bfbe711b5ae03f672f8848999a8eb2711efc7f98.1509609304.git.luto@kernel.org> (raw)
In-Reply-To: <cover.1509609304.git.luto@kernel.org>
In-Reply-To: <cover.1509609304.git.luto@kernel.org>

Xen PV is fundamentally incompatible with our fancy NMI code: it
doesn't use IST at all, and Xen entries clobber two stack slots
below the hardware frame.

Drop Xen PV support from our NMI code entirely.

Cc: Juergen Gross <jgross@suse.com>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Reviewed-by: Borislav Petkov <bp@suse.de>
Acked-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Andy Lutomirski <luto@kernel.org>
---
 arch/x86/entry/entry_64.S | 30 ++++++++++++++++++------------
 1 file changed, 18 insertions(+), 12 deletions(-)

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index b4df83177d14..b58fb6335850 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -1240,9 +1240,13 @@ ENTRY(error_exit)
 	jmp	retint_user
 END(error_exit)
 
-/* Runs on exception stack */
+/*
+ * Runs on exception stack.  Xen PV does not go through this path at all,
+ * so we can use real assembly here.
+ */
 ENTRY(nmi)
 	UNWIND_HINT_IRET_REGS
+
 	/*
 	 * We allow breakpoints in NMIs. If a breakpoint occurs, then
 	 * the iretq it performs will take us out of NMI context.
@@ -1300,7 +1304,7 @@ ENTRY(nmi)
 	 * stacks lest we corrupt the "NMI executing" variable.
 	 */
 
-	SWAPGS_UNSAFE_STACK
+	swapgs
 	cld
 	movq	%rsp, %rdx
 	movq	PER_CPU_VAR(cpu_current_top_of_stack), %rsp
@@ -1465,7 +1469,7 @@ nested_nmi_out:
 	popq	%rdx
 
 	/* We are returning to kernel mode, so this cannot result in a fault. */
-	INTERRUPT_RETURN
+	iretq
 
 first_nmi:
 	/* Restore rdx. */
@@ -1496,7 +1500,7 @@ first_nmi:
 	pushfq			/* RFLAGS */
 	pushq	$__KERNEL_CS	/* CS */
 	pushq	$1f		/* RIP */
-	INTERRUPT_RETURN	/* continues at repeat_nmi below */
+	iretq			/* continues at repeat_nmi below */
 	UNWIND_HINT_IRET_REGS
 1:
 #endif
@@ -1571,20 +1575,22 @@ nmi_restore:
 	/*
 	 * Clear "NMI executing".  Set DF first so that we can easily
 	 * distinguish the remaining code between here and IRET from
-	 * the SYSCALL entry and exit paths.  On a native kernel, we
-	 * could just inspect RIP, but, on paravirt kernels,
-	 * INTERRUPT_RETURN can translate into a jump into a
-	 * hypercall page.
+	 * the SYSCALL entry and exit paths.
+	 *
+	 * We arguably should just inspect RIP instead, but I (Andy) wrote
+	 * this code when I had the misapprehension that Xen PV supported
+	 * NMIs, and Xen PV would break that approach.
 	 */
 	std
 	movq	$0, 5*8(%rsp)		/* clear "NMI executing" */
 
 	/*
-	 * INTERRUPT_RETURN reads the "iret" frame and exits the NMI
-	 * stack in a single instruction.  We are returning to kernel
-	 * mode, so this cannot result in a fault.
+	 * iretq reads the "iret" frame and exits the NMI stack in a
+	 * single instruction.  We are returning to kernel mode, so this
+	 * cannot result in a fault.  Similarly, we don't need to worry
+	 * about espfix64 on the way back to kernel mode.
 	 */
-	INTERRUPT_RETURN
+	iretq
 END(nmi)
 
 ENTRY(ignore_sysret)
-- 
2.13.6

  parent reply index

Thread overview: 48+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-11-02  7:58 [PATCH v2 00/20] Pile o' entry/exit/sp0 changes Andy Lutomirski
2017-11-02  7:58 ` [PATCH v2 01/20] x86/asm/64: Remove the restore_c_regs_and_iret label Andy Lutomirski
2017-11-02 10:49   ` [tip:x86/asm] x86/entry/64: " tip-bot for Andy Lutomirski
2017-11-02  7:58 ` [PATCH v2 02/20] x86/asm/64: Split the iret-to-user and iret-to-kernel paths Andy Lutomirski
2017-11-02 10:49   ` [tip:x86/asm] x86/entry/64: Split the IRET-to-user and IRET-to-kernel paths tip-bot for Andy Lutomirski
2017-11-02 10:50   ` [PATCH v2 02/20] x86/asm/64: Split the iret-to-user and iret-to-kernel paths Borislav Petkov
2017-11-02 12:09     ` [PATCH] x86/entry/64: Shorten TEST instructions Borislav Petkov
2017-11-02 12:48       ` [tip:x86/asm] " tip-bot for Borislav Petkov
2017-11-02  7:59 ` [PATCH v2 03/20] x86/asm/64: Move SWAPGS into the common iret-to-usermode path Andy Lutomirski
2017-11-02 10:49   ` [tip:x86/asm] x86/entry/64: Move SWAPGS into the common IRET-to-usermode path tip-bot for Andy Lutomirski
2017-11-02  7:59 ` [PATCH v2 04/20] x86/asm/64: Simplify reg restore code in the standard IRET paths Andy Lutomirski
2017-11-02 10:50   ` [tip:x86/asm] x86/entry/64: " tip-bot for Andy Lutomirski
2017-11-02  7:59 ` [PATCH v2 05/20] x86/asm/64: Shrink paranoid_exit_restore and make labels local Andy Lutomirski
2017-11-02 10:50   ` [tip:x86/asm] x86/entry/64: " tip-bot for Andy Lutomirski
2017-11-02  7:59 ` [PATCH v2 06/20] x86/asm/64: Use pop instead of movq in syscall_return_via_sysret Andy Lutomirski
2017-11-02 10:51   ` [tip:x86/asm] x86/entry/64: " tip-bot for Andy Lutomirski
2017-11-02  7:59 ` [PATCH v2 07/20] x86/asm/64: Merge the fast and slow SYSRET paths Andy Lutomirski
2017-11-02 10:51   ` [tip:x86/asm] x86/entry/64: " tip-bot for Andy Lutomirski
2017-11-02  7:59 ` [PATCH v2 08/20] x86/entry/64: Use POP instead of MOV to restore regs on NMI return Andy Lutomirski
2017-11-02 10:51   ` [tip:x86/asm] " tip-bot for Andy Lutomirski
2017-11-02  7:59 ` [PATCH v2 09/20] x86/entry/64: Remove the RESTORE_..._REGS infrastructure Andy Lutomirski
2017-11-02 10:52   ` [tip:x86/asm] " tip-bot for Andy Lutomirski
2017-11-02  7:59 ` [PATCH v2 10/20] xen: add xen nmi trap entry Andy Lutomirski
2017-11-02 10:52   ` [tip:x86/asm] xen, x86/entry/64: Add xen NMI " tip-bot for Juergen Gross
2017-11-02  7:59 ` Andy Lutomirski [this message]
2017-11-02 10:53   ` [tip:x86/asm] x86/entry/64: De-Xen-ify our NMI code tip-bot for Andy Lutomirski
2017-11-02  7:59 ` [PATCH v2 12/20] x86/asm/32: Pull MSR_IA32_SYSENTER_CS update code out of native_load_sp0() Andy Lutomirski
2017-11-02 10:53   ` [tip:x86/asm] x86/entry/32: Pull the " tip-bot for Andy Lutomirski
2017-11-02  7:59 ` [PATCH v2 13/20] x86/asm/64: Pass sp0 directly to load_sp0() Andy Lutomirski
2017-11-02  9:48   ` Ingo Molnar
2017-11-02  9:53     ` Ingo Molnar
2017-11-02 10:32     ` Andy Lutomirski
2017-11-02 10:53   ` [tip:x86/asm] x86/entry/64: Pass SP0 " tip-bot for Andy Lutomirski
2017-11-02  7:59 ` [PATCH v2 14/20] x86/asm: Add task_top_of_stack() to find the top of a task's stack Andy Lutomirski
2017-11-02 10:54   ` [tip:x86/asm] x86/entry: " tip-bot for Andy Lutomirski
2017-11-02  7:59 ` [PATCH v2 15/20] x86/xen/64: Clean up SP code in cpu_initialize_context() Andy Lutomirski
2017-11-02  9:56   ` Juergen Gross
2017-11-02 10:54   ` [tip:x86/asm] x86/xen/64, x86/entry/64: " tip-bot for Andy Lutomirski
2017-11-02  7:59 ` [PATCH v2 16/20] x86/boot/64: Stop initializing TSS.sp0 at boot Andy Lutomirski
2017-11-02 10:55   ` [tip:x86/asm] x86/entry/64: " tip-bot for Andy Lutomirski
2017-11-02  7:59 ` [PATCH v2 17/20] x86/asm/64: Remove all remaining direct thread_struct::sp0 reads Andy Lutomirski
2017-11-02 10:55   ` [tip:x86/asm] x86/entry/64: " tip-bot for Andy Lutomirski
2017-11-02  7:59 ` [PATCH v2 18/20] x86/boot/32: Fix cpu_current_top_of_stack initialization at boot Andy Lutomirski
2017-11-02 10:56   ` [tip:x86/asm] x86/entry/32: " tip-bot for Andy Lutomirski
2017-11-02  7:59 ` [PATCH v2 19/20] x86/asm/64: Remove thread_struct::sp0 Andy Lutomirski
2017-11-02 10:56   ` [tip:x86/asm] x86/entry/64: " tip-bot for Andy Lutomirski
2017-11-02  7:59 ` [PATCH v2 20/20] x86/traps: Use a new on_thread_stack() helper to clean up an assertion Andy Lutomirski
2017-11-02 10:56   ` [tip:x86/asm] " tip-bot for Andy Lutomirski

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