linux-kernel - Re: [PATCH 30/43] x86/mm/kaiser: Map espfix structures

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Date:   Mon, 27 Nov 2017 16:35:13 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Ingo Molnar <mingo@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "H . Peter Anvin" <hpa@...or.com>, Borislav Petkov <bp@...en8.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 30/43] x86/mm/kaiser: Map espfix structures

On Mon, Nov 27, 2017 at 10:14:24AM +0100, Peter Zijlstra wrote:

> But if we can freely spill here, should we not do the kernel switch
> instead of doing this user mapping? The way I understand things, the
> less of these magic mappings we have the better.

Turns out, we don't need more scratch regs at all.

The below seems to survive tools/testing/selftests/x86/sigreturn_64
which exercises the ESPFIX crud.

---
 arch/x86/entry/entry_64.S   | 11 ++++++++---
 arch/x86/kernel/espfix_64.c | 10 ++--------
 2 files changed, 10 insertions(+), 11 deletions(-)

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index df0152bee8a8..289ba2680952 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -825,7 +825,9 @@ ENTRY(native_iret)
 	 */
 
 	pushq	%rdi				/* Stash user RDI */
-	SWAPGS
+	SWAPGS					/* to kernel GS */
+	SWITCH_TO_KERNEL_CR3 scratch_reg=%rdi	/* to kernel CR3 */
+
 	movq	PER_CPU_VAR(espfix_waddr), %rdi
 	movq	%rax, (0*8)(%rdi)		/* user RAX */
 	movq	(1*8)(%rsp), %rax		/* user RIP */
@@ -841,7 +843,6 @@ ENTRY(native_iret)
 	/* Now RAX == RSP. */
 
 	andl	$0xffff0000, %eax		/* RAX = (RSP & 0xffff0000) */
-	popq	%rdi				/* Restore user RDI */
 
 	/*
 	 * espfix_stack[31:16] == 0.  The page tables are set up such that
@@ -852,7 +853,11 @@ ENTRY(native_iret)
 	 * still points to an RO alias of the ESPFIX stack.
 	 */
 	orq	PER_CPU_VAR(espfix_stack), %rax
-	SWAPGS
+
+	SWITCH_TO_USER_CR3 scratch_reg=%rdi	/* to user CR3 */
+	SWAPGS					/* to user GS */
+	popq	%rdi				/* Restore user RDI */
+
 	movq	%rax, %rsp
 	UNWIND_HINT_IRET_REGS offset=8
 
diff --git a/arch/x86/kernel/espfix_64.c b/arch/x86/kernel/espfix_64.c
index 8bb116d73aaa..8826475d786c 100644
--- a/arch/x86/kernel/espfix_64.c
+++ b/arch/x86/kernel/espfix_64.c
@@ -61,8 +61,8 @@
 #define PGALLOC_GFP (GFP_KERNEL | __GFP_NOTRACK | __GFP_ZERO)
 
 /* This contains the *bottom* address of the espfix stack */
-DEFINE_PER_CPU_USER_MAPPED(unsigned long, espfix_stack);
-DEFINE_PER_CPU_USER_MAPPED(unsigned long, espfix_waddr);
+DEFINE_PER_CPU_READ_MOSTLY(unsigned long, espfix_stack);
+DEFINE_PER_CPU_READ_MOSTLY(unsigned long, espfix_waddr);
 
 /* Initialization mutex - should this be a spinlock? */
 static DEFINE_MUTEX(espfix_init_mutex);
@@ -225,10 +225,4 @@ void init_espfix_ap(int cpu)
 	per_cpu(espfix_stack, cpu) = addr;
 	per_cpu(espfix_waddr, cpu) = (unsigned long)stack_page
 				      + (addr & ~PAGE_MASK);
-	/*
-	 * _PAGE_GLOBAL is not really required.  This is not a hot
-	 * path, but we do it here for consistency.
-	 */
-	kaiser_add_mapping((unsigned long)stack_page, PAGE_SIZE,
-			__PAGE_KERNEL | _PAGE_GLOBAL);
 }