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Subject[patch 17/60] x86/entry/64: Use a per-CPU trampoline stack for IDT entries
From: Andy Lutomirski <luto@kernel.org>

Historically, IDT entries from usermode have always gone directly
to the running task's kernel stack. Rearrange it so that we enter on
a per-CPU trampoline stack and then manually switch to the task's stack.
This touches a couple of extra cachelines, but it gives us a chance
to run some code before we touch the kernel stack.

The asm isn't exactly beautiful, but I think that fully refactoring
it can wait.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Rik van Riel <riel@redhat.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lkml.kernel.org/r/fa3958723a1a85baeaf309c735b775841205800e.1511497875.git.luto@kernel.org

---
arch/x86/entry/entry_64.S | 67 +++++++++++++++++++++++++++++----------
arch/x86/entry/entry_64_compat.S | 5 ++
arch/x86/include/asm/switch_to.h | 3 -
arch/x86/include/asm/traps.h | 1
arch/x86/kernel/cpu/common.c | 6 ++-
arch/x86/kernel/traps.c | 12 +++---
6 files changed, 65 insertions(+), 29 deletions(-)

--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -564,6 +564,13 @@ END(irq_entries_start)
/* 0(%rsp): ~(interrupt number) */
.macro interrupt func
cld
+
+ testb $3, CS-ORIG_RAX(%rsp)
+ jz 1f
+ SWAPGS
+ call switch_to_thread_stack
+1:
+
ALLOC_PT_GPREGS_ON_STACK
SAVE_C_REGS
SAVE_EXTRA_REGS
@@ -573,12 +580,8 @@ END(irq_entries_start)
jz 1f

/*
- * IRQ from user mode. Switch to kernel gsbase and inform context
- * tracking that we're in kernel mode.
- */
- SWAPGS
-
- /*
+ * IRQ from user mode.
+ *
* We need to tell lockdep that IRQs are off. We can't do this until
* we fix gsbase, and we should do it before enter_from_user_mode
* (which can take locks). Since TRACE_IRQS_OFF idempotent,
@@ -832,6 +835,32 @@ apicinterrupt IRQ_WORK_VECTOR irq_work
*/
#define CPU_TSS_IST(x) PER_CPU_VAR(cpu_tss) + (TSS_ist + ((x) - 1) * 8)

+/*
+ * Switch to the thread stack. This is called with the IRET frame and
+ * orig_ax on the stack. (That is, RDI..R12 are not on the stack and
+ * space has not been allocated for them.)
+ */
+ENTRY(switch_to_thread_stack)
+ UNWIND_HINT_FUNC
+
+ pushq %rdi
+ movq %rsp, %rdi
+ movq PER_CPU_VAR(cpu_current_top_of_stack), %rsp
+ UNWIND_HINT sp_offset=16 sp_reg=ORC_REG_DI
+
+ pushq 7*8(%rdi) /* regs->ss */
+ pushq 6*8(%rdi) /* regs->rsp */
+ pushq 5*8(%rdi) /* regs->eflags */
+ pushq 4*8(%rdi) /* regs->cs */
+ pushq 3*8(%rdi) /* regs->ip */
+ pushq 2*8(%rdi) /* regs->orig_ax */
+ pushq 8(%rdi) /* return address */
+ UNWIND_HINT_FUNC
+
+ movq (%rdi), %rdi
+ ret
+END(switch_to_thread_stack)
+
.macro idtentry sym do_sym has_error_code:req paranoid=0 shift_ist=-1
ENTRY(\sym)
UNWIND_HINT_IRET_REGS offset=\has_error_code*8
@@ -849,11 +878,12 @@ ENTRY(\sym)

ALLOC_PT_GPREGS_ON_STACK

- .if \paranoid
- .if \paranoid == 1
+ .if \paranoid < 2
testb $3, CS(%rsp) /* If coming from userspace, switch stacks */
- jnz 1f
+ jnz .Lfrom_usermode_switch_stack_\@
.endif
+
+ .if \paranoid
call paranoid_entry
.else
call error_entry
@@ -895,20 +925,15 @@ ENTRY(\sym)
jmp error_exit
.endif

- .if \paranoid == 1
+ .if \paranoid < 2
/*
- * Paranoid entry from userspace. Switch stacks and treat it
+ * Entry from userspace. Switch stacks and treat it
* as a normal entry. This means that paranoid handlers
* run in real process context if user_mode(regs).
*/
-1:
+.Lfrom_usermode_switch_stack_\@:
call error_entry

-
- movq %rsp, %rdi /* pt_regs pointer */
- call sync_regs
- movq %rax, %rsp /* switch stack */
-
movq %rsp, %rdi /* pt_regs pointer */

.if \has_error_code
@@ -1171,6 +1196,14 @@ ENTRY(error_entry)
SWAPGS

.Lerror_entry_from_usermode_after_swapgs:
+ /* Put us onto the real thread stack. */
+ popq %r12 /* save return addr in %12 */
+ movq %rsp, %rdi /* arg0 = pt_regs pointer */
+ call sync_regs
+ movq %rax, %rsp /* switch stack */
+ ENCODE_FRAME_POINTER
+ pushq %r12
+
/*
* We need to tell lockdep that IRQs are off. We can't do this until
* we fix gsbase, and we should do it before enter_from_user_mode
--- a/arch/x86/entry/entry_64_compat.S
+++ b/arch/x86/entry/entry_64_compat.S
@@ -306,8 +306,11 @@ ENTRY(entry_INT80_compat)
*/
movl %eax, %eax

- /* Construct struct pt_regs on stack (iret frame is already on stack) */
pushq %rax /* pt_regs->orig_ax */
+
+ /* switch to thread stack expects orig_ax to be pushed */
+ call switch_to_thread_stack
+
pushq %rdi /* pt_regs->di */
pushq %rsi /* pt_regs->si */
pushq %rdx /* pt_regs->dx */
--- a/arch/x86/include/asm/switch_to.h
+++ b/arch/x86/include/asm/switch_to.h
@@ -90,10 +90,9 @@ static inline void refresh_sysenter_cs(s
/* This is used when switching tasks or entering/exiting vm86 mode. */
static inline void update_sp0(struct task_struct *task)
{
+ /* On x86_64, sp0 always points to the entry trampoline stack, which is constant: */
#ifdef CONFIG_X86_32
load_sp0(task->thread.sp0);
-#else
- load_sp0(task_top_of_stack(task));
#endif
}

--- a/arch/x86/include/asm/traps.h
+++ b/arch/x86/include/asm/traps.h
@@ -75,7 +75,6 @@ dotraplinkage void do_segment_not_presen
dotraplinkage void do_stack_segment(struct pt_regs *, long);
#ifdef CONFIG_X86_64
dotraplinkage void do_double_fault(struct pt_regs *, long);
-asmlinkage struct pt_regs *sync_regs(struct pt_regs *);
#endif
dotraplinkage void do_general_protection(struct pt_regs *, long);
dotraplinkage void do_page_fault(struct pt_regs *, unsigned long);
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -1648,11 +1648,13 @@ void cpu_init(void)
setup_cpu_entry_area(cpu);

/*
- * Initialize the TSS. Don't bother initializing sp0, as the initial
- * task never enters user mode.
+ * Initialize the TSS. sp0 points to the entry trampoline stack
+ * regardless of what task is running.
*/
set_tss_desc(cpu, &get_cpu_entry_area(cpu)->tss.x86_tss);
load_TR_desc();
+ load_sp0((unsigned long)&get_cpu_entry_area(cpu)->tss +
+ offsetofend(struct tss_struct, SYSENTER_stack));

load_mm_ldt(&init_mm);

--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -631,7 +631,7 @@ NOKPROBE_SYMBOL(do_int3);
*/
asmlinkage __visible notrace struct pt_regs *sync_regs(struct pt_regs *eregs)
{
- struct pt_regs *regs = task_pt_regs(current);
+ struct pt_regs *regs = (struct pt_regs *)this_cpu_read(cpu_current_top_of_stack) - 1;
*regs = *eregs;
return regs;
}
@@ -648,13 +648,13 @@ struct bad_iret_stack *fixup_bad_iret(st
/*
* This is called from entry_64.S early in handling a fault
* caused by a bad iret to user mode. To handle the fault
- * correctly, we want move our stack frame to task_pt_regs
- * and we want to pretend that the exception came from the
- * iret target.
+ * correctly, we want to move our stack frame to where it would
+ * be had we entered directly on the entry stack (rather than
+ * just below the IRET frame) and we want to pretend that the
+ * exception came from the IRET target.
*/
struct bad_iret_stack *new_stack =
- container_of(task_pt_regs(current),
- struct bad_iret_stack, regs);
+ (struct bad_iret_stack *)this_cpu_read(cpu_tss.x86_tss.sp0) - 1;

/* Copy the IRET target to the new stack. */
memmove(&new_stack->regs.ip, (void *)s->regs.sp, 5*8);

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