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[Xen-devel] Re: [RFC] [PATCH] Dom0: Don't switch back to user space stack in syscall entry
On 01/28/2010 02:26 AM, Jiang, Yunhong wrote:
Jeremy, this patch is a RFC for MCE injection issue as discussed in
http://lists.xensource.com/archives/html/xen-devel/2009-12/msg00849.html .
Currently, when syscall into dom0, the stack is kernel stack. Dom0 kernel will
firstly switch to user space stack, to create context same as physical syscall
entry and then call kernel's common syscall entry. In kernel's common syscall
entry, it will return back to kernel stack.
Does this just apply to dom0? Can a domU kernel ever get an NMI? (Not
that it makes any difference since a solution for one will be a solution
for both, but I'm wondering if there's a reason you're specifically
talking about dom0).
This give a small security windows for MCE. If a vMCE is injected into dom0
when dom0 is in the syscalll entry, but is still using the user space stack, it
will cause great trouble (check above URL for detailed information).
I can think out two options for this issue:
a) The method this patch took. Dom0 didn't try to switch to user space stack.
Instead, it will use kernel statck directly and jump to kenerl entry.
Seems simple enough. The patch looks pretty sound in concept, but I
have some comments inline.
b) Add IST support to xen hypervisor, Considering the ptrace issue in
http://lists.xensource.com/archives/html/xen-devel/2009-05/msg00200.html and
also Ingo's comments in
http://lkml.indiana.edu/hypermail/linux/kernel/0905.1/00087.html, maybe it
still have value to add such support. But not sure if that is accepetable for
xen hypervisor.
That would depend on Keir's thoughts, which I would guess depend on what
the patch and ABI would look like. It's certainly the most general
solution if it can be done in a reasonable way.
As RFC, this patch is just tested to make sure the system can still boot
succesffully, I did't try ptrace or something else. If the idea is ok, I will
do more testing on it.
Did you try booting it native as well? And test 32-bit compat usermode?
Thanks
Yunhong Jiang
From 6f8e8019b6cee92b23a3f02787376e0ab3a4244f Mon Sep 17 00:00:00 2001
From: Jiang, Yunhong<yunhong.jiang@xxxxxxxxx>
Date: Thu, 28 Jan 2010 03:43:41 +0800
Subject: [PATCH] Change the syscall/sysexit entry to use the kernel stack
directly
---
arch/x86/ia32/ia32entry.S | 2 +-
arch/x86/include/asm/irqflags.h | 2 ++
arch/x86/kernel/entry_64.S | 2 +-
arch/x86/xen/xen-asm_64.S | 37 +++++++++++++++++++++++++------------
4 files changed, 29 insertions(+), 14 deletions(-)
diff --git a/arch/x86/ia32/ia32entry.S b/arch/x86/ia32/ia32entry.S
index b09502d..2f8e942 100644
--- a/arch/x86/ia32/ia32entry.S
+++ b/arch/x86/ia32/ia32entry.S
@@ -284,6 +284,7 @@ ENTRY(ia32_cstar_target)
movl %esp,%r8d
CFI_REGISTER rsp,r8
movq PER_CPU_VAR(kernel_stack),%rsp
+ENTRY(ia32_cstart_after_switch_stack)
"cstar" I think.
/*
* No need to follow this irqs on/off section: the syscall
* disabled irqs and here we enable it straight after entry:
@@ -337,7 +338,6 @@ sysretl_from_sys_call:
xorq %r9,%r9
xorq %r8,%r8
TRACE_IRQS_ON
- movl RSP-ARGOFFSET(%rsp),%esp
CFI_RESTORE rsp
You need to update/move this CFI annotation.
USERGS_SYSRET32
diff --git a/arch/x86/include/asm/irqflags.h b/arch/x86/include/asm/irqflags.h
index c6ccbe7..7e62aed 100644
--- a/arch/x86/include/asm/irqflags.h
+++ b/arch/x86/include/asm/irqflags.h
@@ -133,9 +133,11 @@ static inline unsigned long __raw_local_irq_save(void)
#define INTERRUPT_RETURN iretq
#define USERGS_SYSRET64 \
+ movq PER_CPU_VAR(old_rsp), %rsp \
swapgs; \
sysretq;
#define USERGS_SYSRET32 \
+ movl RSP-ARGOFFSET(%rsp), %esp \
swapgs; \
sysretl
OK, so you're redefining sysret32/64 to also include the stack switch.
Makes sense.
#define ENABLE_INTERRUPTS_SYSEXIT32 \
diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
index c251be7..5bc75fe 100644
--- a/arch/x86/kernel/entry_64.S
+++ b/arch/x86/kernel/entry_64.S
@@ -472,6 +472,7 @@ ENTRY(system_call_after_swapgs)
* No need to follow this irqs off/on section - it's straight
* and short:
*/
+ENTRY(system_call_after_switch_stack)
ENABLE_INTERRUPTS(CLBR_NONE)
SAVE_ARGS 8,1
movq %rax,ORIG_RAX-ARGOFFSET(%rsp)
@@ -510,7 +511,6 @@ sysret_check:
CFI_REGISTER rip,rcx
RESTORE_ARGS 0,-ARG_SKIP,1
/*CFI_REGISTER rflags,r11*/
- movq PER_CPU_VAR(old_rsp), %rsp
USERGS_SYSRET64
CFI_RESTORE_STATE
diff --git a/arch/x86/xen/xen-asm_64.S b/arch/x86/xen/xen-asm_64.S
index 53adefd..c953d14 100644
--- a/arch/x86/xen/xen-asm_64.S
+++ b/arch/x86/xen/xen-asm_64.S
@@ -12,6 +12,7 @@
*/
#include<asm/errno.h>
+#include<asm/calling.h>
#include<asm/percpu.h>
#include<asm/processor-flags.h>
#include<asm/segment.h>
@@ -68,9 +69,7 @@ ENTRY(xen_sysret64)
* We're already on the usermode stack at this point, but
* still with the kernel gs, so we can easily switch back
*/
This comment needs update/deletion.
- movq %rsp, PER_CPU_VAR(old_rsp)
- movq PER_CPU_VAR(kernel_stack), %rsp
-
+ movq PER_CPU_VAR(kernel_stack), %rsp
Looks like your indentation is funny; this file uses 8-width tabs.
pushq $__USER_DS
pushq PER_CPU_VAR(old_rsp)
pushq %r11
@@ -84,12 +83,15 @@ RELOC(xen_sysret64, 1b+1)
ENTRY(xen_sysret32)
/*
- * We're already on the usermode stack at this point, but
+ * We're still on the kernel mode stack at this point, but
* still with the kernel gs, so we can easily switch back
*/
May as well delete the whole comment; it doesn't make any sense in this
form.
- movq %rsp, PER_CPU_VAR(old_rsp)
- movq PER_CPU_VAR(kernel_stack), %rsp
-
+ /* The ARGS is restored, so don't clobber anything */
+ pushq %rax
+ movq RSP-ARGOFFSET(%rsp), %rax
+ movq %rax, PER_CPU_VAR(old_rsp)
+ popq %rax
Rather than going via PER_CPU_VAR(old_rsp), maybe it would make more
sense to restructure the iret frame construction to something like:
sub $6*8, %rsp
mov $0, 0*8(%rsp)
mov %rcx, 1*8(%rsp)
mov $__USER32_CS, 2*8(%rsp)
mov %r11, 3*8(%rsp)
mov PER_CPU_VAR(old_rsp), %rcx
mov %rcx, 4*8(%rsp)
mov $__USER32_DS, 5*8(%rsp)
jmp hypercall_iret
+ movq PER_CPU_VAR(kernel_stack), %rsp
Why do you need to do this? Isn't the point that rsp is already the
kernel stack?
pushq $__USER32_DS
pushq PER_CPU_VAR(old_rsp)
pushq %r11
@@ -116,27 +118,38 @@ RELOC(xen_sysret32, 1b+1)
* rsp->rcx
*
* In all the entrypoints, we undo all that to make it look like a
- * CPU-generated syscall/sysenter and jump to the normal entrypoint.
+ * CPU-generated syscall/sysenter and jump to the normal entrypoint,
+ * but we will not switch stack
*/
.macro undo_xen_syscall
+ /* Clobber rcx is ok */
+ movq 5*8(%rsp), %rcx
+ movq %rcx, PER_CPU_VAR(old_rsp)
mov 0*8(%rsp), %rcx
mov 1*8(%rsp), %r11
- mov 5*8(%rsp), %rsp
+ sub $56, %rsp
What's this for? Where does "56" come from?
.endm
/* Normal 64-bit system call target */
ENTRY(xen_syscall_target)
undo_xen_syscall
- jmp system_call_after_swapgs
+ jmp system_call_after_switch_stack
ENDPROC(xen_syscall_target)
#ifdef CONFIG_IA32_EMULATION
+.macro undo_xen_ia32_syscall
+ mov 0*8(%rsp), %rcx
+ mov 1*8(%rsp), %r11
+ mov 5*8(%esp), %r8d
+ sub $56, %rsp
+.endm
/* 32-bit compat syscall target */
ENTRY(xen_syscall32_target)
- undo_xen_syscall
- jmp ia32_cstar_target
+ undo_xen_ia32_syscall
+ movl %esp,%r8d
+ jmp ia32_cstart_after_switch_stack
ENDPROC(xen_syscall32_target)
/* 32-bit compat sysenter target */
Thanks,
J
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