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Subject[PATCH 09/31] x86/entry/32: Leave the kernel via trampoline stack
Date
From: Joerg Roedel <jroedel@suse.de>

Switch back to the trampoline stack before returning to
userspace.

Signed-off-by: Joerg Roedel <jroedel@suse.de>
---
arch/x86/entry/entry_32.S | 55 +++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 55 insertions(+)

diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S
index e714b53..ba3d8c2 100644
--- a/arch/x86/entry/entry_32.S
+++ b/arch/x86/entry/entry_32.S
@@ -338,6 +338,59 @@
.endm

/*
+ * Switch back from the kernel stack to the entry stack.
+ *
+ * The %esp register must point to pt_regs on the task stack. It will
+ * first calculate the size of the stack-frame to copy, depending on
+ * whether we return to VM86 mode or not. With that it uses 'rep movsb'
+ * to copy the contents of the stack over to the entry stack.
+ *
+ * We must be very careful here, as we can't trust the contents of the
+ * task-stack once we switched to the entry-stack. When an NMI happens
+ * while on the entry-stack, the NMI handler will switch back to the top
+ * of the task stack, overwriting our stack-frame we are about to copy.
+ * Therefore we switch the stack only after everything is copied over.
+ */
+.macro SWITCH_TO_ENTRY_STACK
+
+ ALTERNATIVE "", "jmp .Lend_\@", X86_FEATURE_XENPV
+
+ /* Bytes to copy */
+ movl $PTREGS_SIZE, %ecx
+
+#ifdef CONFIG_VM86
+ testl $(X86_EFLAGS_VM), PT_EFLAGS(%esp)
+ jz .Lcopy_pt_regs_\@
+
+ /* Additional 4 registers to copy when returning to VM86 mode */
+ addl $(4 * 4), %ecx
+
+.Lcopy_pt_regs_\@:
+#endif
+
+ /* Initialize source and destination for movsb */
+ movl PER_CPU_VAR(cpu_tss_rw + TSS_sp0), %edi
+ subl %ecx, %edi
+ movl %esp, %esi
+
+ /* Save future stack pointer in %ebx */
+ movl %edi, %ebx
+
+ /* Copy over the stack-frame */
+ cld
+ rep movsb
+
+ /*
+ * Switch to entry-stack - needs to happen after everything is
+ * copied because the NMI handler will overwrite the task-stack
+ * when on entry-stack
+ */
+ movl %ebx, %esp
+
+.Lend_\@:
+.endm
+
+/*
* %eax: prev task
* %edx: next task
*/
@@ -578,6 +631,7 @@ ENTRY(entry_SYSENTER_32)

/* Opportunistic SYSEXIT */
TRACE_IRQS_ON /* User mode traces as IRQs on. */
+ SWITCH_TO_ENTRY_STACK /* Switch to per-cpu entry stack */
movl PT_EIP(%esp), %edx /* pt_regs->ip */
movl PT_OLDESP(%esp), %ecx /* pt_regs->sp */
1: mov PT_FS(%esp), %fs
@@ -665,6 +719,7 @@ ENTRY(entry_INT80_32)

restore_all:
TRACE_IRQS_IRET
+ SWITCH_TO_ENTRY_STACK
.Lrestore_all_notrace:
CHECK_AND_APPLY_ESPFIX
.Lrestore_nocheck:
--
2.7.4
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