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Subject[PATCH 14/39] x86/entry/32: Add PTI cr3 switches to NMI handler code
Date
From: Joerg Roedel <jroedel@suse.de>

The NMI handler is special, as it needs to leave with the
same cr3 as it was entered with. We need to do this because
we could enter the NMI handler from kernel code with
user-cr3 already loaded.

Signed-off-by: Joerg Roedel <jroedel@suse.de>
---
arch/x86/entry/entry_32.S | 41 +++++++++++++++++++++++++++++++++++------
1 file changed, 35 insertions(+), 6 deletions(-)

diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S
index 311aefa..a368583 100644
--- a/arch/x86/entry/entry_32.S
+++ b/arch/x86/entry/entry_32.S
@@ -77,6 +77,8 @@
#endif
.endm

+#define PTI_SWITCH_MASK (1 << PAGE_SHIFT)
+
/*
* User gs save/restore
*
@@ -213,8 +215,19 @@

.endm

-.macro SAVE_ALL_NMI
+.macro SAVE_ALL_NMI cr3_reg:req
SAVE_ALL
+
+ /*
+ * Now switch the CR3 when PTI is enabled.
+ *
+ * We can enter with either user or kernel cr3, the code will
+ * store the old cr3 in \cr3_reg and switches to the kernel cr3
+ * if necessary.
+ */
+ SWITCH_TO_KERNEL_CR3 scratch_reg=\cr3_reg
+
+.Lend_\@:
.endm
/*
* This is a sneaky trick to help the unwinder find pt_regs on the stack. The
@@ -262,7 +275,23 @@
POP_GS_EX
.endm

-.macro RESTORE_ALL_NMI pop=0
+.macro RESTORE_ALL_NMI cr3_reg:req pop=0
+ /*
+ * Now switch the CR3 when PTI is enabled.
+ *
+ * We enter with kernel cr3 and switch the cr3 to the value
+ * stored on \cr3_reg, which is either a user or a kernel cr3.
+ */
+ ALTERNATIVE "jmp .Lswitched_\@", "", X86_FEATURE_PTI
+
+ testl $PTI_SWITCH_MASK, \cr3_reg
+ jz .Lswitched_\@
+
+ /* User cr3 in \cr3_reg - write it to hardware cr3 */
+ movl \cr3_reg, %cr3
+
+.Lswitched_\@:
+
RESTORE_REGS pop=\pop
.endm

@@ -1333,7 +1362,7 @@ ENTRY(nmi)
#endif

pushl %eax # pt_regs->orig_ax
- SAVE_ALL_NMI
+ SAVE_ALL_NMI cr3_reg=%edi
ENCODE_FRAME_POINTER
xorl %edx, %edx # zero error code
movl %esp, %eax # pt_regs pointer
@@ -1361,7 +1390,7 @@ ENTRY(nmi)

.Lnmi_return:
CHECK_AND_APPLY_ESPFIX
- RESTORE_ALL_NMI pop=4
+ RESTORE_ALL_NMI cr3_reg=%edi pop=4
jmp .Lirq_return

#ifdef CONFIG_X86_ESPFIX32
@@ -1377,12 +1406,12 @@ ENTRY(nmi)
pushl 16(%esp)
.endr
pushl %eax
- SAVE_ALL_NMI
+ SAVE_ALL_NMI cr3_reg=%edi
ENCODE_FRAME_POINTER
FIXUP_ESPFIX_STACK # %eax == %esp
xorl %edx, %edx # zero error code
call do_nmi
- RESTORE_ALL_NMI
+ RESTORE_ALL_NMI cr3_reg=%edi
lss 12+4(%esp), %esp # back to espfix stack
jmp .Lirq_return
#endif
--
2.7.4
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