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Subject[PATCH 39/39] x86/entry/32: Add debug code to check entry/exit cr3
Date
From: Joerg Roedel <jroedel@suse.de>

Add a config option that enabled code to check that we enter
and leave the kernel with the correct cr3. This is needed
because we have no NX protection of user-addresses in the
kernel-cr3 on x86-32 and wouldn't notice that type of bug
otherwise.

Signed-off-by: Joerg Roedel <jroedel@suse.de>
---
arch/x86/Kconfig.debug | 12 ++++++++++++
arch/x86/entry/entry_32.S | 43 +++++++++++++++++++++++++++++++++++++++++++
2 files changed, 55 insertions(+)

diff --git a/arch/x86/Kconfig.debug b/arch/x86/Kconfig.debug
index c6dd1d9..6eaca2d 100644
--- a/arch/x86/Kconfig.debug
+++ b/arch/x86/Kconfig.debug
@@ -340,6 +340,18 @@ config X86_DEBUG_FPU

If unsure, say N.

+config X86_DEBUG_ENTRY_CR3
+ bool "Debug CR3 for Kernel entry/exit"
+ depends on X86_32 && PAGE_TABLE_ISOLATION
+ help
+ Add instructions to the x86-32 entry code to check whether the kernel
+ is entered and left with the correct CR3. When PTI is enabled, this
+ checks whether we enter the kernel with the user-space cr3 when
+ coming from user-mode and if we leave with user-cr3 back to
+ user-space.
+
+ If unsure, say N.
+
config PUNIT_ATOM_DEBUG
tristate "ATOM Punit debug driver"
depends on PCI
diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S
index a368583..d8d9a54 100644
--- a/arch/x86/entry/entry_32.S
+++ b/arch/x86/entry/entry_32.S
@@ -166,6 +166,24 @@
.Lend_\@:
.endm

+.macro BUG_IF_WRONG_CR3 no_user_check=0
+#ifdef CONFIG_X86_DEBUG_ENTRY_CR3
+ ALTERNATIVE "jmp .Lend_\@", "", X86_FEATURE_PTI
+ .if \no_user_check == 0
+ /* coming from usermode? */
+ testl $SEGMENT_RPL_MASK, PT_CS(%esp)
+ jz .Lend_\@
+ .endif
+ /* On user-cr3? */
+ movl %cr3, %eax
+ testl $PTI_SWITCH_MASK, %eax
+ jnz .Lend_\@
+ /* From userspace with kernel cr3 - BUG */
+ ud2
+.Lend_\@:
+#endif
+.endm
+
/*
* Switch to kernel cr3 if not already loaded and return current cr3 in
* \scratch_reg
@@ -218,6 +236,8 @@
.macro SAVE_ALL_NMI cr3_reg:req
SAVE_ALL

+ BUG_IF_WRONG_CR3
+
/*
* Now switch the CR3 when PTI is enabled.
*
@@ -229,6 +249,7 @@

.Lend_\@:
.endm
+
/*
* This is a sneaky trick to help the unwinder find pt_regs on the stack. The
* frame pointer is replaced with an encoded pointer to pt_regs. The encoding
@@ -292,6 +313,8 @@

.Lswitched_\@:

+ BUG_IF_WRONG_CR3
+
RESTORE_REGS pop=\pop
.endm

@@ -362,6 +385,8 @@

ALTERNATIVE "", "jmp .Lend_\@", X86_FEATURE_XENPV

+ BUG_IF_WRONG_CR3
+
SWITCH_TO_KERNEL_CR3 scratch_reg=%eax

/*
@@ -803,6 +828,7 @@ ENTRY(entry_SYSENTER_32)
*/
pushfl
pushl %eax
+ BUG_IF_WRONG_CR3 no_user_check=1
SWITCH_TO_KERNEL_CR3 scratch_reg=%eax
popl %eax
popfl
@@ -897,6 +923,7 @@ ENTRY(entry_SYSENTER_32)
* whereas POPF does not.)
*/
btrl $X86_EFLAGS_IF_BIT, (%esp)
+ BUG_IF_WRONG_CR3 no_user_check=1
popfl
popl %eax

@@ -974,6 +1001,8 @@ restore_all:
/* Switch back to user CR3 */
SWITCH_TO_USER_CR3 scratch_reg=%eax

+ BUG_IF_WRONG_CR3
+
/* Restore user state */
RESTORE_REGS pop=4 # skip orig_eax/error_code
.Lirq_return:
@@ -987,6 +1016,7 @@ restore_all:
restore_all_kernel:
TRACE_IRQS_IRET
PARANOID_EXIT_TO_KERNEL_MODE
+ BUG_IF_WRONG_CR3
RESTORE_REGS 4
jmp .Lirq_return

@@ -994,6 +1024,19 @@ restore_all_kernel:
ENTRY(iret_exc )
pushl $0 # no error code
pushl $do_iret_error
+
+#ifdef CONFIG_X86_DEBUG_ENTRY_CR3
+ /*
+ * The stack-frame here is the one that iret faulted on, so its a
+ * return-to-user frame. We are on kernel-cr3 because we come here from
+ * the fixup code. This confuses the CR3 checker, so switch to user-cr3
+ * as the checker expects it.
+ */
+ pushl %eax
+ SWITCH_TO_USER_CR3 scratch_reg=%eax
+ popl %eax
+#endif
+
jmp common_exception
.previous
_ASM_EXTABLE(.Lirq_return, iret_exc)
--
2.7.4
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