[PATCH 35/35] x86/entry/32: Add debug code to check entry/exit cr3

[Joerg Roedel]

From: Joerg Roedel <jroedel@xxxxxxx>

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@xxxxxxx>
---
 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 672441c..b8a772c 100644
--- a/arch/x86/Kconfig.debug
+++ b/arch/x86/Kconfig.debug
@@ -345,6 +345,18 @@ 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 0250b79..ea19bcf 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
 
 	/*
@@ -804,6 +829,7 @@ 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
@@ -898,6 +924,7 @@ ENTRY(entry_SYSENTER_32)
 	 * whereas POPF does not.)
 	 */
 	btr	$X86_EFLAGS_IF_BIT, (%esp)
+	BUG_IF_WRONG_CR3 no_user_check=1
 	popfl
 	popl	%eax
 
@@ -975,6 +1002,8 @@ 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:
@@ -983,6 +1012,7 @@ restore_all:
 restore_all_kernel:
 	TRACE_IRQS_IRET
 	PARANOID_EXIT_TO_KERNEL_MODE
+	BUG_IF_WRONG_CR3
 	RESTORE_REGS 4
 	jmp	.Lirq_return
 
@@ -990,6 +1020,19 @@ 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