From: Joerg Roedel <jroe...@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 <jroe...@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 192e4d2..a57f556 100644
--- a/arch/x86/Kconfig.debug
+++ b/arch/x86/Kconfig.debug
@@ -337,6 +337,18 @@ 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 f47e535..6b371a9 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.)
         */
        btr     $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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