x86/entry/32: Add debug code to check entry/exit CR3

Add code to check whether the kernel is entered and left with the correct
CR3 and make it depend on CONFIG_DEBUG_ENTRY.  This is needed because there
is no NX protection of user-addresses in the kernel-CR3 on x86-32 and that
type of bug would not be detected otherwise.

Signed-off-by: Joerg Roedel <jroedel@suse.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Pavel Machek <pavel@ucw.cz>
Cc: "H . Peter Anvin" <hpa@zytor.com>
Cc: linux-mm@kvack.org
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Juergen Gross <jgross@suse.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Jiri Kosina <jkosina@suse.cz>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: David Laight <David.Laight@aculab.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Eduardo Valentin <eduval@amazon.com>
Cc: Greg KH <gregkh@linuxfoundation.org>
Cc: Will Deacon <will.deacon@arm.com>
Cc: aliguori@amazon.com
Cc: daniel.gruss@iaik.tugraz.at
Cc: hughd@google.com
Cc: keescook@google.com
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Waiman Long <llong@redhat.com>
Cc: "David H . Gutteridge" <dhgutteridge@sympatico.ca>
Cc: joro@8bytes.org
Link: https://lkml.kernel.org/r/1531906876-13451-40-git-send-email-joro@8bytes.org

diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S
index b1541c74c71aff3477d98139a8313d76566ab0c6..010cdb41e3c73237a471a8dd4c748a462579ebf8 100644 (file)
--- 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_DEBUG_ENTRY
+       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
@@ -213,6 +231,8 @@
 .macro SAVE_ALL_NMI cr3_reg:req
        SAVE_ALL
 
+       BUG_IF_WRONG_CR3
+
        /*
         * Now switch the CR3 when PTI is enabled.
         *
@@ -224,6 +244,7 @@
 
 .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
@@ -287,6 +308,8 @@
 
 .Lswitched_\@:
 
+       BUG_IF_WRONG_CR3
+
        RESTORE_REGS pop=\pop
 .endm
 
@@ -357,6 +380,8 @@
 
        ALTERNATIVE     "", "jmp .Lend_\@", X86_FEATURE_XENPV
 
+       BUG_IF_WRONG_CR3
+
        SWITCH_TO_KERNEL_CR3 scratch_reg=%eax
 
        /*
@@ -799,6 +824,7 @@ 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
@@ -893,6 +919,7 @@ 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
 
@@ -970,6 +997,8 @@ 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_DEBUG_ENTRY
+       /*
+        * 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)