[tip:x86/boot] x86/KASLR: Clarify identity map interface

[tip-bot for Kees Cook]

Commit-ID:  11fdf97a3cd1a5a27625f820ceb74e1caba4fd26
Gitweb:     http://git.kernel.org/tip/11fdf97a3cd1a5a27625f820ceb74e1caba4fd26
Author:     Kees Cook <keescook@xxxxxxxxxxxx>
AuthorDate: Wed, 25 May 2016 15:45:31 -0700
Committer:  Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Sun, 26 Jun 2016 12:32:04 +0200

x86/KASLR: Clarify identity map interface

This extracts the call to prepare_level4() into a top-level function
that the user of the pagetable.c interface must call to initialize
the new page tables. For clarity and to match the "finalize" function,
it has been renamed to initialize_identity_maps(). This function also
gains the initialization of mapping_info so we don't have to do it each
time in add_identity_map().

Additionally add copyright notice to the top, to make it clear that the
bulk of the pagetable.c code was written by Yinghai, and that I just
added bugs later. :)

Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: Baoquan He <bhe@xxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: Brian Gerst <brgerst@xxxxxxxxx>
Cc: Denys Vlasenko <dvlasenk@xxxxxxxxxx>
Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
Cc: H. Peter Anvin <hpa@xxxxxxxxx>
Cc: H.J. Lu <hjl.tools@xxxxxxxxx>
Cc: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Yinghai Lu <yinghai@xxxxxxxxxx>
Link: http://lkml.kernel.org/r/1464216334-17200-3-git-send-email-keescook@xxxxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
 arch/x86/boot/compressed/kaslr.c     |  3 +++
 arch/x86/boot/compressed/misc.h      |  3 +++
 arch/x86/boot/compressed/pagetable.c | 26 ++++++++++++++++----------
 3 files changed, 22 insertions(+), 10 deletions(-)

diff --git a/arch/x86/boot/compressed/kaslr.c b/arch/x86/boot/compressed/kaslr.c
index dff4217..54037c9 100644
--- a/arch/x86/boot/compressed/kaslr.c
+++ b/arch/x86/boot/compressed/kaslr.c
@@ -478,6 +478,9 @@ unsigned char *choose_random_location(unsigned long input,
 
 	boot_params->hdr.loadflags |= KASLR_FLAG;
 
+	/* Prepare to add new identity pagetables on demand. */
+	initialize_identity_maps();
+
 	/* Record the various known unsafe memory ranges. */
 	mem_avoid_init(input, input_size, output);
 
diff --git a/arch/x86/boot/compressed/misc.h b/arch/x86/boot/compressed/misc.h
index b6fec1f..09c4ddd 100644
--- a/arch/x86/boot/compressed/misc.h
+++ b/arch/x86/boot/compressed/misc.h
@@ -85,10 +85,13 @@ unsigned char *choose_random_location(unsigned long input_ptr,
 #endif
 
 #ifdef CONFIG_X86_64
+void initialize_identity_maps(void);
 void add_identity_map(unsigned long start, unsigned long size);
 void finalize_identity_maps(void);
 extern unsigned char _pgtable[];
 #else
+static inline void initialize_identity_maps(void)
+{ }
 static inline void add_identity_map(unsigned long start, unsigned long size)
 { }
 static inline void finalize_identity_maps(void)
diff --git a/arch/x86/boot/compressed/pagetable.c b/arch/x86/boot/compressed/pagetable.c
index 34b95df..6e31a6a 100644
--- a/arch/x86/boot/compressed/pagetable.c
+++ b/arch/x86/boot/compressed/pagetable.c
@@ -2,6 +2,9 @@
  * This code is used on x86_64 to create page table identity mappings on
  * demand by building up a new set of page tables (or appending to the
  * existing ones), and then switching over to them when ready.
+ *
+ * Copyright (C) 2015-2016  Yinghai Lu
+ * Copyright (C)      2016  Kees Cook
  */
 
 /*
@@ -59,9 +62,21 @@ static struct alloc_pgt_data pgt_data;
 /* The top level page table entry pointer. */
 static unsigned long level4p;
 
+/*
+ * Mapping information structure passed to kernel_ident_mapping_init().
+ * Due to relocation, pointers must be assigned at run time not build time.
+ */
+static struct x86_mapping_info mapping_info = {
+	.pmd_flag       = __PAGE_KERNEL_LARGE_EXEC,
+};
+
 /* Locates and clears a region for a new top level page table. */
-static void prepare_level4(void)
+void initialize_identity_maps(void)
 {
+	/* Init mapping_info with run-time function/buffer pointers. */
+	mapping_info.alloc_pgt_page = alloc_pgt_page;
+	mapping_info.context = &pgt_data;
+
 	/*
 	 * It should be impossible for this not to already be true,
 	 * but since calling this a second time would rewind the other
@@ -96,17 +111,8 @@ static void prepare_level4(void)
  */
 void add_identity_map(unsigned long start, unsigned long size)
 {
-	struct x86_mapping_info mapping_info = {
-		.alloc_pgt_page	= alloc_pgt_page,
-		.context	= &pgt_data,
-		.pmd_flag	= __PAGE_KERNEL_LARGE_EXEC,
-	};
 	unsigned long end = start + size;
 
-	/* Make sure we have a top level page table ready to use. */
-	if (!level4p)
-		prepare_level4();
-
 	/* Align boundary to 2M. */
 	start = round_down(start, PMD_SIZE);
 	end = round_up(end, PMD_SIZE);