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Date: Tue, 23 Jun 2020 23:54:50 -0700
From: Kees Cook <>
To: Kristen Carlson Accardi <>
	"H. Peter Anvin" <>,,,,
Subject: Re: [PATCH v3 07/10] x86/boot/compressed: change definition of STATIC

On Tue, Jun 23, 2020 at 10:23:24AM -0700, Kristen Carlson Accardi wrote:
> In preparation for changes to the upcoming fgkaslr commit, change misc.c
> to not define STATIC as static, but instead set STATIC to "". This allows
> memptr to become accessible to multiple files.

Thanks for splitting this out!

After looking at the results, I think I finally understand the issue
getting solved. tl;dr: I think your patch isn't the right solution
(basically making "malloc_ptr" a global so that multiple static copies
of malloc() all behave the same). I think the right solution is to avoid
the multiple copies of malloc()/free().

I think this patch should be used instead:

Subject: [PATCH] x86/boot/compressed: Avoid duplicate malloc() implementations

The preboot malloc() (and free()) implementation in
include/linux/decompress/mm.h (which is also included by the
static decompressors) is static. This is fine when the only thing
interested in using malloc() is the decompression code, but the
x86 preboot environment uses malloc() in a couple places, leading to a
potential collision when the static copies of the available memory
region ("malloc_ptr") gets reset to the global "free_mem_ptr" value.
As it happens, the existing usage pattern happened to be safe because each
user did 1 malloc() and 1 free() before returning and were not nested:

extract_kernel() (misc.c)
	choose_random_location() (kaslr.c)
	parse_elf() (misc.c)

Adding FGKASLR, however, will insert additional malloc() calls local to
fgkaslr.c in the middle of parse_elf()'s malloc()/free() pair:

	parse_elf() (misc.c)
		if (...) {
			layout_randomized_image(output, &ehdr, phdrs);
				malloc() <- boom
			layout_image(output, &ehdr, phdrs);

To avoid collisions, there must be a single implementation of malloc().
Adjust include/linux/decompress/mm.h so that visibility can be
controlled, provide prototypes in misc.h, and implement the functions in
misc.c. This also results in a small size savings:

$ size vmlinux.before vmlinux.after
   text    data     bss     dec     hex filename
8842314     468  178320 9021102  89a6ae vmlinux.before
8842240     468  178320 9021028  89a664 vmlinux.after

Signed-off-by: Kees Cook <>
 arch/x86/boot/compressed/kaslr.c |    4 ----
 arch/x86/boot/compressed/misc.c  |    3 +++
 arch/x86/boot/compressed/misc.h  |    2 ++
 include/linux/decompress/mm.h    |   12 ++++++++++--
 4 files changed, 15 insertions(+), 6 deletions(-)

diff --git a/arch/x86/boot/compressed/misc.c b/arch/x86/boot/compressed/misc.c
index 7667415417dc..ae6691e8bb08 100644
--- a/arch/x86/boot/compressed/misc.c
+++ b/arch/x86/boot/compressed/misc.c
@@ -44,12 +44,12 @@ void *memmove(void *dest, const void *src, size_t n);
 struct boot_params *boot_params;
+/* Initial heap area used to initialize malloc()/free() internals.*/
 memptr free_mem_ptr;
 memptr free_mem_end_ptr;
+/* Global internals for malloc()/free() implementations. */
 unsigned long malloc_ptr;
 int malloc_count;
 static char *vidmem;
 static int vidport;
diff --git a/arch/x86/boot/compressed/misc.h b/arch/x86/boot/compressed/misc.h
index 86a5f00b018f..45644056572b 100644
--- a/arch/x86/boot/compressed/misc.h
+++ b/arch/x86/boot/compressed/misc.h
@@ -36,15 +36,14 @@
 #define memptr unsigned
-/* misc.c */
-extern memptr free_mem_ptr;
-extern memptr free_mem_end_ptr;
-#define STATIC
+/* malloc()/free() */
+#define STATIC static
 #define STATIC_RW_DATA extern
 #include <linux/decompress/mm.h>
+/* misc.c */
+extern memptr free_mem_ptr;
+extern memptr free_mem_end_ptr;
 extern struct boot_params *boot_params;
 void __putstr(const char *s);
 void __puthex(unsigned long value);

However, after all that, I actually think the correct way to solve this
is actually to have only one implementation of malloc()/free(). i.e.
replace this patch with this:

diff --git a/arch/x86/boot/compressed/kaslr.c b/arch/x86/boot/compressed/kaslr.c
index d7408af55738..6f596bd5b6e5 100644
--- a/arch/x86/boot/compressed/kaslr.c
+++ b/arch/x86/boot/compressed/kaslr.c
@@ -39,10 +39,6 @@
 #include <generated/utsrelease.h>
 #include <asm/efi.h>
-/* Macros used by the included decompressor code below. */
-#define STATIC
-#include <linux/decompress/mm.h>
 #ifdef CONFIG_X86_5LEVEL
 unsigned int __pgtable_l5_enabled;
 unsigned int pgdir_shift __ro_after_init = 39;
diff --git a/arch/x86/boot/compressed/misc.c b/arch/x86/boot/compressed/misc.c
index 1c8b8aa5539f..f52150ec3ee7 100644
--- a/arch/x86/boot/compressed/misc.c
+++ b/arch/x86/boot/compressed/misc.c
@@ -28,6 +28,9 @@
 /* Macros used by the included decompressor code below. */
 #define STATIC		static
+/* Define an externally visible malloc()/free(). */
+#include <linux/decompress/mm.h>
  * Use normal definitions of mem*() from string.c. There are already
diff --git a/arch/x86/boot/compressed/misc.h b/arch/x86/boot/compressed/misc.h
index 214ef31db468..fd0c63cfaa4a 100644
--- a/arch/x86/boot/compressed/misc.h
+++ b/arch/x86/boot/compressed/misc.h
@@ -39,6 +39,8 @@
 /* misc.c */
 extern memptr free_mem_ptr;
 extern memptr free_mem_end_ptr;
+extern void *malloc(int size);
+extern void free(void *where);
 extern struct boot_params *boot_params;
 void __putstr(const char *s);
 void __puthex(unsigned long value);
diff --git a/include/linux/decompress/mm.h b/include/linux/decompress/mm.h
index 868e9eacd69e..9192986b1a73 100644
--- a/include/linux/decompress/mm.h
+++ b/include/linux/decompress/mm.h
@@ -25,13 +25,21 @@
 #define STATIC_RW_DATA static
+ * When an architecture needs to share the malloc()/free() implementation
+ * between compilation units, it needs to have non-local visibility.
+ */
+#define MALLOC_VISIBLE static
 /* A trivial malloc implementation, adapted from
  *  malloc by Hannu Savolainen 1993 and Matthias Urlichs 1994
 STATIC_RW_DATA unsigned long malloc_ptr;
 STATIC_RW_DATA int malloc_count;
-static void *malloc(int size)
+MALLOC_VISIBLE void *malloc(int size)
 	void *p;
@@ -52,7 +60,7 @@ static void *malloc(int size)
 	return p;
-static void free(void *where)
+MALLOC_VISIBLE void free(void *where)
 	if (!malloc_count)

Kees Cook

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