[PATCH] mm: Revert x86_64 and arm64 ELF_ET_DYN_BASE base

[Kees Cook]

Moving the x86_64 and arm64 PIE base from 0x555555554000 to 0x000100000000
broke AddressSanitizer. This is a partial revert of:

  commit eab09532d400 ("binfmt_elf: use ELF_ET_DYN_BASE only for PIE")
  commit 02445990a96e ("arm64: move ELF_ET_DYN_BASE to 4GB / 4MB")

The AddressSanitizer tool has hard-coded expectations about where
executable mappings are loaded. The motivation for changing the PIE
base in the above commits was to avoid the Stack-Clash CVEs that
allowed executable mappings to get too close to heap and stack. This
was mainly a problem on 32-bit, but the 64-bit bases were moved too,
in an effort to proactively protect those systems (proofs of concept
do exist that show 64-bit collisions, but other recent changes to fix
stack accounting and setuid behaviors will minimize the impact).

The new 32-bit PIE base is fine for ASan (since it matches the ET_EXEC
base), so only the 64-bit PIE base needs to be reverted to let x86 and
arm64 ASan binaries run again. Future changes to the 64-bit PIE base on
these architectures can be made optional once a more dynamic method for
dealing with AddressSanitizer is found. (e.g. always loading PIE into
the mmap region for marked binaries.)

Reported-by: Kostya Serebryany <kcc@xxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
 arch/arm64/include/asm/elf.h | 4 ++--
 arch/x86/include/asm/elf.h   | 4 ++--
 2 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/arch/arm64/include/asm/elf.h b/arch/arm64/include/asm/elf.h
index acae781f7359..3288c2b36731 100644
--- a/arch/arm64/include/asm/elf.h
+++ b/arch/arm64/include/asm/elf.h
@@ -114,10 +114,10 @@
 
 /*
  * This is the base location for PIE (ET_DYN with INTERP) loads. On
- * 64-bit, this is raised to 4GB to leave the entire 32-bit address
+ * 64-bit, this is above 4GB to leave the entire 32-bit address
  * space open for things that want to use the area for 32-bit pointers.
  */
-#define ELF_ET_DYN_BASE		0x100000000UL
+#define ELF_ET_DYN_BASE		(2 * TASK_SIZE_64 / 3)
 
 #ifndef __ASSEMBLY__
 
diff --git a/arch/x86/include/asm/elf.h b/arch/x86/include/asm/elf.h
index 1c18d83d3f09..9aeb91935ce0 100644
--- a/arch/x86/include/asm/elf.h
+++ b/arch/x86/include/asm/elf.h
@@ -247,11 +247,11 @@ extern int force_personality32;
 
 /*
  * This is the base location for PIE (ET_DYN with INTERP) loads. On
- * 64-bit, this is raised to 4GB to leave the entire 32-bit address
+ * 64-bit, this is above 4GB to leave the entire 32-bit address
  * space open for things that want to use the area for 32-bit pointers.
  */
 #define ELF_ET_DYN_BASE		(mmap_is_ia32() ? 0x000400000UL : \
-						  0x100000000UL)
+						  (TASK_SIZE / 3 * 2))
 
 /* This yields a mask that user programs can use to figure out what
    instruction set this CPU supports.  This could be done in user space,
-- 
2.7.4


-- 
Kees Cook
Pixel Security