[Xenial SRU][PATCH 1/1] mm: revert x86_64 and arm64 ELF_ET_DYN_BASE base changes

[Kleber Sacilotto de Souza]

From: Kees Cook <keescook at chromium.org>

BugLink: http://bugs.launchpad.net/bugs/1715636

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

  eab09532d400 ("binfmt_elf: use ELF_ET_DYN_BASE only for PIE")
  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.)

Link: http://lkml.kernel.org/r/20170807201542.GA21271@beast
Fixes: eab09532d400 ("binfmt_elf: use ELF_ET_DYN_BASE only for PIE")
Fixes: 02445990a96e ("arm64: move ELF_ET_DYN_BASE to 4GB / 4MB")
Signed-off-by: Kees Cook <keescook at chromium.org>
Reported-by: Kostya Serebryany <kcc at google.com>
Acked-by: Will Deacon <will.deacon at arm.com>
Cc: Ingo Molnar <mingo at elte.hu>
Cc: "H. Peter Anvin" <hpa at zytor.com>
Cc: Thomas Gleixner <tglx at linutronix.de>
Cc: <stable at vger.kernel.org>
Signed-off-by: Andrew Morton <akpm at linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
(cherry picked from commit c715b72c1ba406f133217b509044c38d8e714a37)
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza at canonical.com>
---
 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 9e11dbe1cec3..329c127e13dc 100644
--- a/arch/arm64/include/asm/elf.h
+++ b/arch/arm64/include/asm/elf.h
@@ -121,10 +121,10 @@ typedef struct user_fpsimd_state elf_fpregset_t;
 
 /*
  * 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)
 
 /*
  * When the program starts, a1 contains a pointer to a function to be
diff --git a/arch/x86/include/asm/elf.h b/arch/x86/include/asm/elf.h
index 07cf288b692e..bcd3d6199464 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,
-- 
2.14.1