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Subject[PATCH] ASLRv3: randomize_va_space=3 preventing offset2lib attack
[PATCH] ASLRv3: randomize_va_space=3 preventing offset2lib attack

The issue appears on PIE linked executables when all memory areas of
a process are randomized (randomize_va_space=2). In this case, the
attack "offset2lib" de-randomizes all library areas on 64 bit Linux
systems in less than one second.

Further details of the PoC attack at:
http://cybersecurity.upv.es/attacks/offset2lib/offset2lib.html

PIE linked applications are loaded side by side with the dynamic
libraries, which is exploited by the offset2lib attack. Moving away
the executable from the mmap_base area (libraries area) prevents the
attack.

This patch loads the PIE linked executable in a different area than
the libraries when randomize_va_space=3.

Patch implementation details:

- The ELF_ET_DYN_BASE address is used as the base to load randomly
the PIE executable.

- The executable image has the same entropy than
randomize_va_space=2.


If the randomize_va_space is set to 2 then this patch does not change
any behavior when loading new processes.

The patch has been tested on x86_64/32 and ARM/ARM64.


diff --git a/arch/arm/mm/mmap.c b/arch/arm/mm/mmap.c
index 5e85ed3..6602f5e 100644
--- a/arch/arm/mm/mmap.c
+++ b/arch/arm/mm/mmap.c
@@ -10,6 +10,7 @@
#include <linux/personality.h>
#include <linux/random.h>
#include <asm/cachetype.h>
+#include <asm/elf.h>

#define COLOUR_ALIGN(addr,pgoff) \
((((addr)+SHMLBA-1)&~(SHMLBA-1)) + \
@@ -19,6 +20,14 @@
#define MIN_GAP (128*1024*1024UL)
#define MAX_GAP ((TASK_SIZE)/6*5)

+#if ELF_EXEC_PAGESIZE > PAGE_SIZE
+#define ELF_MIN_ALIGN ELF_EXEC_PAGESIZE
+#else
+#define ELF_MIN_ALIGN PAGE_SIZE
+#endif
+
+#define ELF_PAGESTART(_v) ((_v) & ~(unsigned long)(ELF_MIN_ALIGN-1))
+
static int mmap_is_legacy(void)
{
if (current->personality & ADDR_COMPAT_LAYOUT)
@@ -184,6 +193,9 @@ void arch_pick_mmap_layout(struct mm_struct *mm)
} else {
mm->mmap_base = mmap_base(random_factor);
mm->get_unmapped_area = arch_get_unmapped_area_topdown;
+ if (randomize_va_space > 2)
+ mm->exec_base = ELF_PAGESTART(ELF_ET_DYN_BASE -
+ ((get_random_int() % (1 << 8)) << PAGE_SHIFT));
}
}

diff --git a/arch/arm64/mm/mmap.c b/arch/arm64/mm/mmap.c
index 1d73662..32be3fd 100644
--- a/arch/arm64/mm/mmap.c
+++ b/arch/arm64/mm/mmap.c
@@ -28,6 +28,16 @@
#include <linux/random.h>

#include <asm/cputype.h>
+#include <asm/elf.h>
+
+
+#if ELF_EXEC_PAGESIZE > PAGE_SIZE
+#define ELF_MIN_ALIGN ELF_EXEC_PAGESIZE
+#else
+#define ELF_MIN_ALIGN PAGE_SIZE
+#endif
+
+#define ELF_PAGESTART(_v) ((_v) & ~(unsigned long)(ELF_MIN_ALIGN-1))

/*
* Leave enough space between the mmap area and the stack to honour
ulimit in
@@ -93,6 +103,8 @@ void arch_pick_mmap_layout(struct mm_struct *mm)
} else {
mm->mmap_base = mmap_base();
mm->get_unmapped_area = arch_get_unmapped_area_topdown;
+ if (randomize_va_space > 2)
+ mm->exec_base = ELF_PAGESTART(ELF_ET_DYN_BASE - mmap_rnd());
}
}
EXPORT_SYMBOL_GPL(arch_pick_mmap_layout);
diff --git a/arch/x86/mm/mmap.c b/arch/x86/mm/mmap.c
index 919b912..8cb9855 100644
--- a/arch/x86/mm/mmap.c
+++ b/arch/x86/mm/mmap.c
@@ -31,6 +31,14 @@
#include <linux/sched.h>
#include <asm/elf.h>

+#if ELF_EXEC_PAGESIZE > PAGE_SIZE
+#define ELF_MIN_ALIGN ELF_EXEC_PAGESIZE
+#else
+#define ELF_MIN_ALIGN PAGE_SIZE
+#endif
+
+#define ELF_PAGESTART(_v) ((_v) & ~(unsigned long)(ELF_MIN_ALIGN-1))
+
struct va_alignment __read_mostly va_align = {
.flags = -1,
};
@@ -120,5 +128,7 @@ void arch_pick_mmap_layout(struct mm_struct *mm)
mm->get_unmapped_area = arch_get_unmapped_area;
} else {
mm->get_unmapped_area = arch_get_unmapped_area_topdown;
+ if (randomize_va_space > 2)
+ mm->exec_base = ELF_PAGESTART(ELF_ET_DYN_BASE - mmap_rnd());
}
}
diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
index d8fc060..6f319c1 100644
--- a/fs/binfmt_elf.c
+++ b/fs/binfmt_elf.c
@@ -804,8 +804,11 @@ static int load_elf_binary(struct linux_binprm *bprm)
* load_bias value in order to establish proper
* non-randomized mappings.
*/
- if (current->flags & PF_RANDOMIZE)
+ if (current->flags & PF_RANDOMIZE) {
load_bias = 0;
+ if (randomize_va_space > 2)
+ load_bias = current->mm->exec_base;
+ }
else
load_bias = ELF_PAGESTART(ELF_ET_DYN_BASE - vaddr);
#else
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 6e0b286..dd052ec 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -353,6 +353,7 @@ struct mm_struct {
#endif
unsigned long mmap_base; /* base of mmap area */
unsigned long mmap_legacy_base; /* base of mmap area in
bottom-up allocations */
+ unsigned long exec_base; /* base of exec area */
unsigned long task_size; /* size of task vm space */
unsigned long highest_vm_end; /* highest vma end address */
pgd_t * pgd;
diff --git a/kernel/fork.c b/kernel/fork.c
index 9b7d746..1fd4553 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -551,6 +551,7 @@ static void mm_init_owner(struct mm_struct *mm,
struct task_struct *p)
static struct mm_struct *mm_init(struct mm_struct *mm, struct
task_struct *p)
{
mm->mmap = NULL;
+ mm->exec_base = 0;
mm->mm_rb = RB_ROOT;
mm->vmacache_seqnum = 0;
atomic_set(&mm->mm_users, 1);

Signed-off-by: Hector Marco-Gisbert <hecmargi@upv.es>
Signed-off-by: Ismael Ripoll <iripoll@upv.es>


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