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Date: Fri, 5 Dec 2014 14:00:34 -0800
From: Andy Lutomirski <luto@...capital.net>
To: Hector Marco <hecmargi@....es>
Cc: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
Andrew Morton <akpm@...ux-foundation.org>,
Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...hat.com>,
"H. Peter Anvin" <hpa@...or.com>,
Russell King <linux@....linux.org.uk>,
Catalin Marinas <catalin.marinas@....com>,
Will Deacon <will.deacon@....com>,
Oleg Nesterov <oleg@...hat.com>,
Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH] ASLRv3: randomize_va_space=3 preventing offset2lib attack
On Thu, Dec 4, 2014 at 4:07 PM, Hector Marco <hecmargi@....es> wrote:
> [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.
Can we do much better if we add randomly-sized guard regions between
all libraries? I don't understand why libraries are mapped
consecutively, other than to save a tiny bit of RAM by using fewer
page tables.
--Andy
>
> 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@....es>
> Signed-off-by: Ismael Ripoll <iripoll@....es>
--
Andy Lutomirski
AMA Capital Management, LLC
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