Re: [PATCH v3 04/11] arm64, mm: Move generic mmap layout functions to mm

From: Kees Cook
Date: Thu Apr 18 2019 - 01:25:39 EST

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(

On Wed, Apr 17, 2019 at 12:27 AM Alexandre Ghiti <alex@xxxxxxxx> wrote:
>
> arm64 handles top-down mmap layout in a way that can be easily reused
> by other architectures, so make it available in mm.
> It then introduces a new config ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT
> that can be set by other architectures to benefit from those functions.
> Note that this new config depends on MMU being enabled, if selected
> without MMU support, a warning will be thrown.
>
> Suggested-by: Christoph Hellwig <hch@xxxxxxxxxxxxx>
> Signed-off-by: Alexandre Ghiti <alex@xxxxxxxx>
> ---
> arch/Kconfig | 8 ++++
> arch/arm64/Kconfig | 1 +
> arch/arm64/include/asm/processor.h | 2 -
> arch/arm64/mm/mmap.c | 76 ------------------------------
> kernel/sysctl.c | 6 ++-
> mm/util.c | 74 ++++++++++++++++++++++++++++-
> 6 files changed, 86 insertions(+), 81 deletions(-)
>
> diff --git a/arch/Kconfig b/arch/Kconfig
> index 33687dddd86a..7c8965c64590 100644
> --- a/arch/Kconfig
> +++ b/arch/Kconfig
> @@ -684,6 +684,14 @@ config HAVE_ARCH_COMPAT_MMAP_BASES
> and vice-versa 32-bit applications to call 64-bit mmap().
> Required for applications doing different bitness syscalls.
>
> +# This allows to use a set of generic functions to determine mmap base
> +# address by giving priority to top-down scheme only if the process
> +# is not in legacy mode (compat task, unlimited stack size or
> +# sysctl_legacy_va_layout).
> +config ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT
> + bool
> + depends on MMU

I'd prefer the comment were moved to the help text. I would include
any details about what the arch still needs to define. For example
right now, I think STACK_RND_MASK is still needed. (Though I think a
common one could be added for this series too...)

> +
> config HAVE_COPY_THREAD_TLS
> bool
> help
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index 7e34b9eba5de..670719a26b45 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -66,6 +66,7 @@ config ARM64
> select ARCH_SUPPORTS_INT128 if GCC_VERSION >= 50000 || CC_IS_CLANG
> select ARCH_SUPPORTS_NUMA_BALANCING
> select ARCH_WANT_COMPAT_IPC_PARSE_VERSION
> + select ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT
> select ARCH_WANT_FRAME_POINTERS
> select ARCH_HAS_UBSAN_SANITIZE_ALL
> select ARM_AMBA
> diff --git a/arch/arm64/include/asm/processor.h b/arch/arm64/include/asm/processor.h
> index 5d9ce62bdebd..4de2a2fd605a 100644
> --- a/arch/arm64/include/asm/processor.h
> +++ b/arch/arm64/include/asm/processor.h
> @@ -274,8 +274,6 @@ static inline void spin_lock_prefetch(const void *ptr)
> "nop") : : "p" (ptr));
> }
>
> -#define HAVE_ARCH_PICK_MMAP_LAYOUT
> -
> #endif
>
> extern unsigned long __ro_after_init signal_minsigstksz; /* sigframe size */
> diff --git a/arch/arm64/mm/mmap.c b/arch/arm64/mm/mmap.c
> index ac89686c4af8..c74224421216 100644
> --- a/arch/arm64/mm/mmap.c
> +++ b/arch/arm64/mm/mmap.c
> @@ -31,82 +31,6 @@
>
> #include <asm/cputype.h>
>
> -/*
> - * Leave enough space between the mmap area and the stack to honour ulimit in
> - * the face of randomisation.
> - */

This comment goes missing in the move...

> -#define MIN_GAP (SZ_128M)
> -#define MAX_GAP (STACK_TOP/6*5)
> -
> -static int mmap_is_legacy(struct rlimit *rlim_stack)
> -{
> - if (current->personality & ADDR_COMPAT_LAYOUT)
> - return 1;
> -
> - if (rlim_stack->rlim_cur == RLIM_INFINITY)
> - return 1;
> -
> - return sysctl_legacy_va_layout;
> -}
> -
> -unsigned long arch_mmap_rnd(void)
> -{
> - unsigned long rnd;
> -
> -#ifdef CONFIG_COMPAT
> - if (is_compat_task())
> - rnd = get_random_long() & ((1UL << mmap_rnd_compat_bits) - 1);
> - else
> -#endif
> - rnd = get_random_long() & ((1UL << mmap_rnd_bits) - 1);
> - return rnd << PAGE_SHIFT;
> -}
> -
> -static unsigned long mmap_base(unsigned long rnd, struct rlimit *rlim_stack)
> -{
> - unsigned long gap = rlim_stack->rlim_cur;
> - unsigned long pad = stack_guard_gap;
> -
> - /* Account for stack randomization if necessary */
> - if (current->flags & PF_RANDOMIZE)
> - pad += (STACK_RND_MASK << PAGE_SHIFT);
> -
> - /* Values close to RLIM_INFINITY can overflow. */
> - if (gap + pad > gap)
> - gap += pad;
> -
> - if (gap < MIN_GAP)
> - gap = MIN_GAP;
> - else if (gap > MAX_GAP)
> - gap = MAX_GAP;
> -
> - return PAGE_ALIGN(STACK_TOP - gap - rnd);
> -}
> -
> -/*
> - * This function, called very early during the creation of a new process VM
> - * image, sets up which VM layout function to use:
> - */
> -void arch_pick_mmap_layout(struct mm_struct *mm, struct rlimit *rlim_stack)
> -{
> - unsigned long random_factor = 0UL;
> -
> - if (current->flags & PF_RANDOMIZE)
> - random_factor = arch_mmap_rnd();
> -
> - /*
> - * Fall back to the standard layout if the personality bit is set, or
> - * if the expected stack growth is unlimited:
> - */
> - if (mmap_is_legacy(rlim_stack)) {
> - mm->mmap_base = TASK_UNMAPPED_BASE + random_factor;
> - mm->get_unmapped_area = arch_get_unmapped_area;
> - } else {
> - mm->mmap_base = mmap_base(random_factor, rlim_stack);
> - mm->get_unmapped_area = arch_get_unmapped_area_topdown;
> - }
> -}
> -
> /*
> * You really shouldn't be using read() or write() on /dev/mem. This might go
> * away in the future.
> diff --git a/kernel/sysctl.c b/kernel/sysctl.c
> index e5da394d1ca3..eb3414e78986 100644
> --- a/kernel/sysctl.c
> +++ b/kernel/sysctl.c
> @@ -269,7 +269,8 @@ extern struct ctl_table epoll_table[];
> extern struct ctl_table firmware_config_table[];
> #endif
>
> -#ifdef HAVE_ARCH_PICK_MMAP_LAYOUT
> +#if defined(HAVE_ARCH_PICK_MMAP_LAYOUT) || \
> + defined(CONFIG_ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT)
> int sysctl_legacy_va_layout;
> #endif
>
> @@ -1564,7 +1565,8 @@ static struct ctl_table vm_table[] = {
> .proc_handler = proc_dointvec,
> .extra1 = &zero,
> },
> -#ifdef HAVE_ARCH_PICK_MMAP_LAYOUT
> +#if defined(HAVE_ARCH_PICK_MMAP_LAYOUT) || \
> + defined(CONFIG_ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT)
> {
> .procname = "legacy_va_layout",
> .data = &sysctl_legacy_va_layout,
> diff --git a/mm/util.c b/mm/util.c
> index a54afb9b4faa..5c3393d32ed1 100644
> --- a/mm/util.c
> +++ b/mm/util.c
> @@ -15,7 +15,12 @@
> #include <linux/vmalloc.h>
> #include <linux/userfaultfd_k.h>
> #include <linux/elf.h>
> +#include <linux/elf-randomize.h>
> +#include <linux/personality.h>
> #include <linux/random.h>
> +#include <linux/processor.h>
> +#include <linux/sizes.h>
> +#include <linux/compat.h>
>
> #include <linux/uaccess.h>
>
> @@ -313,7 +318,74 @@ unsigned long randomize_stack_top(unsigned long stack_top)
> #endif
> }
>
> -#if defined(CONFIG_MMU) && !defined(HAVE_ARCH_PICK_MMAP_LAYOUT)
> +#ifdef CONFIG_ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT
> +#ifdef CONFIG_ARCH_HAS_ELF_RANDOMIZE

I think CONFIG_ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT should select
CONFIG_ARCH_HAS_ELF_RANDOMIZE. It would mean moving
arch_randomize_brk() into this patch set too. For arm64 and arm, this
is totally fine: they have identical logic. On MIPS this would mean
bumping the randomization up: arm64 uses SZ_32M for 32-bit and SZ_1G
for 64-bit. MIPS is 8M and 256M respectively. I don't see anything
that indicates this would be a problem. *cross fingers*

It looks like x86 would need bumping too: it uses 32M on both 32-bit
and 64-bit. STACK_RND_MASK is the same though.

> +unsigned long arch_mmap_rnd(void)
> +{
> + unsigned long rnd;
> +
> +#ifdef CONFIG_COMPAT
> + if (is_compat_task())
> + rnd = get_random_long() & ((1UL << mmap_rnd_compat_bits) - 1);
> + else
> +#endif /* CONFIG_COMPAT */

The ifdefs on is_compat_task() are not needed: is_compat_task()
returns 0 in the !CONFIG_COMPAT case.

> + rnd = get_random_long() & ((1UL << mmap_rnd_bits) - 1);
> +
> + return rnd << PAGE_SHIFT;
> +}
> +#endif /* CONFIG_ARCH_HAS_ELF_RANDOMIZE */
> +
> +static int mmap_is_legacy(struct rlimit *rlim_stack)
> +{
> + if (current->personality & ADDR_COMPAT_LAYOUT)
> + return 1;
> +
> + if (rlim_stack->rlim_cur == RLIM_INFINITY)
> + return 1;
> +
> + return sysctl_legacy_va_layout;
> +}
> +
> +#define MIN_GAP (SZ_128M)
> +#define MAX_GAP (STACK_TOP / 6 * 5)
> +
> +static unsigned long mmap_base(unsigned long rnd, struct rlimit *rlim_stack)
> +{
> + unsigned long gap = rlim_stack->rlim_cur;
> + unsigned long pad = stack_guard_gap;
> +
> + /* Account for stack randomization if necessary */
> + if (current->flags & PF_RANDOMIZE)
> + pad += (STACK_RND_MASK << PAGE_SHIFT);
> +
> + /* Values close to RLIM_INFINITY can overflow. */
> + if (gap + pad > gap)
> + gap += pad;
> +
> + if (gap < MIN_GAP)
> + gap = MIN_GAP;
> + else if (gap > MAX_GAP)
> + gap = MAX_GAP;
> +
> + return PAGE_ALIGN(STACK_TOP - gap - rnd);
> +}
> +
> +void arch_pick_mmap_layout(struct mm_struct *mm, struct rlimit *rlim_stack)
> +{
> + unsigned long random_factor = 0UL;
> +
> + if (current->flags & PF_RANDOMIZE)
> + random_factor = arch_mmap_rnd();
> +
> + if (mmap_is_legacy(rlim_stack)) {
> + mm->mmap_base = TASK_UNMAPPED_BASE + random_factor;
> + mm->get_unmapped_area = arch_get_unmapped_area;
> + } else {
> + mm->mmap_base = mmap_base(random_factor, rlim_stack);
> + mm->get_unmapped_area = arch_get_unmapped_area_topdown;
> + }
> +}
> +#elif defined(CONFIG_MMU) && !defined(HAVE_ARCH_PICK_MMAP_LAYOUT)
> void arch_pick_mmap_layout(struct mm_struct *mm, struct rlimit *rlim_stack)
> {
> mm->mmap_base = TASK_UNMAPPED_BASE;
> --
> 2.20.1
>

--
Kees Cook

Next message: Oleksij Rempel: "[PATCH v2 2/3] MIPS: ath79: ar9331: add Ethernet nodes"
Previous message: Alex Ghiti: "Re: [PATCH v3 03/11] arm64: Consider stack randomization for mmap base only when necessary"
In reply to: Alexandre Ghiti: "[PATCH v3 04/11] arm64, mm: Move generic mmap layout functions to mm"
Next in thread: Alex Ghiti: "Re: [PATCH v3 04/11] arm64, mm: Move generic mmap layout functions to mm"
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