Re: Security fix for remapping of page 0 (was [PATCH] Change ZERO_SIZE_PTR to point at unmapped space)

Eric Paris
2009-06-03T20:16:04Z

On Wed, Jun 3, 2009 at 4:04 PM, Christoph Lameter
<cl@linux-foundation.org> wrote:
> On Wed, 3 Jun 2009, Eric Paris wrote:
>
>> The 'right'est fix is as Alan suggested, duplicate the code
>>
>> from security/capability.c::cap_file_mmap()
>> to include/linux/security.h::securitry_file_mmap()
>
> Thats easy to do but isnt it a bit weird now to configure mmap_min_addr?

??

> A security model may give it a different interpretation?

Not sure what you mean. Yes, each security model is allowed to decide
what permissions are needed to pass a given security check. SELinux
decided that CAP_SYS_RAWIO was not needed, but the selinux permission
mmap_zero was. Had there been a more specific capability to use
SELinux might have been happy using a capability...

> What about round_hint_to_min()?

not sure what you mean....

>
> Use mmap_min_addr indepedently of security models
>
> This patch removes the dependency of mmap_min_addr on CONFIG_SECURITY.
> It also sets a default mmap_min_addr of 4096.
>
> mmapping of addresses below 4096 will only be possible for processes
> with CAP_SYS_RAWIO.

<pedantic nit> "or the appropriate permission for the given LSM </pedantic nit>

> Signed-off-by: Christoph Lameter <cl@linux-foundation.org>

Clearly lots more cleanup can be done between CONFIG_SECURITY and
!CONFIG_SECURITY like Linus suggested, but

Acked-by: Eric Paris <eparis@redhat.com>

> ---
>  include/linux/mm.h       |    2 --
>  include/linux/security.h |    2 ++
>  kernel/sysctl.c          |    2 --
>  mm/Kconfig               |   19 +++++++++++++++++++
>  mm/mmap.c                |    3 +++
>  security/Kconfig         |   20 --------------------
>  security/security.c      |    3 ---
>  7 files changed, 24 insertions(+), 27 deletions(-)
>
> Index: linux-2.6/include/linux/mm.h
> ===================================================================
> --- linux-2.6.orig/include/linux/mm.h   2009-06-03 15:00:54.000000000 -0500
> +++ linux-2.6/include/linux/mm.h        2009-06-03 15:00:56.000000000 -0500
> @@ -580,12 +580,10 @@ static inline void set_page_links(struct
>  */
>  static inline unsigned long round_hint_to_min(unsigned long hint)
>  {
> -#ifdef CONFIG_SECURITY
>        hint &= PAGE_MASK;
>        if (((void *)hint != NULL) &&
>            (hint < mmap_min_addr))
>                return PAGE_ALIGN(mmap_min_addr);
> -#endif
>        return hint;
>  }
>
> Index: linux-2.6/kernel/sysctl.c
> ===================================================================
> --- linux-2.6.orig/kernel/sysctl.c      2009-06-03 15:00:54.000000000 -0500
> +++ linux-2.6/kernel/sysctl.c   2009-06-03 15:00:56.000000000 -0500
> @@ -1225,7 +1225,6 @@ static struct ctl_table vm_table[] = {
>                .strategy       = &sysctl_jiffies,
>        },
>  #endif
> -#ifdef CONFIG_SECURITY
>        {
>                .ctl_name       = CTL_UNNUMBERED,
>                .procname       = "mmap_min_addr",
> @@ -1234,7 +1233,6 @@ static struct ctl_table vm_table[] = {
>                .mode           = 0644,
>                .proc_handler   = &proc_doulongvec_minmax,
>        },
> -#endif
>  #ifdef CONFIG_NUMA
>        {
>                .ctl_name       = CTL_UNNUMBERED,
> Index: linux-2.6/mm/mmap.c
> ===================================================================
> --- linux-2.6.orig/mm/mmap.c    2009-06-03 15:00:54.000000000 -0500
> +++ linux-2.6/mm/mmap.c 2009-06-03 15:01:18.000000000 -0500
> @@ -87,6 +87,9 @@ int sysctl_overcommit_ratio = 50;     /* def
>  int sysctl_max_map_count __read_mostly = DEFAULT_MAX_MAP_COUNT;
>  struct percpu_counter vm_committed_as;
>
> +/* amount of vm to protect from userspace access */
> +unsigned long mmap_min_addr = CONFIG_DEFAULT_MMAP_MIN_ADDR;
> +
>  /*
>  * Check that a process has enough memory to allocate a new virtual
>  * mapping. 0 means there is enough memory for the allocation to
> Index: linux-2.6/security/security.c
> ===================================================================
> --- linux-2.6.orig/security/security.c  2009-06-03 15:00:54.000000000 -0500
> +++ linux-2.6/security/security.c       2009-06-03 15:00:56.000000000 -0500
> @@ -26,9 +26,6 @@ extern void security_fixup_ops(struct se
>
>  struct security_operations *security_ops;      /* Initialized to NULL */
>
> -/* amount of vm to protect from userspace access */
> -unsigned long mmap_min_addr = CONFIG_SECURITY_DEFAULT_MMAP_MIN_ADDR;
> -
>  static inline int verify(struct security_operations *ops)
>  {
>        /* verify the security_operations structure exists */
> Index: linux-2.6/mm/Kconfig
> ===================================================================
> --- linux-2.6.orig/mm/Kconfig   2009-06-03 15:00:54.000000000 -0500
> +++ linux-2.6/mm/Kconfig        2009-06-03 15:00:56.000000000 -0500
> @@ -226,6 +226,25 @@ config HAVE_MLOCKED_PAGE_BIT
>  config MMU_NOTIFIER
>        bool
>
> +config DEFAULT_MMAP_MIN_ADDR
> +        int "Low address space to protect from user allocation"
> +        default 4096
> +        help
> +         This is the portion of low virtual memory which should be protected
> +         from userspace allocation.  Keeping a user from writing to low pages
> +         can help reduce the impact of kernel NULL pointer bugs.
> +
> +         For most ia64, ppc64 and x86 users with lots of address space
> +         a value of 65536 is reasonable and should cause no problems.
> +         On arm and other archs it should not be higher than 32768.
> +         Programs which use vm86 functionality would either need additional
> +         permissions from either the LSM or the capabilities module or have
> +         this protection disabled.
> +
> +         This value can be changed after boot using the
> +         /proc/sys/vm/mmap_min_addr tunable.
> +
> +
>  config NOMMU_INITIAL_TRIM_EXCESS
>        int "Turn on mmap() excess space trimming before booting"
>        depends on !MMU
> Index: linux-2.6/security/Kconfig
> ===================================================================
> --- linux-2.6.orig/security/Kconfig     2009-06-03 15:00:54.000000000 -0500
> +++ linux-2.6/security/Kconfig  2009-06-03 15:00:56.000000000 -0500
> @@ -113,26 +113,6 @@ config SECURITY_ROOTPLUG
>
>          If you are unsure how to answer this question, answer N.
>
> -config SECURITY_DEFAULT_MMAP_MIN_ADDR
> -        int "Low address space to protect from user allocation"
> -        depends on SECURITY
> -        default 0
> -        help
> -         This is the portion of low virtual memory which should be protected
> -         from userspace allocation.  Keeping a user from writing to low pages
> -         can help reduce the impact of kernel NULL pointer bugs.
> -
> -         For most ia64, ppc64 and x86 users with lots of address space
> -         a value of 65536 is reasonable and should cause no problems.
> -         On arm and other archs it should not be higher than 32768.
> -         Programs which use vm86 functionality would either need additional
> -         permissions from either the LSM or the capabilities module or have
> -         this protection disabled.
> -
> -         This value can be changed after boot using the
> -         /proc/sys/vm/mmap_min_addr tunable.
> -
> -
>  source security/selinux/Kconfig
>  source security/smack/Kconfig
>  source security/tomoyo/Kconfig
> Index: linux-2.6/include/linux/security.h
> ===================================================================
> --- linux-2.6.orig/include/linux/security.h     2009-06-03 15:01:28.000000000 -0500
> +++ linux-2.6/include/linux/security.h  2009-06-03 15:01:42.000000000 -0500
> @@ -2197,6 +2197,8 @@ static inline int security_file_mmap(str
>                                     unsigned long addr,
>                                     unsigned long addr_only)
>  {
> +       if ((addr < mmap_min_addr) && !capable(CAP_SYS_RAWIO))
> +               return -EACCES;
>        return 0;
>  }