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    SubjectRe: Security fix for remapping of page 0 (was [PATCH] Change ZERO_SIZE_PTR to point at unmapped space)
    From
    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;
    >  }
    >
    >
    --
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