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    Subject[PATCH 4.9 10/26] prctl: allow to setup brk for et_dyn executables
    Date
    From: Cyrill Gorcunov <gorcunov@gmail.com>

    commit e1fbbd073137a9d63279f6bf363151a938347640 upstream.

    Keno Fischer reported that when a binray loaded via ld-linux-x the
    prctl(PR_SET_MM_MAP) doesn't allow to setup brk value because it lays
    before mm:end_data.

    For example a test program shows

    | # ~/t
    |
    | start_code 401000
    | end_code 401a15
    | start_stack 7ffce4577dd0
    | start_data 403e10
    | end_data 40408c
    | start_brk b5b000
    | sbrk(0) b5b000

    and when executed via ld-linux

    | # /lib64/ld-linux-x86-64.so.2 ~/t
    |
    | start_code 7fc25b0a4000
    | end_code 7fc25b0c4524
    | start_stack 7fffcc6b2400
    | start_data 7fc25b0ce4c0
    | end_data 7fc25b0cff98
    | start_brk 55555710c000
    | sbrk(0) 55555710c000

    This of course prevent criu from restoring such programs. Looking into
    how kernel operates with brk/start_brk inside brk() syscall I don't see
    any problem if we allow to setup brk/start_brk without checking for
    end_data. Even if someone pass some weird address here on a purpose then
    the worst possible result will be an unexpected unmapping of existing vma
    (own vma, since prctl works with the callers memory) but test for
    RLIMIT_DATA is still valid and a user won't be able to gain more memory in
    case of expanding VMAs via new values shipped with prctl call.

    Link: https://lkml.kernel.org/r/20210121221207.GB2174@grain
    Fixes: bbdc6076d2e5 ("binfmt_elf: move brk out of mmap when doing direct loader exec")
    Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>
    Reported-by: Keno Fischer <keno@juliacomputing.com>
    Acked-by: Andrey Vagin <avagin@gmail.com>
    Tested-by: Andrey Vagin <avagin@gmail.com>
    Cc: Dmitry Safonov <0x7f454c46@gmail.com>
    Cc: Kirill Tkhai <ktkhai@virtuozzo.com>
    Cc: Eric W. Biederman <ebiederm@xmission.com>
    Cc: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
    Cc: Alexander Mikhalitsyn <alexander.mikhalitsyn@virtuozzo.com>
    Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    kernel/sys.c | 7 -------
    1 file changed, 7 deletions(-)

    --- a/kernel/sys.c
    +++ b/kernel/sys.c
    @@ -1775,13 +1775,6 @@ static int validate_prctl_map(struct prc
    error = -EINVAL;

    /*
    - * @brk should be after @end_data in traditional maps.
    - */
    - if (prctl_map->start_brk <= prctl_map->end_data ||
    - prctl_map->brk <= prctl_map->end_data)
    - goto out;
    -
    - /*
    * Neither we should allow to override limits if they set.
    */
    if (check_data_rlimit(rlimit(RLIMIT_DATA), prctl_map->brk,

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