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    Subject[2.6.20.17 review 22/58] Fix user struct leakage with locked IPC shem segment
    When user locks an ipc shmem segmant with SHM_LOCK ctl and the segment is
    already locked the shmem_lock() function returns 0. After this the
    subsequent code leaks the existing user struct:

    == ipc/shm.c: sys_shmctl() ==
    ...
    err = shmem_lock(shp->shm_file, 1, user);
    if (!err) {
    shp->shm_perm.mode |= SHM_LOCKED;
    shp->mlock_user = user;
    }
    ...
    ==

    Other results of this are:
    1. the new shp->mlock_user is not get-ed and will point to freed
    memory when the task dies.
    2. the RLIMIT_MEMLOCK is screwed on both user structs.

    The exploit looks like this:

    ==
    id = shmget(...);
    setresuid(uid, 0, 0);
    shmctl(id, SHM_LOCK, NULL);
    setresuid(uid + 1, 0, 0);
    shmctl(id, SHM_LOCK, NULL);
    ==

    My solution is to return 0 to the userspace and do not change the
    segment's user.

    Signed-off-by: Pavel Emelianov <xemul@openvz.org>
    Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
    Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
    Signed-off-by: Willy Tarreau <w@1wt.eu>
    ---
    ipc/shm.c | 2 +-
    1 files changed, 1 insertions(+), 1 deletions(-)

    diff --git a/ipc/shm.c b/ipc/shm.c
    index f8e10a2..10b7a2c 100644
    --- a/ipc/shm.c
    +++ b/ipc/shm.c
    @@ -652,7 +652,7 @@ asmlinkage long sys_shmctl (int shmid, int cmd, struct shmid_ds __user *buf)
    struct user_struct * user = current->user;
    if (!is_file_hugepages(shp->shm_file)) {
    err = shmem_lock(shp->shm_file, 1, user);
    - if (!err) {
    + if (!err && !(shp->shm_perm.mode & SHM_LOCKED)){
    shp->shm_perm.mode |= SHM_LOCKED;
    shp->mlock_user = user;
    }
    --
    1.5.2.5
    --
    -
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