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    Subject[PATCH 3.16 102/305] dm ioctl: harden copy_params()'s copy_from_user() from malicious users
    3.16.63-rc1 review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Wenwen Wang <wang6495@umn.edu>

    commit 800a7340ab7dd667edf95e74d8e4f23a17e87076 upstream.

    In copy_params(), the struct 'dm_ioctl' is first copied from the user
    space buffer 'user' to 'param_kernel' and the field 'data_size' is
    checked against 'minimum_data_size' (size of 'struct dm_ioctl' payload
    up to its 'data' member). If the check fails, an error code EINVAL will be
    returned. Otherwise, param_kernel->data_size is used to do a second copy,
    which copies from the same user-space buffer to 'dmi'. After the second
    copy, only 'dmi->data_size' is checked against 'param_kernel->data_size'.
    Given that the buffer 'user' resides in the user space, a malicious
    user-space process can race to change the content in the buffer between
    the two copies. This way, the attacker can inject inconsistent data
    into 'dmi' (versus previously validated 'param_kernel').

    Fix redundant copying of 'minimum_data_size' from user-space buffer by
    using the first copy stored in 'param_kernel'. Also remove the
    'data_size' check after the second copy because it is now unnecessary.

    Signed-off-by: Wenwen Wang <wang6495@umn.edu>
    Signed-off-by: Mike Snitzer <snitzer@redhat.com>
    Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
    ---
    drivers/md/dm-ioctl.c | 18 ++++++------------
    1 file changed, 6 insertions(+), 12 deletions(-)

    --- a/drivers/md/dm-ioctl.c
    +++ b/drivers/md/dm-ioctl.c
    @@ -1681,8 +1681,7 @@ static void free_params(struct dm_ioctl
    }

    static int copy_params(struct dm_ioctl __user *user, struct dm_ioctl *param_kernel,
    - int ioctl_flags,
    - struct dm_ioctl **param, int *param_flags)
    + int ioctl_flags, struct dm_ioctl **param, int *param_flags)
    {
    struct dm_ioctl *dmi;
    int secure_data;
    @@ -1730,18 +1729,13 @@ static int copy_params(struct dm_ioctl _
    return -ENOMEM;
    }

    - if (copy_from_user(dmi, user, param_kernel->data_size))
    - goto bad;
    + /* Copy from param_kernel (which was already copied from user) */
    + memcpy(dmi, param_kernel, minimum_data_size);

    -data_copied:
    - /*
    - * Abort if something changed the ioctl data while it was being copied.
    - */
    - if (dmi->data_size != param_kernel->data_size) {
    - DMERR("rejecting ioctl: data size modified while processing parameters");
    + if (copy_from_user(&dmi->data, (char __user *)user + minimum_data_size,
    + param_kernel->data_size - minimum_data_size))
    goto bad;
    - }
    -
    +data_copied:
    /* Wipe the user buffer so we do not return it to userspace */
    if (secure_data && clear_user(user, param_kernel->data_size))
    goto bad;
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