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    Subject[patch 24/28] mm_for_maps: simplify, use ptrace_may_access()
    2.6.27-stable review patch.  If anyone has any objections, please let us know.

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

    From: Oleg Nesterov <oleg@redhat.com>

    commit 13f0feafa6b8aead57a2a328e2fca6a5828bf286 upstream.

    It would be nice to kill __ptrace_may_access(). It requires task_lock(),
    but this lock is only needed to read mm->flags in the middle.

    Convert mm_for_maps() to use ptrace_may_access(), this also simplifies
    the code a little bit.

    Also, we do not need to take ->mmap_sem in advance. In fact I think
    mm_for_maps() should not play with ->mmap_sem at all, the caller should
    take this lock.

    With or without this patch, without ->cred_guard_mutex held we can race
    with exec() and get the new ->mm but check old creds.

    Signed-off-by: Oleg Nesterov <oleg@redhat.com>
    Reviewed-by: Serge Hallyn <serue@us.ibm.com>
    Signed-off-by: James Morris <jmorris@namei.org>
    Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

    ---
    fs/proc/base.c | 23 +++++++++++------------
    1 file changed, 11 insertions(+), 12 deletions(-)

    --- a/fs/proc/base.c
    +++ b/fs/proc/base.c
    @@ -242,20 +242,19 @@ struct mm_struct *mm_for_maps(struct tas
    struct mm_struct *mm = get_task_mm(task);
    if (!mm)
    return NULL;
    + if (mm != current->mm) {
    + /*
    + * task->mm can be changed before security check,
    + * in that case we must notice the change after.
    + */
    + if (!ptrace_may_access(task, PTRACE_MODE_READ) ||
    + mm != task->mm) {
    + mmput(mm);
    + return NULL;
    + }
    + }
    down_read(&mm->mmap_sem);
    - task_lock(task);
    - if (task->mm != mm)
    - goto out;
    - if (task->mm != current->mm &&
    - __ptrace_may_access(task, PTRACE_MODE_READ) < 0)
    - goto out;
    - task_unlock(task);
    return mm;
    -out:
    - task_unlock(task);
    - up_read(&mm->mmap_sem);
    - mmput(mm);
    - return NULL;
    }

    static int proc_pid_cmdline(struct task_struct *task, char * buffer)



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