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    SubjectRe: + c-r-prctl-add-ability-to-set-new-mm_struct-exe_file-update-after-mm- num_exe_file_vmas-removal.patch added to -mm tree
    On 04/19, Andrew Morton wrote:
    >
    > From: Konstantin Khlebnikov <khlebnikov@openvz.org>
    > Subject: c/r: prctl: update prctl_set_mm_exe_file() after mm->num_exe_file_vmas removal
    >
    > [ fix for "c-r-prctl-add-ability-to-set-new-mm_struct-exe_file-v2" from mm tree ]
    >
    > After removing mm->num_exe_file_vmas kernel keeps mm->exe_file until final
    > mmput(), it never becomes NULL while task is alive.
    >
    > We can check for other mapped files in mm instead of checking
    > mm->num_exe_file_vmas, and mark mm with flag MMF_EXE_FILE_CHANGED in order
    > to forbid second changing of mm->exe_file.

    I lost the track a long ago.

    Just one question, what does this "forbid second changing" actually mean?

    > * The symlink can be changed only once, just to disallow arbitrary
    > * transitions malicious software might bring in. This means one
    > * could make a snapshot over all processes running and monitor
    > * /proc/pid/exe changes to notice unusual activity if needed.
    > */
    > - down_write(&mm->mmap_sem);
    > - if (likely(!mm->exe_file))
    > - set_mm_exe_file(mm, exe_file);
    > - else
    > - err = -EBUSY;
    > + err = -EPERM;
    > + if (test_and_set_bit(MMF_EXE_FILE_CHANGED, &mm->flags))
    > + goto exit_unlock;
    > +
    > + set_mm_exe_file(mm, exe_file);
    > +exit_unlock:

    OK, I am not arguing, but looking at this code I suspect that you
    also want to forbid the second change after fork too?

    If yes, then you probably need to include MMF_EXE_FILE_CHANGED in
    MMF_INIT_MASK.

    But at the same time, then it should be probably cleared somewhere
    in bprm_mm_init() paths.

    Oleg.



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