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    SubjectRe: [RFC] c/r: prctl: Add ability to set new mm_struct::exe_file v3
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    On Thu, Mar 8, 2012 at 8:51 AM, Cyrill Gorcunov <gorcunov@openvz.org> wrote:
    > When we do restore we would like to have a way to setup
    > a former mm_struct::exe_file so that /proc/pid/exe would
    > point to the original executable file a process had at
    > checkpoint time.
    >
    > For this the PR_SET_MM_EXE_FILE code is introduced.
    > This option takes a file descriptor which will be
    > set as new /proc/$pid/exe symlink.
    >
    > This feature is available iif CONFIG_CHECKPOINT_RESTORE is set.
    > [...]
    > Index: linux-2.6.git/kernel/sys.c
    > ===================================================================
    > --- linux-2.6.git.orig/kernel/sys.c
    > +++ linux-2.6.git/kernel/sys.c
    > [...]
    > +       exe_file = fget(fd);
    > +       if (!exe_file)
    > +               return -EBADF;
    > +
    > +       dentry = exe_file->f_path.dentry;
    > +
    > +       /*
    > +        * Because the original mm->exe_file
    > +        * points to executable file, make sure
    > +        * this one is executable as well to not
    > +        * break "big" picture and proc/pid/exe
    > +        * symlink will be still pointing to
    > +        * executable one.
    > +        */
    > +       err = -EACCES;
    > +       if (!S_ISREG(dentry->d_inode->i_mode)   ||
    > +           exe_file->f_path.mnt->mnt_flags & MNT_NOEXEC)
    > +               goto exit;

    I'm starting to notice that this pattern (testing ISREG and
    MNT_NOEXEC) is getting repeated a few times in the kernel, and at
    least the no-new-privs patch (not yet in -mm but hopefully soon given
    the seccomp_filter work) updates this pattern everywhere. Perhaps this
    should be extracted into a helper first, and then this patch can call
    that helper here? (And then nnp can just update the single helper.)

    -Kees

    --
    Kees Cook
    ChromeOS Security
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