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    SubjectRe: [PATCH 04/24] VFS: Add LSM hooks for filesystem context [ver #7]
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    On 04/24/2018 11:22 AM, David Howells wrote:
    > Stephen Smalley <sds@tycho.nsa.gov> wrote:
    >
    >> Neither fsopen() nor fscontext_fs_write() appear to perform any kind of
    >> up-front permission checking (DAC or MAC), although some security hooks may
    >> be ultimately called to allocate structures, parse security options, etc.
    >> Is there a reason not apply a may_mount() or similar check up front?
    >
    > may_mount() is called by fsmount() at the moment. It may make sense to move
    > this earlier to fsopen(). Note that there's also going to be something that
    > looks like:
    >
    > fd = fspick("/mnt");
    > fsmount(fd, "/a", MNT_NOEXEC); // ie. bind mount
    >
    > or:
    >
    > fd = fspick("/mnt");
    > write(fd, "o intr");
    > write(fd, "x reconfigure"); // ie. something like remount
    > close(fd);
    >
    > I guess we'd want to call may_mount() in fspick() too. But there's also the
    > possibility of using this to create a query interfact too:
    >
    > fd = fspick("/mnt");
    > write(fd, "q intr");
    > read(fd, value_buffer);

    My concern was that fsopen()/fscontext_fs_write() may expose attack surface (e.g. mount option parsing code) that might not be normally accessible to unprivileged userspace (i.e. gated by may_mount() and security_sb_mount()) prior to your changes.


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