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    SubjectRe: [PATCH v3 0/7] User namespace mount updates
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    On Tue, Nov 17, 2015 at 07:30:12PM +0000, Al Viro wrote:
    > On Tue, Nov 17, 2015 at 02:02:09PM -0500, Austin S Hemmelgarn wrote:
    >
    > > >_Static_ attacks, or change-image-under-mounted-fs attacks?
    > > To properly protect against attacks on mounted filesystems, we'd
    > > need some new concept of a userspace immutable file (that is, one
    > > where nobody can write to it except the kernel, and only the kernel
    > > can change it between regular access and this new state), and then
    > > have the kernel set an image (or block device) to this state when a
    > > filesystem is mounted from it (this introduces all kinds of other
    > > issues too however, for example stuff that allows an online fsck on
    > > the device will stop working, as will many un-deletion tools).
    > >
    > > The only other option would be to force the FS to cache all metadata
    > > in memory, and validate between the cache and what's on disk on
    > > every access, which is not realistic for any real world system.
    >
    > Doctor, it hurt when I do it...
    >
    > IOW, the other option is to refuse attempting this insanity. Fuse probably
    > can be handled, but being able to mount (with kernel-space drivera) an
    > arbitrary ext4 image is equivalent to being able to do anything and it's
    > going to stay that way for the forseeable future.

    What about the filesystems that desktop users commonly mount? (fat,
    isofs, udf?)

    --b.


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