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    SubjectRe: [PATCH] mm/mincore: allow for making sys_mincore() privileged
    On Sat, 5 Jan 2019, Vlastimil Babka wrote:

    > > There are possibilities [1] how mincore() could be used as a converyor of
    > > a sidechannel information about pagecache metadata.
    > >
    > > Provide vm.mincore_privileged sysctl, which makes it possible to mincore()
    > > start returning -EPERM in case it's invoked by a process lacking
    > > CAP_SYS_ADMIN.
    >
    > Haven't checked the details yet, but wouldn't it be safe if anonymous private
    > mincore() kept working, and restrictions were applied only to page cache?

    I was considering that, but then I decided not to do so, as that'd make
    the interface even more confusing and semantics non-obvious in the
    'privileged' case.

    > > The default behavior stays "mincore() can be used by anybody" in order to
    > > be conservative with respect to userspace behavior.
    >
    > What if we lied instead of returned -EPERM, to not break userspace so
    > obviously? I guess false positive would be the safer lie?

    So your proposal basically would be

    if (privileged && !CAP_SYS_ADMIN)
    if (pagecache)
    return false;
    else
    return do_mincore()

    right ?

    I think userspace would hate us for that semantics, but on the other hand
    I can sort of understand the 'mincore() is racy anyway, so what' argument,
    if that's what you are suggesting.

    But then, I have no idea what userspace is using mincore() for.
    https://codesearch.debian.net/search?q=mincore might provide some insight
    I guess (thanks Matthew).

    --
    Jiri Kosina
    SUSE Labs

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