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    SubjectRe: [PATCH] mm/mincore: allow for making sys_mincore() privileged
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    On 5.1.2019 20:24, Jiri Kosina wrote:
    > 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;

    I was thinking about "return true" here, assuming that userspace generally wants
    to ensure itself there won't be page faults when it starts doing something
    critical, and if it sees a "false" it will try to do some kind of prefaulting,
    possibly in a loop. There might be somebody trying to make sure something is out
    of pagecache (it wants to see "false"), but can't think of anything except
    benchmarks?

    > 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).
    >

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