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    SubjectRe: [PATCH] Make /proc/slabinfo 0400
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    On Sun, 2011-03-06 at 13:19 +0000, Alan Cox wrote:
    > > If we had wrappers for the most common cases, then any cases that were
    > > left that used copy_from_user() explicitly could be flagged and
    > > checked by hand, since they would be exception, and not the rule.
    >
    > Arjan's copy_from_user validation code already does verification checks
    > on the copies using gcc magic.
    >
    > Some of the others might be useful - kmalloc_from_user() is a fairly
    > obvious interface, a copy_from_user_into() interface where you pass
    > the destination object and its actual length as well is mostly covered by
    > Arjan's stuff.
    >
    > Alan

    This is all worthwhile discussion, and a good implementation of these
    kinds of features is available as part of grsecurity (PAX_USERCOPY) - it
    provides additional bounds-checking for copy operations into both heap
    and stack buffers. Rather than reinventing the wheel, perhaps it would
    be a better use of time to extract this patch and make it suitable for
    inclusion.

    In the meantime, I'd like to get back to the original patch
    (make /proc/slabinfo 0400), and the subsequent followup patch (randomize
    free objects within a slab). While it's clear that these patches by
    themselves will not entirely prevent kernel heap exploits, they both
    seem to be sane improvements, won't significantly impact performance,
    and shouldn't be more than a very minor inconvenience to some small
    subset of normal users. In addition, the absence of these changes might
    undermine future hardening improvements (e.g. with a more hardened heap,
    the readability of /proc/slabinfo may be more necessary for successful
    exploitation).

    -Dan



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