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    SubjectRe: [PATCH] Make /proc/slabinfo 0400
    On Fri, 2011-03-04 at 22:02 +0200, Pekka Enberg wrote:
    > On Fri, Mar 4, 2011 at 8:14 PM, Matt Mackall <> wrote:
    > >> Of course, as you say, '/proc/meminfo' still does give you the trigger
    > >> for "oh, now somebody actually allocated a new page". That's totally
    > >> independent of slabinfo, though (and knowing the number of active
    > >> slabs would neither help nor hurt somebody who uses meminfo - you
    > >> might as well allocate new sockets in a loop, and use _only_ meminfo
    > >> to see when that allocated a new page).
    > >
    > > I think lying to the user is much worse than changing the permissions.
    > > The cost of the resulting confusion is WAY higher.
    > Yeah, maybe. I've attached a proof of concept patch that attempts to
    > randomize object layout in individual slabs. I'm don't completely
    > understand the attack vector so I don't make any claims if the patch
    > helps or not.
    > Pekka

    Thanks for your work on this. The most general exploitation techniques
    involving kernel SLUB/SLAB corruption involve manipulating heap state
    such that an object that can be overflowed by the attacker resides
    immediately before another object whose contents are worth overwriting,
    or overflowing into the page following the slab. The most common known
    techniques involve overflowing into an allocated object with useful
    contents such as a function pointer and then triggering these (various
    IPC-related structs are often used for this). It's also possible to
    overflow into a free object and overwrite its free pointer, causing
    subsequent allocations to result in a fake heap object residing in
    userland being under an attacker's control.

    This patch makes these techniques more difficult by making it hard to
    know whether the last attacker-allocated object resides before a free or
    allocated object. Especially with vulnerabilities that only allow one
    attempt at exploitation before recovery is needed to avoid trashing too
    much heap state and causing a crash, this could go a long way. I'd
    still argue in favor of removing the ability to know how many objects
    are used in a given slab, since randomizing objects doesn't help if you
    know every object is allocated.

    Of course people more knowledgeable on SLUB should look this over for
    sanity's sake, but it looks good to me.


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