Announcing security hardened kernels for testing
rob at codeweavers.com
Thu Jan 6 15:47:13 CST 2005
Mike Hearn wrote:
>On Thu, 2005-01-06 at 02:28 +0100, pageexec at freemail.hu wrote:
>>>c) Required to implement DCOM universal interface proxies
>>required as in 'cannot be implemented any other way'?
>I'm not sure. These proxies are run-time generated objects. Essentially
>a DCOM universal/typelib marshaller proxy is a COM interface (so an
>array of function pointers) that when called marshal the arguments into
>a packet and dispatch it via the Windows RPC infrastructure. These
>proxies come in three forms:
>- precompiled MIDL/C marshallers
>- precompiled MOPs (these are a custom bytecode language fed to a VM
> which does the marshalling)
>- generated at runtime from type libraries (databases which describe
> the types and interfaces used in a program)
>I don't know if the second needs runtime code geneation but I don't see
>any way we can avoid generating code for the third at runtime.
Actually, we shouldn't be generating assembly code on the fly. If you
have more than say 16 proxies in a process then it is actually cheaper
in terms of memory usage and cache locality to have a set of compiled
entry points that can be shared by all proxies. It is even better if you
consider the fact that we shouldn't be allocating the memory for the
code from the heap, but should be requesting an executable page of
memory for each.
Just for the record, PaX and execshield are trying to solve problems
that are much better solved by other methods that don't break backwards
compatibility. One of the best methods is introducing a terminator
canary value between the return address and variables stored on the
stack. Obviously, this requires compiler support (which GCC currently
lacks, I believe), but it has worked wonders for Microsoft in SP2. It
even prevents exploits that PaX/execshield can't, like "return to libc"
where the return address is overwritten by the address of another
function so that execution jumps into that function.
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