Avoiding programmer invariant violations (was: Re: FreeBSD Kernel buffer overflow)

[gerarra at tin.it]

>I'd suggest that we need to look at this in two ways:
>
>(1) There's a compile-time INVARIANT that needs to be maintained by
>    developers in adding new system calls.  When building the kernel, it
>    would be useful to have a compile-time assertion that causes a kernel
>    compile to fail if an invalid system call is defined.  I.e., when
>    init_sysent.c is generated, it should build in __CTASSERT's that all
>    argument counts are consistent with the requirements of the hardware
>    architecture being built for. 
>
>(2) There's a run-time INVARIANT issue for loadable modules built by third
>    parties who may not understand the limits on arguments on system calls
>    for various architectures.  This can be handled by a check in the
>    system call registration code, although since that's a non-critical
>    performance path, I suggest testing the invariant even if INVARIANTS
>    isn't compiled in.  In some ways, I'd rather handle this at
>    compile-time for the module, but I think the infrastructure for
>    hooking up system calls at compile-time for modules will make that
>    more difficult as compared to statically compiled system calls.
>

Completely agree

>Note that the discussion so far has not addressed the compile-time issue:
>
>which is a much better time to perform the tests -- it's something we can
>test when the kernel is compiled, so why not?.  It also hasn't addressed
>non-i386 systems, such as amd64, which have similar or identical concerns.

I was thinking exactly to it while coding patch, but I'm not so experienced
with SPARC and/or other architectures to do that

>With all due respect to the submitter, I think bugtraq was not the forum
>to post this issue to, as that forum is typically preferred for
>exploitable vulnerabilities.  A follow-up post to clarify that the initial
>post described a possible avenue for programmer error when extending the
>kernel, rather than an immediately exploitable vulnerability, might reduce
>confusion.

You're completely right again. I posted on bugtraq beacause somebody else
could get a good idea to break code, something I not thought...(so I post
this email in hackers@ to let other undestand mine wasn't a exploitable
bug report; nobody told "exploitable bug user -> root" or something like
that).
 

So what we I have to do? remove INVARIANTS dependency?

thanks,
rookie