FreeBSD Security Advisory FreeBSD-SA-03:09.signal

[Robert Watson]

On Sun, 10 Aug 2003, Jason Dambrosio wrote:

> > IV.  Workaround
> > 
> > There is no workaround for the local denial-of-service attack.
> 
>     Wouldn't a possible workaround be, to load a kld module that would
> replace the ptrace(2) system call with a patched one? I remember doing
> such a trick for modifying other system calls using kld modules... 

Yes; it should be fairly trivial to write a kernel module that modifies
the system call vector to wrap the current ptrace() and performs extra
run-time argument checking.  Off-hand, I don't remember if the ptrace() 
argument in question involves an extra copyin() -- if so, a competent
attacker could race the system call wrapper, but if not, it should be
pretty secure.  I was thinking about writing one while driving to work
today; I may get around to it this evening sometime, unless someone else
gets there first.  I know we support ptrace() in the Linux emulation on
-current (maybe also -stable) -- I'm not sure if you'd also need to wrap
that interface or not. 

Robert N M Watson             FreeBSD Core Team, TrustedBSD Projects
robert at fledge.watson.org      Network Associates Laboratories