On Sat, 2003-05-10 at 16:38, Ahmed Masud wrote:

> Case in point, I wrote a security module for Linux that overrides _all_
> 237 systemcalls to audit and control the use of the system calls on a per
> uid basis.  (i.e. if the user was actually allowed to make the system call
> or not) and return -EPERM or jump to system call proper.

I'm pretty sure that auditing by your module can easily be avoided.

examle: pseudocode for the unlink syscall

long your_wrapped_syscall(char *userfilename)
{
    char kernelpointer[something];
    copy_from_user(kernelpointer, usefilename, ...);
    audit_log(kernelpointer);
    return original_syscall(userfilename);
}

now.... the original syscall does ANOTHER copy_from_user().
Eg I can easily fool your logging by having a second thread change the
filename between the time your code copies it and the time the original
syscall copies it again. The chances of getting the timing right are 50%
at least (been there done that ;)

The only solution for this is to check/audit/log things after the ONE
copy. Eg not by overriding the syscall but inside the syscall.