On Wed, Jan 18, 2012 at 4:14 PM, Indan Zupancic <indan@xxxxxx> wrote: > On Wed, January 18, 2012 22:13, Chris Evans wrote: >> On Wed, Jan 18, 2012 at 4:12 AM, Indan Zupancic <indan@xxxxxx> wrote: >>> On Wed, January 18, 2012 06:43, Chris Evans wrote: >>>> 2) Tracee traps >>>> 2b) Tracee could take a SIGKILL here >>>> 3) Tracer looks at registers; bad syscall >>>> 3b) Or tracee could take a SIGKILL here >>>> 4) The only way to stop the bad syscall from executing is to rewrite >>>> orig_eax (PTRACE_CONT + SIGKILL only kills the process after the >>>> syscall has finished) >>> >>> Yes, we rewrite it to -1. >>> >>>> 5) Disaster: the tracee took a SIGKILL so any attempt to address it by >>>> pid (such as PTRACE_SETREGS) fails. >>> >>> I assume that if a task can execute system calls and we get ptrace events >>> for that, that we can do other ptrace operations too. Are you saying that >>> the kernel has this ptrace gap between SIGKILL and task exit where ptrace >>> doesn't work but the task continues executing system calls? That would be >>> a huge bug, but it seems very unlikely too, as the task is stopped and >>> shouldn't be able to disappear till it is continued by the tracer. >>> >>> I mean, really? That would be stupid. > > Okay, I tested this scenario and you're right, we're screwed. > > What the hell guys? Steady on :) ptrace() has never been sold as a technology upon which its safe to build security solutions. > What about other PID checks in the kernel, are they still > safe if the process looks dead but is still active? Or is it a ptrace-only > problem? > >>> If true we have to work around it by disallowing SIGKILL and just sending >>> them ourselves within the jail. Meh. > > I guess this helps a bit. It doesn't prevent external signals, but prisoners > don't have control over that. Well.... a prisoner may be able to play other tricks: - Allocate lots of memory... kernel may start spraying around SIGKILLs - Sending SIGKILL via prctl() - Sending SIGKILL via fcntl() - Sending SIGKILL via clone() > > Is this SIGKILL specific or is it true for all task ending signals? Can't remember - try it? > >>> How will you avoid file path races with BPF? >> >> There is typically no need for file-path based access control in an FTP server. >> Take for example anonymous FTP, which will typically be inside a >> chroot() to /var/ftp. Inside that filesystem tree -- if you can open() >> it, you can have it. > > Ah, you count on having root access. We don't. > > Do you know any more crazy security destroying holes? Try spraying SIGCONT and / or SIGSTOP at tracees. It may be possible to confuse the tracer about whether a SIGTRAP event is syscall entry or exit. Try doing an execve() that fails. May cause similar state confusion in the tracer. Cheers Chris > > Thanks, > > Indan > > -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
