On Thu, January 19, 2012 02:19, Andrew Lutomirski wrote: > On Wed, Jan 18, 2012 at 5:06 PM, Indan Zupancic <indan@xxxxxx> wrote: >> On Wed, January 18, 2012 02:01, Andrew Lutomirski wrote: >>> On Tue, Jan 17, 2012 at 4:56 PM, Indan Zupancic <indan@xxxxxx> wrote: >>> I don't know what your ptrace jailer does. But a task can switch >>> itself between 32-bit and 64-bit execution at will, and there's >>> nothing the kernel can do about it. (That isn't quite true -- in >>> theory the kernel could fiddle with the GDT, but that would be >>> expensive and wouldn't work on Xen.) >> >> That's why we don't cache the CS value but check it for every system call. >> But you said elsewhere that checking CS isn't always correct either. >> I grepped arch/x86 for "user_64bit_mode", but couldn't find anything, >> but maybe my kernel sources are too old, I haven't updated this system >> for almost a year. The current code only handles 0x23 and 0x33 and kills >> the jail if it encounters anything else. > > I think you're hosed on Xen, then. Xen regularly runs with a > different Xen-specific cs value. That's fine as long as a cs value of 0x23 or 0x33 gives reliable information. Not running is highly prefered above running insecurely. Security first, functionality second. Greetings, 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
