Messages in this thread | | | Date | 22 Feb 2008 13:55:01 +0100 | Date | Fri, 22 Feb 2008 13:55:00 +0100 | From | Andi Kleen <> | Subject | Re: arch/x86/kernel/vsyscall_64.c: overeager NOP of syscalls |
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On Thu, Feb 21, 2008 at 09:59:24PM +0100, Thomas Gleixner wrote: > On Thu, 21 Feb 2008, Thomas Gleixner wrote: > > > That or we need to do the NOP/un-NOP part in the update_vsyscall code > > > dependent on if the current clocksource supports vread, instead of on > > > the /proc entry access. > > > > That won't fly. We need to sychronize the CPUs when we patch the code, > > which is not possible from update_wall_time with interrupts disabled. > > Also this is utterly stupid as we keep the syscall in cases where we > do not have vread anyway, so we keep the attack point open for a lot > of existing machines due to TSC wreckage and HPET unavailability.
Yes that is true, but I didn't find a cheap way around it without breaking binary compatibility.
I found an expensive way (essentially just putting a trampoline to a variable mapped vsyscall page on the old static address), but since it would have added quite a lot of complexity (vsyscall is inside the kernel mapping and would need to be rewritten at context switch) and memory overhead in page tables and one more page per process I didn't do that.
I also considered doing boot time randomization only (which would avoid a lot of these problems), but it didn't seem worth it.
Also one must say I don't consider it a big security improvement on most systems. That is because most programs are not PIC, but linked at a fixed address and usually if you grep the larger binary for the few bytes needed for a system call you'll find it at some known offset as part of another instruction sequence.
-Andi
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