On Sat, Jul 25, 2015 at 10:42:14AM -0700, Andy Lutomirski wrote: > On Sat, Jul 25, 2015 at 9:33 AM, Willy Tarreau <w...@1wt.eu> wrote: > > On Sat, Jul 25, 2015 at 09:08:39AM -0700, Andy Lutomirski wrote: > >> There's one thing that I think is incomplete here. Currently, espfix > >> triggers if SS points to the LDT. It's possible for SS to point to > >> the LDT even with modify_ldt disabled, and there's a decent amount of > >> attack surface there. > >> > >> Can we improve this? Two ideas: > >> > >> 1. In the asm, patch out or otherwise disable espfix if that sysctl > >> has never been set. (Ick.) > >> > >> 2. When modify_ldt is runtime-disabled (or compile-time disabled, > >> perhaps), disallow setting the LDT bit in SS in the handful of places > >> that would allow it (ptrace and sigreturn off the top of my head). We > >> don't need to worry about (regs->ss & 4) being set on kernel entry > >> because we'll never be in user mode with that bit set if the LDT is > >> disabled, but that bit could still be set using kernel APIs. (In > >> fact, my sigreturn test does exactly that.) > >> > >> Hmm. With synchronous LDT, we could plausibly check at runtime in the > >> espfix code, too. We used to use LAR to do this, but hpa removed it > >> when he realized that it was racy. It shouldn't be racy any more, > >> because, with my patches applied, the LDT never changes while > >> interrupts are off. > > > > I understand it's not complete but I'm a bit bothered with conflating > > this sysctl with other setting methods, because if the purpose of the > > sysctl is to disable the syscall, it should do that only. I'd rather > > document that it's less complete than the Kconfig method and continue > > to recommend using your option whenever possible (eg: all my kernels > > will use it just as I've already disabled X86_16BIT everywhere). > > > > Agreed. We can certainly tighten up the espfix code later. > > > Also one benefit of having both options is that it will mechanically > > make LDT a much less interesting target for future attacks, since it > > will significantly reduce the likeliness of success, hence the motivation > > for writing exploits that only work in conferences. > > > > Patch looks fine to me.
OK thanks. Willy -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
