Date: Tue, 23 Jan 2018 09:01:01 +0100 From: Ingo Molnar <mingo@...nel.org> To: Linus Torvalds <torvalds@...ux-foundation.org> Cc: Andy Lutomirski <luto@...nel.org>, the arch/x86 maintainers <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, Alan Cox <alan@...ux.intel.com>, Jann Horn <jannh@...gle.com>, Samuel Neves <samuel.c.p.neves@...il.com>, Dan Williams <dan.j.williams@...el.com>, Kernel Hardening <kernel-hardening@...ts.openwall.com>, Borislav Petkov <bp@...en8.de> Subject: Re: [PATCH] x86/retpoline/entry: Disable the entire SYSCALL64 fast path with retpolines on * Linus Torvalds <torvalds@...ux-foundation.org> wrote: > On Mon, Jan 22, 2018 at 10:04 AM, Andy Lutomirski <luto@...nel.org> wrote: > > The existing retpoline code carefully and awkwardly retpolinifies > > the SYSCALL64 slow path. This stops the fast path from being > > particularly fast, and it's IMO rather messy. > > I'm not convinced your patch isn't messier still.. It's certainly > subtle. I had to look at that ptregs stub generator thing twice. > > Honestly, I'd rather get rid of the fast-path entirely. Compared to > all the PTI mess, it's not even noticeable. > > And if we ever get CPU's that have this all fixed, we can re-visit > introducing the fastpath. But this is all very messy and it doesn't > seem worth it right now. > > If we get rid of the fastpath, we can lay out the slow path slightly > better, and get rid of some of those jump-overs. And we'd get rid of > the ptregs hooks entirely. > > So we can try to make the "slow" path better while at it, but I really > don't think it matters much now in the post-PTI era. Sadly. Note that there's another advantage to your proposal: should other vulnerabilities arise in the future, requiring changes in the syscall entry path, we'd be more flexible to address them in the C space than in the assembly space. In hindsight a _LOT_ of the PTI complexity and fragility centered around interacting with x86 kernel entry assembly code - which entry code fortunately got much simpler (and easier to review) in the past 1-2 years due to the thorough cleanups and the conversion of most of it to C. But it was still painful. So I'm fully in favor of that. Thanks, Ingo
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