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Date: Thu, 10 Jan 2019 15:40:04 -0800
From: Dave Hansen <dave.hansen@...el.com>
To: Khalid Aziz <khalid.aziz@...cle.com>, juergh@...il.com, tycho@...ho.ws,
jsteckli@...zon.de, ak@...ux.intel.com, torvalds@...ux-foundation.org,
liran.alon@...cle.com, keescook@...gle.com, konrad.wilk@...cle.com
Cc: deepa.srinivasan@...cle.com, chris.hyser@...cle.com,
tyhicks@...onical.com, dwmw@...zon.co.uk, andrew.cooper3@...rix.com,
jcm@...hat.com, boris.ostrovsky@...cle.com, kanth.ghatraju@...cle.com,
joao.m.martins@...cle.com, jmattson@...gle.com, pradeep.vincent@...cle.com,
john.haxby@...cle.com, tglx@...utronix.de, kirill.shutemov@...ux.intel.com,
hch@....de, steven.sistare@...cle.com, kernel-hardening@...ts.openwall.com,
linux-mm@...ck.org, linux-kernel@...r.kernel.org,
Andy Lutomirski <luto@...nel.org>, Peter Zijlstra <peterz@...radead.org>
Subject: Re: [RFC PATCH v7 00/16] Add support for eXclusive Page Frame
Ownership
First of all, thanks for picking this back up. It looks to be going in
a very positive direction!
On 1/10/19 1:09 PM, Khalid Aziz wrote:
> I implemented a solution to reduce performance penalty and
> that has had large impact. When XPFO code flushes stale TLB entries,
> it does so for all CPUs on the system which may include CPUs that
> may not have any matching TLB entries or may never be scheduled to
> run the userspace task causing TLB flush.
...
> A rogue process can launch a ret2dir attack only from a CPU that has
> dual mapping for its pages in physmap in its TLB. We can hence defer
> TLB flush on a CPU until a process that would have caused a TLB
> flush is scheduled on that CPU.
This logic is a bit suspect to me. Imagine a situation where we have
two attacker processes: one which is causing page to go from
kernel->user (and be unmapped from the kernel) and a second process that
*was* accessing that page.
The second process could easily have the page's old TLB entry. It could
abuse that entry as long as that CPU doesn't context switch
(switch_mm_irqs_off()) or otherwise flush the TLB entry.
As for where to flush the TLB... As you know, using synchronous IPIs is
obviously the most bulletproof from a mitigation perspective. If you
can batch the IPIs, you can get the overhead down, but you need to do
the flushes for a bunch of pages at once, which I think is what you were
exploring but haven't gotten working yet.
Anything else you do will have *some* reduced mitigation value, which
isn't a deal-breaker (to me at least). Some ideas:
Take a look at the SWITCH_TO_KERNEL_CR3 in head_64.S. Every time that
gets called, we've (potentially) just done a user->kernel transition and
might benefit from flushing the TLB. We're always doing a CR3 write (on
Meltdown-vulnerable hardware) and it can do a full TLB flush based on if
X86_CR3_PCID_NOFLUSH_BIT is set. So, when you need a TLB flush, you
would set a bit that ADJUST_KERNEL_CR3 would see on the next
user->kernel transition on *each* CPU. Potentially, multiple TLB
flushes could be coalesced this way. The downside of this is that
you're exposed to the old TLB entries if a flush is needed while you are
already *in* the kernel.
You could also potentially do this from C code, like in the syscall
entry code, or in sensitive places, like when you're returning from a
guest after a VMEXIT in the kvm code.
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