From: Josh Poimboeuf <jpoimboe@redhat.com>
To: speck@linutronix.de
Subject: [MODERATED] Re: LVI
Date: Tue, 19 Nov 2019 15:00:07 -0600 [thread overview]
Message-ID: <20191119210007.zrweuamvb44x7evv@treble> (raw)
In-Reply-To: <f5afdb69-0108-f8f3-daac-73863e4206c3@citrix.com>
On Tue, Nov 19, 2019 at 06:39:34PM +0000, speck for Andrew Cooper wrote:
> On 19/11/2019 18:27, speck for Josh Poimboeuf wrote:
> >>> To protect the kernel, we'd presumably need to look for places where
> >>> users can trigger a faulting/assisting load. For example,
> >>> copy_from_user().
> >>>
> >>> copy_from_user() has an LFENCE between the access_ok() check and the
> >>> actual copy to protect against Spectre v1. What if we move that LFENCE
> >>> to *after* the copy? I think that would protect against both Spectre v1
> >>> and LVI.
> >>>
> >>> Thoughts?
> >> The lfence before protects from speculating into copy_from_user() with a
> >> bad pointer. This protection is still necessary.
> > What's the harm of speculating the copy with a bad pointer if we can
> > confirm there are no gadgets between the copy and the LFENCE?
>
> Because the other hyperthread can read the data directly out of the
> cache using L1TF/MDS/other as applicable.
>
> True - this is applicable to any speculative OoB read on the first
> thread so isn't unique to copy_from_user(), but copy_from_user() is very
> easy for an attacker to control.
Ok, I guess had just assumed we didn't care about SMT at this point, we
should at least try to do a best effort there.
Does this match your thinking?
if (SMT && (BUG_L1TF || BUG_MDS || BUG_TAA))
lfence();
... do the copy ...
if (BUG_LVI)
lfence();
--
Josh
next prev parent reply other threads:[~2019-11-19 21:00 UTC|newest]
Thread overview: 34+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-11-19 17:40 [MODERATED] LVI Josh Poimboeuf
2019-11-19 17:51 ` [MODERATED] LVI Andrew Cooper
2019-11-19 18:27 ` Josh Poimboeuf
2019-11-19 19:26 ` Andrew Cooper
2019-11-20 9:52 ` Paolo Bonzini
2019-11-19 18:12 ` Greg KH
2019-11-19 18:21 ` Josh Poimboeuf
2019-11-19 18:46 ` Greg KH
2019-11-19 18:21 ` Paolo Bonzini
2019-11-19 18:22 ` Andrew Cooper
2019-11-19 18:27 ` Josh Poimboeuf
2019-11-19 18:36 ` Luck, Tony
2019-11-20 17:02 ` Greg KH
2019-11-19 18:39 ` Andrew Cooper
2019-11-19 21:00 ` Josh Poimboeuf [this message]
2019-11-19 21:03 ` Josh Poimboeuf
2019-11-20 14:11 ` Andrew Cooper
2019-11-20 8:04 ` Peter Zijlstra
2019-11-20 9:49 ` Andrew Cooper
2019-11-20 17:13 ` Josh Poimboeuf
2019-11-20 17:25 ` Greg KH
2019-11-20 17:29 ` Tyler Hicks
2019-11-20 17:30 ` Andrew Cooper
2019-11-20 17:46 ` Greg KH
2019-11-20 19:09 ` Peter Zijlstra
2019-11-20 19:19 ` Greg KH
2019-11-21 0:50 ` LVI Thomas Gleixner
2019-11-21 13:45 ` [MODERATED] LVI Greg KH
2019-11-26 0:54 ` Andi Kleen
2019-11-26 10:37 ` Greg KH
2019-11-26 18:23 ` Andi Kleen
2019-11-27 7:38 ` Greg KH
2019-11-26 10:55 ` Paolo Bonzini
2019-11-26 18:28 ` Andi Kleen
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