From: David Woodhouse <dwmw2@infradead.org>
To: David Laight <David.Laight@ACULAB.COM>,
"riel@redhat.com" <riel@redhat.com>,
"tglx@linutronix.de" <tglx@linutronix.de>,
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"jpoimboe@redhat.com" <jpoimboe@redhat.com>,
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<linux-tip-commits@vger.kernel.org>
Subject: Re: [tip:x86/pti] x86/retpoline: Fill RSB on context switch for affected CPUs
Date: Mon, 15 Jan 2018 15:39:26 +0100 [thread overview]
Message-ID: <1516027166.22147.183.camel@infradead.org> (raw)
In-Reply-To: <a4e50e6701554400afd44767c1812ccd@AcuMS.aculab.com>
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On Mon, 2018-01-15 at 14:35 +0000, David Laight wrote:
> From: David Woodhouse
> >
> > Sent: 14 January 2018 17:04
> > x86/retpoline: Fill RSB on context switch for affected CPUs
> >
> > On context switch from a shallow call stack to a deeper one, as the CPU
> > does 'ret' up the deeper side it may encounter RSB entries (predictions for
> > where the 'ret' goes to) which were populated in userspace.
> >
> > This is problematic if neither SMEP nor KPTI (the latter of which marks
> > userspace pages as NX for the kernel) are active, as malicious code in
> > userspace may then be executed speculatively.
> ...
>
> Do we have a guarantee that all cpu actually detect the related RSB underflow?
>
> It wouldn't surprise me if at least some cpu just let it wrap.
>
> This would means that userspace would see return predictions based
> on the values the kernel 'stuffed' into the RSB to fill it.
>
> Potentially this leaks a kernel address to userspace.
Yeah, KASLR is dead unless we do a full IBPB before *every* VMLAUNCH or
return to userspace anyway, isn't it? With KPTI we could put the RSB-
stuffer into the syscall trampoline page perhaps...
For this to be a concern for userspace, I think it does have to be true
that only the lower bits are used, which adds a little complexity but
probably isn't insurmountable?
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next prev parent reply other threads:[~2018-01-15 14:39 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-01-12 17:49 [PATCH] x86/retpoline: Fill RSB on context switch for affected CPUs David Woodhouse
2018-01-12 18:02 ` Andi Kleen
2018-01-12 18:23 ` David Woodhouse
2018-01-12 18:05 ` Andrew Cooper
2018-01-12 18:56 ` David Woodhouse
2018-01-12 23:41 ` Josh Poimboeuf
2018-01-14 11:39 ` Thomas Gleixner
2018-01-14 17:04 ` [tip:x86/pti] " tip-bot for David Woodhouse
2018-01-15 14:35 ` David Laight
2018-01-15 14:39 ` David Woodhouse [this message]
2018-01-15 14:42 ` Arjan van de Ven
2018-01-15 20:03 ` Kees Cook
2018-01-14 23:37 ` tip-bot for David Woodhouse
2018-01-15 0:05 ` Andi Kleen
2018-01-15 0:09 ` Andi Kleen
2018-01-15 10:13 ` David Woodhouse
2018-03-09 13:12 ` Maciej S. Szmigiero
2018-03-09 15:14 ` Andi Kleen
2018-03-09 15:33 ` Maciej S. Szmigiero
2018-03-09 15:38 ` Woodhouse, David
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