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Message-ID: <CAPcyv4iBHTahi66deUT1iSPpA2W2wjYZSaiUYUjpdK10d5Rt7g@mail.gmail.com>
Date: Fri, 19 Jan 2018 22:58:44 -0800
From: Dan Williams <dan.j.williams@...el.com>
To: Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Cc: Mark Rutland <mark.rutland@....com>,
Kernel Hardening <kernel-hardening@...ts.openwall.com>, Peter Zijlstra <peterz@...radead.org>,
Catalin Marinas <catalin.marinas@....com>, Will Deacon <will.deacon@....com>,
"H. Peter Anvin" <hpa@...or.com>, Elena Reshetova <elena.reshetova@...el.com>,
linux-arch <linux-arch@...r.kernel.org>, Andi Kleen <ak@...ux.intel.com>,
Jonathan Corbet <corbet@....net>, X86 ML <x86@...nel.org>, Russell King <linux@...linux.org.uk>,
Ingo Molnar <mingo@...hat.com>, Andrew Honig <ahonig@...gle.com>, Alan Cox <alan@...ux.intel.com>,
Tom Lendacky <thomas.lendacky@....com>, Kees Cook <keescook@...omium.org>,
Al Viro <viro@...iv.linux.org.uk>, Andy Lutomirski <luto@...nel.org>,
Thomas Gleixner <tglx@...utronix.de>, Andrew Morton <akpm@...ux-foundation.org>,
Jim Mattson <jmattson@...gle.com>, Christian Lamparter <chunkeey@...il.com>,
Greg KH <gregkh@...uxfoundation.org>,
Linux Wireless List <linux-wireless@...r.kernel.org>, stable@...r.kernel.org,
Paolo Bonzini <pbonzini@...hat.com>, Johannes Berg <johannes@...solutions.net>,
Linus Torvalds <torvalds@...ux-foundation.org>, "David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH v4 00/10] prevent bounds-check bypass via speculative execution
On Thu, Jan 18, 2018 at 4:01 PM, Dan Williams <dan.j.williams@...el.com> wrote:
> Changes since v3 [1]
> * Drop 'ifence_array_ptr' and associated compile-time + run-time
> switching and just use the masking approach all the time.
>
> * Convert 'get_user' to use pointer sanitization via masking rather than
> lfence. '__get_user' and associated paths still rely on
> lfence. (Linus)
>
> "Basically, the rule is trivial: find all 'stac' users, and use
> address masking if those users already integrate the limit
> check, and lfence they don't."
>
> * At syscall entry sanitize the syscall number under speculation
> to remove a user controlled pointer de-reference in kernel
> space. (Linus)
>
> * Fix a raw lfence in the kvm code (added for v4.15-rc8) to use
> 'array_ptr'.
>
> * Propose 'array_idx' as a way to sanitize user input that is
> later used as an array index, but where the validation is
> happening in a different code block than the array reference.
> (Christian).
>
> * Fix grammar in speculation.txt (Kees)
>
> ---
>
> Quoting Mark's original RFC:
>
> "Recently, Google Project Zero discovered several classes of attack
> against speculative execution. One of these, known as variant-1, allows
> explicit bounds checks to be bypassed under speculation, providing an
> arbitrary read gadget. Further details can be found on the GPZ blog [2]
> and the Documentation patch in this series."
>
> A precondition of using this attack on the kernel is to get a user
> controlled pointer de-referenced (under speculation) in privileged code.
> The primary source of user controlled pointers in the kernel is the
> arguments passed to 'get_user' and '__get_user'. An example of other
> user controlled pointers are user-controlled array / pointer offsets.
>
> Better tooling is needed to find more arrays / pointers with user
> controlled indices / offsets that can be converted to use 'array_ptr' or
> 'array_idx'. A few are included in this set, and these are not expected
> to be complete. That said, the 'get_user' protections raise the bar on
> finding a vulnerable gadget in the kernel.
>
> These patches are also available via the 'nospec-v4' git branch here:
>
> git://git.kernel.org/pub/scm/linux/kernel/git/djbw/linux nospec-v4
I've pushed out a nospec-v4.1 with the below minor cleanup, a fixup of
the changelog for "kvm, x86: fix spectre-v1 mitigation", and added
Paolo's ack.
git://git.kernel.org/pub/scm/linux/kernel/git/djbw/linux nospec-v4.1
diff --git a/include/linux/nospec.h b/include/linux/nospec.h
index 8af35be1869e..b8a9222e34d1 100644
--- a/include/linux/nospec.h
+++ b/include/linux/nospec.h
@@ -37,7 +37,7 @@ static inline unsigned long array_ptr_mask(unsigned
long idx, unsigned long sz)
unsigned long _i = (idx); \
unsigned long _mask = array_ptr_mask(_i, (sz)); \
\
- __u._ptr = _arr + (_i & _mask); \
+ __u._ptr = _arr + _i; \
__u._bit &= _mask; \
__u._ptr; \
})
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