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Date: Sat, 20 Jan 2018 08:56:34 -0800
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Dan Williams <dan.j.williams@...el.com>
Cc: Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
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 Fri, Jan 19, 2018 at 10:58:44PM -0800, Dan Williams wrote:
> 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; \
hmm. I'm not sure it's the right thing to do, since the macro
is forcing cpu to speculate subsequent load from null instead
of valid pointer.
As Linus said: "
So that __u._bit masking wasn't masking
the pointer, it was masking the value that was *read* from the
pointer, so that you could know that an invalid access returned
0/NULL, not just the first value in the array.
"
imo just
return _arr + (_i & _mask);
is enough. No need for union games.
The cpu will speculate the load from _arr[0] if _i is out of bounds
which is the same as if user passed _i == 0 which would have passed
bounds check anyway, so I don't see any data leak from populating
cache with _arr[0] data. In-bounds access can do that just as well
without any speculation.
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