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  > > * 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  > > 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 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 data. In-bounds access can do that just as well without any speculation.
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