From: Dan Williams <dan.j.williams@intel.com>
To: Will Deacon <will.deacon@arm.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
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Subject: Re: [PATCH v2 00/19] prevent bounds-check bypass via speculative execution
Date: Thu, 18 Jan 2018 08:58:08 -0800 [thread overview]
Message-ID: <CAPcyv4gPLx74CAHGGrC3R-fgrh0vUmCbLNXZ0f7PTiKi0f+hCQ@mail.gmail.com> (raw)
In-Reply-To: <20180118131837.GA20783@arm.com>
On Thu, Jan 18, 2018 at 5:18 AM, Will Deacon <will.deacon@arm.com> wrote:
> Hi Dan, Linus,
>
> On Thu, Jan 11, 2018 at 05:41:08PM -0800, Dan Williams wrote:
>> On Thu, Jan 11, 2018 at 5:19 PM, Linus Torvalds
>> <torvalds@linux-foundation.org> wrote:
>> > On Thu, Jan 11, 2018 at 4:46 PM, Dan Williams <dan.j.williams@intel.com> wrote:
>> >>
>> >> This series incorporates Mark Rutland's latest ARM changes and adds
>> >> the x86 specific implementation of 'ifence_array_ptr'. That ifence
>> >> based approach is provided as an opt-in fallback, but the default
>> >> mitigation, '__array_ptr', uses a 'mask' approach that removes
>> >> conditional branches instructions, and otherwise aims to redirect
>> >> speculation to use a NULL pointer rather than a user controlled value.
>> >
>> > Do you have any performance numbers and perhaps example code
>> > generation? Is this noticeable? Are there any microbenchmarks showing
>> > the difference between lfence use and the masking model?
>>
>> I don't have performance numbers, but here's a sample code generation
>> from __fcheck_files, where the 'and; lea; and' sequence is portion of
>> array_ptr() after the mask generation with 'sbb'.
>>
>> fdp = array_ptr(fdt->fd, fd, fdt->max_fds);
>> 8e7: 8b 02 mov (%rdx),%eax
>> 8e9: 48 39 c7 cmp %rax,%rdi
>> 8ec: 48 19 c9 sbb %rcx,%rcx
>> 8ef: 48 8b 42 08 mov 0x8(%rdx),%rax
>> 8f3: 48 89 fe mov %rdi,%rsi
>> 8f6: 48 21 ce and %rcx,%rsi
>> 8f9: 48 8d 04 f0 lea (%rax,%rsi,8),%rax
>> 8fd: 48 21 c8 and %rcx,%rax
>>
>>
>> > Having both seems good for testing, but wouldn't we want to pick one in the end?
>>
>> I was thinking we'd keep it as a 'just in case' sort of thing, at
>> least until the 'probably safe' assumption of the 'mask' approach has
>> more time to settle out.
>
> From the arm64 side, the only concern I have (and this actually applies to
> our CSDB sequence as well) is the calculation of the array size by the
> caller. As Linus mentioned at the end of [1], if the determination of the
> size argument is based on a conditional branch, then masking doesn't help
> because you bound within the wrong range under speculation.
>
> We ran into this when trying to use masking to protect our uaccess routines
> where the conditional bound is either KERNEL_DS or USER_DS. It's possible
> that a prior conditional set_fs(KERNEL_DS) could defeat the masking and so
> we'd need to throw some heavy barriers in set_fs to make it robust.
At least in the conditional mask case near set_fs() usage the approach
we are taking is to use a barrier. I.e. the following guidance from
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."
...which translates to narrow the pointer for get_user() and use a
barrier for __get_user().
next prev parent reply other threads:[~2018-01-18 16:58 UTC|newest]
Thread overview: 59+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-01-12 0:46 [PATCH v2 00/19] prevent bounds-check bypass via speculative execution Dan Williams
2018-01-12 0:46 ` [PATCH v2 01/19] Documentation: document array_ptr Dan Williams
2018-01-12 10:38 ` Geert Uytterhoeven
2018-01-16 21:01 ` Kees Cook
2018-01-12 0:46 ` [PATCH v2 02/19] arm64: implement ifence_array_ptr() Dan Williams
2018-01-12 0:46 ` [PATCH v2 03/19] arm: " Dan Williams
2018-01-12 0:46 ` [PATCH v2 04/19] x86: implement ifence() Dan Williams
2018-01-12 2:27 ` Eric W. Biederman
2018-01-12 3:39 ` Dan Williams
2018-01-12 0:46 ` [PATCH v2 05/19] x86: implement ifence_array_ptr() and array_ptr_mask() Dan Williams
2018-01-12 0:46 ` [PATCH v2 06/19] asm-generic/barrier: mask speculative execution flows Dan Williams
2018-01-12 2:42 ` Eric W. Biederman
2018-01-12 9:12 ` Peter Zijlstra
2018-01-13 0:41 ` Dan Williams
2018-01-15 8:46 ` Peter Zijlstra
2018-01-12 0:47 ` [PATCH v2 07/19] x86: introduce __uaccess_begin_nospec and ASM_IFENCE Dan Williams
2018-01-12 17:51 ` Josh Poimboeuf
2018-01-12 18:21 ` Dan Williams
2018-01-12 18:58 ` Josh Poimboeuf
2018-01-12 19:26 ` Dan Williams
2018-01-12 20:01 ` Linus Torvalds
2018-01-12 20:41 ` Josh Poimboeuf
2018-01-12 0:47 ` [PATCH v2 08/19] x86: use __uaccess_begin_nospec and ASM_IFENCE in get_user paths Dan Williams
2018-01-12 1:11 ` Linus Torvalds
2018-01-12 1:14 ` Dan Williams
2018-01-12 0:47 ` [PATCH v2 09/19] ipv6: prevent bounds-check bypass via speculative execution Dan Williams
2018-01-12 0:47 ` [PATCH v2 10/19] ipv4: " Dan Williams
2018-01-12 7:59 ` Greg KH
2018-01-12 18:47 ` Dan Williams
2018-01-13 8:56 ` Greg KH
2018-01-12 0:47 ` [PATCH v2 11/19] vfs, fdtable: " Dan Williams
2018-01-12 0:47 ` [PATCH v2 12/19] userns: " Dan Williams
2018-01-12 0:47 ` [PATCH v2 13/19] udf: " Dan Williams
2018-01-15 10:32 ` Jan Kara
2018-01-15 17:49 ` Dan Williams
2018-01-12 0:47 ` [PATCH v2 14/19] [media] uvcvideo: " Dan Williams
2018-08-06 21:40 ` Laurent Pinchart
2018-01-12 0:47 ` [PATCH v2 15/19] carl9170: " Dan Williams
2018-01-12 14:42 ` Christian Lamparter
2018-01-12 18:39 ` Dan Williams
2018-01-12 20:01 ` Christian Lamparter
2018-01-12 23:05 ` Dan Williams
2018-01-12 0:47 ` [PATCH v2 16/19] p54: " Dan Williams
2018-01-12 0:47 ` [PATCH v2 17/19] qla2xxx: " Dan Williams
2018-01-12 1:19 ` James Bottomley
2018-01-12 5:38 ` Dan Williams
2018-01-12 6:05 ` James Bottomley
2018-01-12 0:48 ` [PATCH v2 18/19] cw1200: " Dan Williams
2018-01-12 0:48 ` [PATCH v2 19/19] net: mpls: " Dan Williams
2018-01-12 1:19 ` [PATCH v2 00/19] " Linus Torvalds
2018-01-12 1:41 ` Dan Williams
2018-01-18 13:18 ` Will Deacon
2018-01-18 16:58 ` Dan Williams [this message]
2018-01-18 17:05 ` Will Deacon
2018-01-18 21:41 ` Laurent Pinchart
2018-01-13 0:15 ` Tony Luck
2018-01-13 18:51 ` Linus Torvalds
2018-01-16 19:21 ` Tony Luck
2018-01-12 10:02 ` Russell King - ARM Linux
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