From: Dan Williams <dan.j.williams@intel.com>
To: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
linux-arch@vger.kernel.org, Andi Kleen <ak@linux.intel.com>,
Kees Cook <keescook@chromium.org>,
kernel-hardening@lists.openwall.com, X86 ML <x86@kernel.org>,
Ingo Molnar <mingo@redhat.com>, Al Viro <viro@zeniv.linux.org.uk>,
"H. Peter Anvin" <hpa@zytor.com>,
Thomas Gleixner <tglx@linutronix.de>,
Linus Torvalds <torvalds@linux-foundation.org>,
Andrew Morton <akpm@linux-foundation.org>,
Alan Cox <alan@linux.intel.com>
Subject: Re: [PATCH v2 07/19] x86: introduce __uaccess_begin_nospec and ASM_IFENCE
Date: Fri, 12 Jan 2018 10:21:43 -0800 [thread overview]
Message-ID: <CAPcyv4heW2FqxpCa0NZeTsa5V7pgpzA-MdHYJA+y9C5L2GTP9w@mail.gmail.com> (raw)
In-Reply-To: <20180112175109.yoz4jaaipztdj34k@treble>
On Fri, Jan 12, 2018 at 9:51 AM, Josh Poimboeuf <jpoimboe@redhat.com> wrote:
> On Thu, Jan 11, 2018 at 04:47:02PM -0800, Dan Williams wrote:
>> For 'get_user' paths, do not allow the kernel to speculate on the value
>> of a user controlled pointer. In addition to the 'stac' instruction for
>> Supervisor Mode Access Protection, an 'ifence' causes the 'access_ok'
>> result to resolve in the pipeline before the cpu might take any
>> speculative action on the pointer value.
>
> So I understand the need to "patch first and ask questions later". I
> also understand that usercopy is an obvious attack point for speculative
> bugs. However, I'm still hopelessly confused about what exactly this
> patch (and the next one) are supposed to accomplish.
>
> I can't figure out if:
>
> a) I'm missing something completely obvious;
> b) this is poorly described; or
> c) it doesn't actually fix/protect/harden anything.
>
> The commit log doesn't help me at all. In fact, it confuses me more.
> For example, this paragraph:
>
>> Since this is a major kernel interface that deals with user controlled
>> data, the '__uaccess_begin_nospec' mechanism will prevent speculative
>> execution past an 'access_ok' permission check. While speculative
>> execution past 'access_ok' is not enough to lead to a kernel memory
>> leak, it is a necessary precondition.
>
> That just sounds wrong. What if the speculation starts *after* the
> access_ok() check? Then the barrier has no purpose.
>
> Most access_ok/get_user/copy_from_user calls are like this:
>
> if (copy_from_user(...uptr..)) /* or access_ok() or get_user() */
> return -EFAULT;
>
> So in other words, the usercopy function is called *before* the branch.
>
> But to halt speculation, the lfence needs to come *after* the branch.
> So putting lfences *before* the branch doesn't solve anything.
>
> So what am I missing?
We're trying to prevent a pointer under user control from being
de-referenced inside the kernel, before we know it has been limited to
something safe. In the following sequence the branch we are worried
about speculating is the privilege check:
if (access_ok(uptr)) /* <--- Privelege Check */
if (copy_from_user_(uptr))
The cpu can speculatively skip that access_ok() check and cause a read
of kernel memory.
next prev parent reply other threads:[~2018-01-12 18:21 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 [this message]
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
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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