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Message-ID: <CAPcyv4g94iysWqz64KNk=HDdx6+b2e0O-rRrnFZDqfNSR3Xrjg@mail.gmail.com>
Date: Tue, 16 Jan 2018 20:30:17 -0800
From: Dan Williams <dan.j.williams@...el.com>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, linux-arch@...r.kernel.org,
Andi Kleen <ak@...ux.intel.com>, Kees Cook <keescook@...omium.org>,
kernel-hardening@...ts.openwall.com,
Greg Kroah-Hartman <gregkh@...uxfoundation.org>, "the arch/x86 maintainers" <x86@...nel.org>,
Ingo Molnar <mingo@...hat.com>, Al Viro <viro@...iv.linux.org.uk>,
"H. Peter Anvin" <hpa@...or.com>, Thomas Gleixner <tglx@...utronix.de>,
Andrew Morton <akpm@...ux-foundation.org>, Alan Cox <alan@...ux.intel.com>
Subject: Re: [PATCH v3 8/9] x86: use __uaccess_begin_nospec and ASM_IFENCE in
get_user paths
On Tue, Jan 16, 2018 at 2:23 PM, Dan Williams <dan.j.williams@...el.com> wrote:
> On Sat, Jan 13, 2018 at 11:33 AM, Linus Torvalds
[..]
> I'll respin this set along those lines, and drop the ifence bits.
So now I'm not so sure. Yes, get_user_{1,2,4,8} can mask the pointer
with the address limit result, but this doesn't work for the
access_ok() + __get_user() case. We can either change the access_ok()
calling convention to return a properly masked pointer to be used in
subsequent calls to __get_user(), or go with lfence on every
__get_user call. There seem to be several drivers that open code
copy_from_user() with __get_user loops, so the 'fence every
__get_user' approach might have noticeable overhead. On the other hand
the access_ok conversion, while it could be scripted with coccinelle,
is ~300 sites (VERIFY_READ), if you're concerned about having
something small to merge for 4.15.
I think the access_ok() conversion to return a speculation sanitized
pointer or NULL is the way to go unless I'm missing something simpler.
Other ideas?
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