linux-kernel - Re: [PATCH v3 8/9] x86: use __uaccess_begin_nospec and ASM_IFENCE in get

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Message-ID: <CAPcyv4gHVSZjCHckGF+aTK3pN6FRY7KcsBtsMvXLZrZvzFwMVg@mail.gmail.com>
Date:   Wed, 17 Jan 2018 12:14:22 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     Alan Cox <alan@...ux.intel.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        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>,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v3 8/9] x86: use __uaccess_begin_nospec and ASM_IFENCE in
 get_user paths

On Wed, Jan 17, 2018 at 12:05 PM, Al Viro <viro@...iv.linux.org.uk> wrote:
> On Wed, Jan 17, 2018 at 11:54:12AM -0800, Dan Williams wrote:
>> On Wed, Jan 17, 2018 at 10:52 AM, Al Viro <viro@...iv.linux.org.uk> wrote:
>> > On Wed, Jan 17, 2018 at 02:17:26PM +0000, Alan Cox wrote:
>> [..]
>> >         Incidentally, what about copy_to_iter() and friends?  They
>> > check iov_iter flavour and go either into the "copy to kernel buffer"
>> > or "copy to userland" paths.  Do we need to deal with mispredictions
>> > there?  We are calling a bunch of those on read()...
>> >
>>
>> Those should be protected by the conversion of __uaccess_begin to
>> __uaccess_begin_nospec that includes the lfence.
>
> Huh?  What the hell does it do to speculative execution of "memcpy those
> suckers" branch?

'raw_copy_from_user 'is changed to use 'uaccess_begin_nospec' instead
of plain 'uacess_begin'. The only difference between those being that
the former includes an lfence. So with this sequence.

        if (access_ok(VERIFY_READ, from, n)) {
                kasan_check_write(to, n);
                n = raw_copy_from_user(to, from, n);
        }
        return n;

...'from' is guaranteed to be within the address limit with respect to
speculative execution, or otherwise never de-referenced.