linux-kernel - Re: [PATCH] x86/speculation: Add document to describe Spectre and its mitigations

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Message-ID: <20190114120157.GB21544@amd>
Date:   Mon, 14 Jan 2019 13:01:57 +0100
From:   Pavel Machek <pavel@....cz>
To:     Jiri Kosina <jikos@...nel.org>
Cc:     Tim Chen <tim.c.chen@...ux.intel.com>,
        Jonathan Corbet <corbet@....net>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Tom Lendacky <thomas.lendacky@....com>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        David Woodhouse <dwmw@...zon.co.uk>,
        Andi Kleen <ak@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Asit Mallick <asit.k.mallick@...el.com>,
        Arjan van de Ven <arjan@...ux.intel.com>,
        Jon Masters <jcm@...hat.com>,
        Waiman Long <longman9394@...il.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Borislav Petkov <bp@...en8.de>, linux-kernel@...r.kernel.org,
        x86@...nel.org, stable@...r.kernel.org
Subject: Re: [PATCH] x86/speculation: Add document to describe Spectre and
 its mitigations

On Mon 2019-01-14 00:12:59, Jiri Kosina wrote:
> On Mon, 14 Jan 2019, Pavel Machek wrote:
> 
> > That one really is Intel-specific (not even all x86s are affectd). Same 
> > for Meltdown.
> 
> At least for Meltdown, your claim is simply not correct.

You are right, there may be few ARM chips affected by meltdown.

I don't know about any non-Intel affected by l1tf.

...and its documentation is just plain wrong, explaining I'm protected
when I'm not...

commit f372cd79be31382ae6030a1f15638cc7fe9eeb9f
Author: Pavel <pavel@....cz>
Date:   Thu Jan 3 00:48:40 2019 +0100

    Ok, I guess L1TF was a lot of fun, and there was not time for a good
    documentation.
    
    There's admin guide that is written as an advertisment, and
    unfortunately is slightly "inaccurate" at places (to the point of
    lying).
    
    Plus, I believe it should go to x86/ directory, as this is really
    Intel issue, and not anything ARM (or RISC-V) people need to know.
    
    Signed-off-by: Pavel Machek <pavel@....cz>

diff --git a/Documentation/admin-guide/l1tf.rst b/Documentation/admin-guide/l1tf.rst
index 9af9773..05c5422 100644
--- a/Documentation/admin-guide/l1tf.rst
+++ b/Documentation/admin-guide/l1tf.rst
@@ -1,10 +1,11 @@
 L1TF - L1 Terminal Fault
 ========================
 
-L1 Terminal Fault is a hardware vulnerability which allows unprivileged
-speculative access to data which is available in the Level 1 Data Cache
-when the page table entry controlling the virtual address, which is used
-for the access, has the Present bit cleared or other reserved bits set.
+L1 Terminal Fault is a hardware vulnerability on most recent Intel x86
+CPUs which allows unprivileged speculative access to data which is
+available in the Level 1 Data Cache when the page table entry
+controlling the virtual address, which is used for the access, has the
+Present bit cleared or other reserved bits set.
 
 Affected processors
 -------------------
@@ -76,12 +77,14 @@ Attack scenarios
    deterministic and more practical.
 
    The Linux kernel contains a mitigation for this attack vector, PTE
-   inversion, which is permanently enabled and has no performance
-   impact. The kernel ensures that the address bits of PTEs, which are not
-   marked present, never point to cacheable physical memory space.
-
-   A system with an up to date kernel is protected against attacks from
-   malicious user space applications.
+   inversion, which is permanently enabled and has no measurable
+   performance impact in most configurations. The kernel ensures that
+   the address bits of PTEs, which are not marked present, never point
+   to cacheable physical memory space. On x86-32, this physical memory
+   needs to be limited to 2GiB to make mitigation effective.
+
+   Mitigation is present in kernels v4.19 and newer, and in
+   recent -stable kernels.
 
 2. Malicious guest in a virtual machine
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^


-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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