linux-kernel - Re: [patch] Fix up l1ft documentation was Re: Taking a break

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Message-ID: <20190311131341.GA28223@amd>
Date:   Mon, 11 Mar 2019 14:13:41 +0100
From:   Pavel Machek <pavel@....cz>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     corbet@....net, LKML <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>, x86@...nel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Jiri Kosina <jkosina@...e.cz>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        David Woodhouse <dwmw2@...radead.org>,
        Tom Lendacky <thomas.lendacky@....com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Joerg Roedel <joro@...tes.org>,
        Tony Luck <tony.luck@...el.com>,
        Salvatore Bonaccorso <carnil@...ian.org>,
        linux-doc@...r.kernel.org
Subject: Re: [patch] Fix up l1ft documentation was Re: Taking a break - time
 to look back

On Mon 2019-03-11 14:05:07, Thomas Gleixner wrote:
> On Mon, 11 Mar 2019, Pavel Machek wrote:
> > On Thu 2019-01-03 00:51:52, Pavel Machek wrote:
> > > Hi!
> > > 
> > > > The next round of speculation-related issues including the scary L1TF
> > > > hardware bug was a way more "pleasant" experience to work on. While for
> > > > obvious reasons the mitigation development happened behind closed doors in
> > > > a smaller group of people, we were at least able to collaborate in a way
> > > > which is somehow close to what we are used to.
> > > 
> > > 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
> 
> What's advertisement there?

"No problem here, no performance issues, nothing to be seen unless you
are running VM."

> > > unfortunately is slightly "inaccurate" at places (to the point of
> > > lying).
> 
> Huch? Care to tell what's a lie instead of making bold statements?

Take a care to look at the patch I submitted?

Lie:

# A system with an up to date kernel is protected against attacks from
# malicious user space applications.

3GB system running 32bit kernel is not protected. Same is true for for
really big 64bit systems.

If I do what dmesg suggests, this becomes untrue:

# The Linux kernel contains a mitigation for this attack vector, PTE
# inversion, which is permanently enabled and has no performance
# impact.

Limiting memory to 2GB _is_ going to have severe perfomance impact.

								Pavel

commit 9664b4dabdb132433a6843aefe05814953f1342f
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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