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

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Date:   Thu, 3 Jan 2019 00:51:52 +0100
From:   Pavel Machek <pavel@....cz>
To:     Thomas Gleixner <tglx@...utronix.de>, corbet@....net
Cc:     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: [patch] Fix up l1ft documentation was Re: Taking a break - time to
 look back

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
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. (But we already have some urls in printk messages that may need
fixing up..?)

Signed-off-by: Pavel Machek <pavel@....cz>

diff --git a/Documentation/admin-guide/l1tf.rst b/Documentation/admin-guide/l1tf.rst
index b85dd80..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.
+   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.
 
-   A system with an up to date kernel is protected against attacks from
-   malicious user space applications.
+   Mitigation is present in kernels v4.19 and newer, and in
+   recent -stable kernels.
 
 2. Malicious guest in a virtual machine
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -405,6 +408,9 @@ time with the option "l1tf=". The valid arguments for this option are:
 
   off		Disables hypervisor mitigations and doesn't emit any
 		warnings.
+		It also drops the swap size and available RAM limit restrictions
+		on both hypervisor and bare metal.
+
   ============  =============================================================
 
 The default is 'flush'. For details about L1D flushing see :ref:`l1d_flush`.
@@ -576,7 +582,8 @@ Default mitigations
   The kernel default mitigations for vulnerable processors are:
 
   - PTE inversion to protect against malicious user space. This is done
-    unconditionally and cannot be controlled.
+    unconditionally and cannot be controlled. The swap storage is limited
+    to ~16TB.
 
   - L1D conditional flushing on VMENTER when EPT is enabled for
     a guest.




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

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