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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@ucw.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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