LKML Archive on
 help / color / Atom feed
From: Andy Lutomirski <>
Cc: LKML <>,
	Andy Lutomirski <>,,
	Greg Kroah-Hartman <>,
	Borislav Petkov <>,
	Frederic Weisbecker <>,
	Jon Masters <>
Subject: [PATCH 2/2] x86/speculation/mds: Improve CPU buffer clear documentation
Date: Tue, 14 May 2019 13:24:40 -0700
Message-ID: <> (raw)
In-Reply-To: <>

On x86_64, all returns to usermode go through
prepare_exit_to_usermode(), with the sole exception of do_nmi().
This even includes machine checks -- this was added several years
ago to support MCE recovery.  Update the documentation.

Cc: Greg Kroah-Hartman <>
Cc: Borislav Petkov <>
Cc: Frederic Weisbecker <>
Cc: Jon Masters <>
Fixes: 04dcbdb80578 ("x86/speculation/mds: Clear CPU buffers on exit to user")
Signed-off-by: Andy Lutomirski <>
 Documentation/x86/mds.rst | 39 +++++++--------------------------------
 1 file changed, 7 insertions(+), 32 deletions(-)

diff --git a/Documentation/x86/mds.rst b/Documentation/x86/mds.rst
index 0dc812bb9249..5d4330be200f 100644
--- a/Documentation/x86/mds.rst
+++ b/Documentation/x86/mds.rst
@@ -142,38 +142,13 @@ Mitigation points
    The mitigation is invoked in prepare_exit_to_usermode() which covers
-   most of the kernel to user space transitions. There are a few exceptions
-   which are not invoking prepare_exit_to_usermode() on return to user
-   space. These exceptions use the paranoid exit code.
-   - Non Maskable Interrupt (NMI):
-     Access to sensible data like keys, credentials in the NMI context is
-     mostly theoretical: The CPU can do prefetching or execute a
-     misspeculated code path and thereby fetching data which might end up
-     leaking through a buffer.
-     But for mounting other attacks the kernel stack address of the task is
-     already valuable information. So in full mitigation mode, the NMI is
-     mitigated on the return from do_nmi() to provide almost complete
-     coverage.
-   - Machine Check Exception (#MC):
-     Another corner case is a #MC which hits between the CPU buffer clear
-     invocation and the actual return to user. As this still is in kernel
-     space it takes the paranoid exit path which does not clear the CPU
-     buffers. So the #MC handler repopulates the buffers to some
-     extent. Machine checks are not reliably controllable and the window is
-     extremly small so mitigation would just tick a checkbox that this
-     theoretical corner case is covered. To keep the amount of special
-     cases small, ignore #MC.
-   - Debug Exception (#DB):
-     This takes the paranoid exit path only when the INT1 breakpoint is in
-     kernel space. #DB on a user space address takes the regular exit path,
-     so no extra mitigation required.
+   all but one of the kernel to user space transitions.  The exception
+   is when we return from a Non Maskable Interrupt (NMI), which is
+   handled directly in do_nmi().
+   (The reason that NMI is special is that prepare_exit_to_usermode() can
+    enable IRQs.  In NMI context, NMIs are blocked, and we don't want to
+    enable IRQs with NMIs blocked.)
 2. C-State transition

  parent reply index

Thread overview: 5+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-05-14 20:24 [PATCH 0/2] x86/speculation/mds: Minor fixes Andy Lutomirski
2019-05-14 20:24 ` [PATCH 1/2] x86/speculation/mds: Revert CPU buffer clear on double fault exit Andy Lutomirski
2019-05-16  7:10   ` [tip:x86/urgent] " tip-bot for Andy Lutomirski
2019-05-14 20:24 ` Andy Lutomirski [this message]
2019-05-16  7:11   ` [tip:x86/urgent] x86/speculation/mds: Improve CPU buffer clear documentation tip-bot for Andy Lutomirski

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \ \ \ \ \ \ \ \ \ \

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

LKML Archive on

Archives are clonable:
	git clone --mirror lkml/git/0.git
	git clone --mirror lkml/git/1.git
	git clone --mirror lkml/git/2.git
	git clone --mirror lkml/git/3.git
	git clone --mirror lkml/git/4.git
	git clone --mirror lkml/git/5.git
	git clone --mirror lkml/git/6.git
	git clone --mirror lkml/git/7.git
	git clone --mirror lkml/git/8.git
	git clone --mirror lkml/git/9.git

	# If you have public-inbox 1.1+ installed, you may
	# initialize and index your mirror using the following commands:
	public-inbox-init -V2 lkml lkml/ \
	public-inbox-index lkml

Example config snippet for mirrors

Newsgroup available over NNTP:

AGPL code for this site: git clone