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Andrew Cooper 008778f708 x86/kfence: avoid writing L1TF-vulnerable PTEs 
commit b505f19445 upstream.

For native, the choice of PTE is fine.  There's real memory backing the
non-present PTE.  However, for XenPV, Xen complains:

  (XEN) d1 L1TF-vulnerable L1e 8010000018200066 - Shadowing

To explain, some background on XenPV pagetables:

  Xen PV guests are control their own pagetables; they choose the new
  PTE value, and use hypercalls to make changes so Xen can audit for
  safety.

  In addition to a regular reference count, Xen also maintains a type
  reference count.  e.g.  SegDesc (referenced by vGDT/vLDT), Writable
  (referenced with _PAGE_RW) or L{1..4} (referenced by vCR3 or a lower
  pagetable level).  This is in order to prevent e.g.  a page being
  inserted into the pagetables for which the guest has a writable mapping.

  For non-present mappings, all other bits become software accessible,
  and typically contain metadata rather a real frame address.  There is
  nothing that a reference count could sensibly be tied to.  As such, even
  if Xen could recognise the address as currently safe, nothing would
  prevent that frame from changing owner to another VM in the future.

  When Xen detects a PV guest writing a L1TF-PTE, it responds by
  activating shadow paging.  This is normally only used for the live phase
  of migration, and comes with a reasonable overhead.

KFENCE only cares about getting #PF to catch wild accesses; it doesn't
care about the value for non-present mappings.  Use a fully inverted PTE,
to avoid hitting the slow path when running under Xen.

While adjusting the logic, take the opportunity to skip all actions if the
PTE is already in the right state, half the number PVOps callouts, and
skip TLB maintenance on a !P -> P transition which benefits non-Xen cases
too.

Link: https://lkml.kernel.org/r/20260106180426.710013-1-andrew.cooper3@citrix.com
Fixes: 1dc0da6e9e ("x86, kfence: enable KFENCE for x86")
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Tested-by: Marco Elver <elver@google.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Marco Elver <elver@google.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Jann Horn <jannh@google.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
History 2026-01-30 10:28:40 +01:00
arch x86/kfence: avoid writing L1TF-vulnerable PTEs 2026-01-30 10:28:40 +01:00
block block: fix NULL pointer dereference in blk_zone_reset_all_bio_endio() 2026-01-08 10:15:06 +01:00
certs sign-file,extract-cert: use pkcs11 provider for OPENSSL MAJOR >= 3 2024-09-20 19:52:48 +03:00
crypto crypto: authencesn - reject too-short AAD (assoclen<8) to match ESP/ESN spec 2026-01-30 10:28:39 +01:00
Documentation fou: Don't allow 0 for FOU_ATTR_IPPROTO. 2026-01-30 10:28:39 +01:00
drivers scsi: xen: scsiback: Fix potential memory leak in scsiback_remove() 2026-01-30 10:28:40 +01:00
fs btrfs: fix missing fields in superblock backup with BLOCK_GROUP_TREE 2026-01-30 10:28:37 +01:00
include dt-bindings: power: qcom,rpmpd: Add SC8280XP_MXC_AO 2026-01-30 10:28:37 +01:00
init init: handle bootloader identifier in kernel parameters 2025-10-19 16:33:50 +02:00
io_uring io_uring: move local task_work in exit cancel loop 2026-01-23 11:18:42 +01:00
ipc ipc: fix to protect IPCS lookups using RCU 2025-06-27 11:11:22 +01:00
kernel ptp: Add PHC file mode checks. Allow RO adjtime() without FMODE_WRITE. 2026-01-30 10:28:37 +01:00
lib lib/buildid: use __kernel_read() for sleepable context 2026-01-23 11:18:43 +01:00
LICENSES LICENSES: add 0BSD license text 2024-09-01 20:43:24 -07:00
mm mm/page_alloc: prevent pcp corruption with SMP=n 2026-01-23 11:18:52 +01:00
net wifi: mac80211: don't perform DA check on S1G beacon 2026-01-30 10:28:39 +01:00
rust rust: kbuild: workaround rustdoc doctests modifier bug 2025-11-24 10:36:07 +01:00
samples samples/ftrace: Adjust LoongArch register restore order in direct calls 2026-01-08 10:14:50 +01:00
scripts kbuild: fix compilation of dtb specified on command-line without make rule 2026-01-08 10:14:34 +01:00
security tpm2-sessions: Fix out of range indexing in name_size 2026-01-17 16:31:29 +01:00
sound ASoC: codecs: wsa881x: fix unnecessary initialisation 2026-01-23 11:18:45 +01:00
tools tools: ynl: Specify --no-line-number in ynl-regen.sh. 2026-01-30 10:28:39 +01:00
usr kbuild: uapi: Strip comments before size type check 2025-11-13 15:34:35 -05:00
virt KVM: Disallow toggling KVM_MEM_GUEST_MEMFD on an existing memslot 2026-01-08 10:14:21 +01:00
.clang-format clang-format: Update with v6.11-rc1's for_each macro list 2024-08-02 13:20:31 +02:00
.clippy.toml rust: give Clippy the minimum supported Rust version 2025-08-01 09:48:44 +01:00
.cocciconfig scripts: add Linux .cocciconfig for coccinelle 2016-07-22 12:13:39 +02:00
.editorconfig .editorconfig: remove trim_trailing_whitespace option 2024-06-13 16:47:52 +02:00
.get_maintainer.ignore Add Jeff Kirsher to .get_maintainer.ignore 2024-03-08 11:36:54 +00:00
.gitattributes .gitattributes: set diff driver for Rust source code files 2023-05-31 17:48:25 +02:00
.gitignore rust: introduce .clippy.toml 2025-03-13 13:01:42 +01:00
.mailmap mailmap: add entry for Thorsten Blum 2024-11-07 14:14:59 -08:00
.rustfmt.toml rust: add .rustfmt.toml 2022-09-28 09:02:20 +02:00
COPYING COPYING: state that all contributions really are covered by this file 2020-02-10 13:32:20 -08:00
CREDITS MAINTAINERS: Remove self from DSA entry 2024-11-03 12:52:38 -08:00
Kbuild Kbuild updates for v6.1 2022-10-10 12:00:45 -07:00
Kconfig kbuild: ensure full rebuild when the compiler is updated 2020-05-12 13:28:33 +09:00
MAINTAINERS staging: rtl8712: Remove driver using deprecated API wext 2025-12-07 06:25:02 +09:00
Makefile Linux 6.12.67 2026-01-23 11:18:52 +01:00
README README: Fix spelling 2024-03-18 03:36:32 -06:00

README 

Linux kernel
============

There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.

In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``.  The formatted documentation can also be read online at:

    https://www.kernel.org/doc/html/latest/

There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.

Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.