The patch titled
Subject: Documentation/mm: avoid invalid use of addresses from kmap_local_page()
has been added to the -mm mm-unstable branch. Its filename is
documentation-mm-avoid-invalid-use-of-addresses-from-kmap_local_page.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/documentation-mm-avoid-invalid-use-of-addresses-from-kmap_local_page.patch
This patch will later appear in the mm-unstable branch at
git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
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------------------------------------------------------
From: "Fabio M. De Francesco" <fmdefrancesco@xxxxxxxxx>
Subject: Documentation/mm: avoid invalid use of addresses from kmap_local_page()
Date: Thu, 28 Jul 2022 17:48:41 +0200
Users of kmap_local_page() must be absolutely sure to not hand kernel
virtual address obtained calling kmap_local_page() on highmem pages to
other contexts because those pointers are thread local, therefore, they
are no longer valid across different contexts.
Extend the documentation of kmap_local_page() to warn users about the
above-mentioned potential invalid use of pointers returned by
kmap_local_page().
Link: https://lkml.kernel.org/r/20220728154844.10874-5-fmdefrancesco@xxxxxxxxx
Signed-off-by: Fabio M. De Francesco <fmdefrancesco@xxxxxxxxx>
Suggested-by: Ira Weiny <ira.weiny@xxxxxxxxx>
Cc: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
Cc: Mike Rapoport <rppt@xxxxxxxxxxxxx>
Cc: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
Cc: Jonathan Corbet <corbet@xxxxxxx>
Cc: Peter Collingbourne <pcc@xxxxxxxxxx>
Cc: Vlastimil Babka <vbabka@xxxxxxx>
Cc: Will Deacon <will@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
Documentation/mm/highmem.rst | 7 +++++++
1 file changed, 7 insertions(+)
--- a/Documentation/mm/highmem.rst~documentation-mm-avoid-invalid-use-of-addresses-from-kmap_local_page
+++ a/Documentation/mm/highmem.rst
@@ -77,6 +77,13 @@ list shows them in order of preference o
for pages which are known to not come from ZONE_HIGHMEM. However, it is
always safe to use kmap_local_page() / kunmap_local().
+ While it is significantly faster than kmap(), for the higmem case it
+ comes with restrictions about the pointers validity. Contrary to kmap()
+ mappings, the local mappings are only valid in the context of the caller
+ and cannot be handed to other contexts. This implies that users must
+ be absolutely sure to keep the use of the return address local to the
+ thread which mapped it.
+
Nesting kmap_local_page() and kmap_atomic() mappings is allowed to a certain
extent (up to KMAP_TYPE_NR) but their invocations have to be strictly ordered
because the map implementation is stack based. See kmap_local_page() kdocs
_
Patches currently in -mm which might be from fmdefrancesco@xxxxxxxxx are
highmem-remove-unneeded-spaces-in-kmap_local_page-kdocs.patch
highmem-specify-that-kmap_local_page-is-callable-from-interrupts.patch
documentation-mm-dont-kmap-pages-which-cant-come-from-highmem.patch
documentation-mm-avoid-invalid-use-of-addresses-from-kmap_local_page.patch
documentation-mm-prefer-kmap_local_page-and-avoid-kmap.patch
highmem-delete-a-sentence-from-kmap_local_page-kdocs.patch
documentation-mm-add-details-about-kmap_local_page-and-preemption.patch
