* + xarray-fix-index-jumping-backwards-in-xas_find.patch added to mm-hotfixes-unstable branch
@ 2026-09-06 0:35 Andrew Morton
0 siblings, 0 replies; 2+ messages in thread
From: Andrew Morton @ 2026-09-06 0:35 UTC (permalink / raw)
To: mm-commits, willy, stable, rppt, krystianmkaniewski, akpm
The patch titled
Subject: xarray: fix index jumping backwards in xas_find()
has been added to the -mm mm-hotfixes-unstable branch. Its filename is
xarray-fix-index-jumping-backwards-in-xas_find.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/xarray-fix-index-jumping-backwards-in-xas_find.patch
This patch will later appear in the mm-hotfixes-unstable branch at
git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
Before you just go and hit "reply", please:
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------------------------------------------------------
From: Krystian Kaniewski <krystianmkaniewski@gmail.com>
Subject: xarray: fix index jumping backwards in xas_find()
Date: Fri, 4 Sep 2026 14:12:59 +0200
A bug in the XArray iterator xas_find() causes the iterator's index
(xas->xa_index) to jump backwards when iterating over a multi-index entry
(like a THP) that resides in a non-leaf node and is concurrently split.
When iterating over a multi-index entry in a non-leaf node, xas_load()
sets xas->xa_offset to the base offset of the entry, but leaves
xas->xa_index at the requested index. When the caller subsequently wants
to advance to the next entry, xas_find() is called. xas_find() attempts
to synchronize xas->xa_offset with xas->xa_index before advancing.
However, the fixup logic was incorrectly restricted to leaf nodes
(!xas->xa_node->shift). Because the THP resides in a non-leaf node, the
fixup is skipped.
As a result, xas_find() simply increments xas->xa_offset and recalculates
xas->xa_index based on this new offset. This causes xas->xa_index to jump
backwards. If the THP was concurrently split, the entry at the new offset
is a node pointer, so xas_find() descends into it and returns the folio at
the backwards index. The caller (filemap_map_pages()) then calculates the
PTE pointer based on this backwards index, resulting in an invalid memory
access such as an out-of-bounds read or use-after-free on a page-table
page freed via tlb_remove_table_rcu().
A userspace access that faults in a file-backed mapping can trigger this
path. When the index moves backwards, filemap_map_pages() can calculate a
PTE outside the page locked for fault-around and dereference a freed
page-table page, resulting in a KASAN-detected use-after-free read.
To fix this, check if xas->xa_offset matches get_offset(xas->xa_index,
xas->xa_node). If it does not and the node is a non-leaf node, set
xas->xa_offset to get_offset(xas->xa_index, xas->xa_node) before
advancing. Also add test cases in test_xarray to verify xas_find()
behavior when iterating over and splitting multi-index entries.
Link: https://lore.kernel.org/20260904121301.200049-1-krystianmkaniewski@gmail.com
Fixes: b803b42823d0 ("xarray: Add XArray iterators")
Assisted-by: Gemini:gemini-3.7-flash syzbot
Reported-by: syzbot+b72767277f29b6407083@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=b72767277f29b6407083
Link: https://syzkaller.appspot.com/ai_job?id=a01c56bd-74d0-411c-afb4-ee6f0cb6cb61
Signed-off-by: Krystian Kaniewski <krystianmkaniewski@gmail.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---
lib/test_xarray.c | 70 ++++++++++++++++++++++++++++++++++++++++++++
lib/xarray.c | 8 +++--
2 files changed, 75 insertions(+), 3 deletions(-)
--- a/lib/test_xarray.c~xarray-fix-index-jumping-backwards-in-xas_find
+++ a/lib/test_xarray.c
@@ -1247,6 +1247,75 @@ static noinline void check_multi_find_3(
}
}
+static noinline void check_multi_find_4(struct xarray *xa)
+{
+#ifdef CONFIG_XARRAY_MULTI
+ unsigned int order = XA_CHUNK_SHIFT + 1;
+ unsigned long next = 1UL << order;
+ unsigned long start = next - 1;
+ XA_STATE(xas, xa, start);
+ XA_STATE_ORDER(split, xa, 0, 0);
+ void *entry;
+ unsigned long i;
+
+ /* Multi-index entry in two slots of a non-leaf node. */
+ xa_store_order(xa, 0, order, xa_mk_index(0), GFP_KERNEL);
+ XA_BUG_ON(xa, xa_store_index(xa, next, GFP_KERNEL) != NULL);
+
+ rcu_read_lock();
+ entry = xas_find(&xas, ULONG_MAX);
+ XA_BUG_ON(xa, entry != xa_mk_index(0));
+ XA_BUG_ON(xa, xas.xa_index != start);
+
+ entry = xas_find(&xas, ULONG_MAX);
+ XA_BUG_ON(xa, entry != xa_mk_index(next));
+ XA_BUG_ON(xa, xas.xa_index != next);
+
+ entry = xas_find(&xas, ULONG_MAX);
+ XA_BUG_ON(xa, entry != NULL);
+ rcu_read_unlock();
+
+ xa_erase_index(xa, next);
+ xa_erase_index(xa, 0);
+ XA_BUG_ON(xa, !xa_empty(xa));
+
+ /* Split the multi-index entry after a lookup begins inside it. */
+ xa_store_order(xa, 0, order, xa_mk_index(0), GFP_KERNEL);
+ XA_BUG_ON(xa, xa_store_index(xa, next, GFP_KERNEL) != NULL);
+
+ xas_set(&xas, start);
+ rcu_read_lock();
+ entry = xas_find(&xas, ULONG_MAX);
+ XA_BUG_ON(xa, entry != xa_mk_index(0));
+ XA_BUG_ON(xa, xas.xa_index != start);
+ rcu_read_unlock();
+
+ xas_split_alloc(&split, xa_mk_index(0), order, GFP_KERNEL);
+ if (xas_error(&split)) {
+ XA_BUG_ON(xa, true);
+ goto out;
+ }
+ xas_lock(&split);
+ xas_split(&split, xa_mk_index(0), order);
+ for (i = 0; i < next; i++)
+ __xa_store(xa, i, xa_mk_index(i), 0);
+ xas_unlock(&split);
+
+ rcu_read_lock();
+ entry = xas_find(&xas, ULONG_MAX);
+ XA_BUG_ON(xa, entry != xa_mk_index(next));
+ XA_BUG_ON(xa, xas.xa_index != next);
+
+ entry = xas_find(&xas, ULONG_MAX);
+ XA_BUG_ON(xa, entry != NULL);
+ rcu_read_unlock();
+
+out:
+ xa_destroy(xa);
+ XA_BUG_ON(xa, !xa_empty(xa));
+#endif
+}
+
static noinline void check_find_1(struct xarray *xa)
{
unsigned long i, j, k;
@@ -1370,6 +1439,7 @@ static noinline void check_find(struct x
check_multi_find_1(xa, i);
check_multi_find_2(xa);
check_multi_find_3(xa);
+ check_multi_find_4(xa);
}
/* See find_swap_entry() in mm/shmem.c */
--- a/lib/xarray.c~xarray-fix-index-jumping-backwards-in-xas_find
+++ a/lib/xarray.c
@@ -1409,9 +1409,11 @@ void *xas_find(struct xa_state *xas, uns
entry = xas_load(xas);
if (entry || xas_not_node(xas->xa_node))
return entry;
- } else if (!xas->xa_node->shift &&
- xas->xa_offset != (xas->xa_index & XA_CHUNK_MASK)) {
- xas->xa_offset = ((xas->xa_index - 1) & XA_CHUNK_MASK) + 1;
+ } else if (xas->xa_offset != get_offset(xas->xa_index, xas->xa_node)) {
+ if (!xas->xa_node->shift)
+ xas->xa_offset = ((xas->xa_index - 1) & XA_CHUNK_MASK) + 1;
+ else
+ xas->xa_offset = get_offset(xas->xa_index, xas->xa_node);
}
xas_next_offset(xas);
_
Patches currently in -mm which might be from krystianmkaniewski@gmail.com are
xarray-fix-index-jumping-backwards-in-xas_find.patch
^ permalink raw reply [flat|nested] 2+ messages in thread* + xarray-fix-index-jumping-backwards-in-xas_find.patch added to mm-hotfixes-unstable branch
@ 2026-09-03 18:22 Andrew Morton
0 siblings, 0 replies; 2+ messages in thread
From: Andrew Morton @ 2026-09-03 18:22 UTC (permalink / raw)
To: mm-commits, willy, stable, krystianmkaniewski, akpm
The patch titled
Subject: xarray: fix index jumping backwards in xas_find()
has been added to the -mm mm-hotfixes-unstable branch. Its filename is
xarray-fix-index-jumping-backwards-in-xas_find.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/xarray-fix-index-jumping-backwards-in-xas_find.patch
This patch will later appear in the mm-hotfixes-unstable branch at
git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
Before you just go and hit "reply", please:
a) Consider who else should be cc'ed
b) Prefer to cc a suitable mailing list as well
c) Ideally: find the original patch on the mailing list and do a
reply-to-all to that, adding suitable additional cc's
*** Remember to use Documentation/process/submit-checklist.rst when testing your code ***
The -mm tree is included into linux-next via various
branches at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
and is updated there most days
------------------------------------------------------
From: Krystian Kaniewski <krystianmkaniewski@gmail.com>
Subject: xarray: fix index jumping backwards in xas_find()
Date: Thu, 3 Sep 2026 13:48:55 +0000 (UTC)
A bug in the XArray iterator xas_find() causes the iterator's index
(xas->xa_index) to jump backwards when iterating over a multi-index entry
(like a THP) that resides in a non-leaf node and is concurrently split.
This results in a use-after-free in ptep_get(), detected by KASAN [1].
When iterating over a multi-index entry in a non-leaf node, xas_load() sets
xas->xa_offset to the base offset of the entry, but leaves xas->xa_index at
the requested index. When the caller subsequently wants to advance to the
next entry, xas_find() is called. xas_find() attempts to synchronize
xas->xa_offset with xas->xa_index before advancing. However, the fixup
logic was incorrectly restricted to leaf nodes (!xas->xa_node->shift).
Because the THP resides in a non-leaf node, the fixup is skipped.
As a result, xas_find() simply increments xas->xa_offset and recalculates
xas->xa_index based on this new offset. This causes xas->xa_index to jump
backwards. If the THP was concurrently split, the entry at the new offset
is a node pointer, so xas_find() descends into it and returns the folio at
the backwards index. The caller (filemap_map_pages()) then calculates the
PTE pointer based on this backwards index, resulting in an invalid memory
access such as an out-of-bounds read or use-after-free on a page-table page
freed via tlb_remove_table_rcu().
To fix this, check if xas->xa_offset matches get_offset(xas->xa_index,
xas->xa_node). If it does not and the node is a non-leaf node, set
xas->xa_offset to get_offset(xas->xa_index, xas->xa_node) before advancing.
Also add test cases in test_xarray to verify xas_find() behavior when
iterating over and splitting multi-index entries.
Link: https://lore.kernel.org/2992424b-2120-489e-9010-f45f46ed52c8@mail.kernel.org
Fixes: b803b42823d0 ("xarray: Add XArray iterators")
Signed-off-by: Krystian Kaniewski <krystianmkaniewski@gmail.com>
Assisted-by: Gemini:gemini-3.7-flash syzbot
Reported-by: syzbot+b72767277f29b6407083@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=b72767277f29b6407083 [1]
Link: https://syzkaller.appspot.com/ai_job?id=a01c56bd-74d0-411c-afb4-ee6f0cb6cb61
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---
lib/test_xarray.c | 62 ++++++++++++++++++++++++++++++++++++++++++++
lib/xarray.c | 8 +++--
2 files changed, 67 insertions(+), 3 deletions(-)
--- a/lib/test_xarray.c~xarray-fix-index-jumping-backwards-in-xas_find
+++ a/lib/test_xarray.c
@@ -1247,6 +1247,67 @@ static noinline void check_multi_find_3(
}
}
+static noinline void check_multi_find_4(struct xarray *xa)
+{
+#ifdef CONFIG_XARRAY_MULTI
+ XA_STATE(xas, xa, 100);
+ XA_STATE_ORDER(split, xa, 0, 0);
+ void *entry;
+ unsigned long i;
+
+ /* (1) Order-7 entry (0-127) with adjacent entry at 128, starting at 100 */
+ xa_store_order(xa, 0, 7, xa_mk_index(0), GFP_KERNEL);
+ XA_BUG_ON(xa, xa_store_index(xa, 128, GFP_KERNEL) != NULL);
+
+ rcu_read_lock();
+ entry = xas_find(&xas, ULONG_MAX);
+ XA_BUG_ON(xa, entry != xa_mk_index(0));
+ XA_BUG_ON(xa, xas.xa_index != 100);
+
+ entry = xas_find(&xas, ULONG_MAX);
+ XA_BUG_ON(xa, entry != xa_mk_index(128));
+ XA_BUG_ON(xa, xas.xa_index != 128);
+
+ entry = xas_find(&xas, ULONG_MAX);
+ XA_BUG_ON(xa, entry != NULL);
+ rcu_read_unlock();
+
+ xa_erase_index(xa, 128);
+ xa_erase_index(xa, 0);
+ XA_BUG_ON(xa, !xa_empty(xa));
+
+ /* (2) Splitting a multi-index entry after a lookup begins inside it */
+ xa_store_order(xa, 0, 7, xa_mk_index(0), GFP_KERNEL);
+ XA_BUG_ON(xa, xa_store_index(xa, 128, GFP_KERNEL) != NULL);
+
+ xas_set(&xas, 100);
+ rcu_read_lock();
+ entry = xas_find(&xas, ULONG_MAX);
+ XA_BUG_ON(xa, entry != xa_mk_index(0));
+ XA_BUG_ON(xa, xas.xa_index != 100);
+ rcu_read_unlock();
+
+ xas_split_alloc(&split, xa_mk_index(0), 7, GFP_KERNEL);
+ xas_lock(&split);
+ xas_split(&split, xa_mk_index(0), 7);
+ for (i = 0; i < 128; i++)
+ __xa_store(xa, i, xa_mk_index(i), 0);
+ xas_unlock(&split);
+
+ rcu_read_lock();
+ entry = xas_find(&xas, ULONG_MAX);
+ XA_BUG_ON(xa, entry != xa_mk_index(128));
+ XA_BUG_ON(xa, xas.xa_index != 128);
+
+ entry = xas_find(&xas, ULONG_MAX);
+ XA_BUG_ON(xa, entry != NULL);
+ rcu_read_unlock();
+
+ xa_destroy(xa);
+ XA_BUG_ON(xa, !xa_empty(xa));
+#endif
+}
+
static noinline void check_find_1(struct xarray *xa)
{
unsigned long i, j, k;
@@ -1370,6 +1431,7 @@ static noinline void check_find(struct x
check_multi_find_1(xa, i);
check_multi_find_2(xa);
check_multi_find_3(xa);
+ check_multi_find_4(xa);
}
/* See find_swap_entry() in mm/shmem.c */
--- a/lib/xarray.c~xarray-fix-index-jumping-backwards-in-xas_find
+++ a/lib/xarray.c
@@ -1409,9 +1409,11 @@ void *xas_find(struct xa_state *xas, uns
entry = xas_load(xas);
if (entry || xas_not_node(xas->xa_node))
return entry;
- } else if (!xas->xa_node->shift &&
- xas->xa_offset != (xas->xa_index & XA_CHUNK_MASK)) {
- xas->xa_offset = ((xas->xa_index - 1) & XA_CHUNK_MASK) + 1;
+ } else if (xas->xa_offset != get_offset(xas->xa_index, xas->xa_node)) {
+ if (!xas->xa_node->shift)
+ xas->xa_offset = ((xas->xa_index - 1) & XA_CHUNK_MASK) + 1;
+ else
+ xas->xa_offset = get_offset(xas->xa_index, xas->xa_node);
}
xas_next_offset(xas);
_
Patches currently in -mm which might be from krystianmkaniewski@gmail.com are
xarray-fix-index-jumping-backwards-in-xas_find.patch
^ permalink raw reply [flat|nested] 2+ messages in thread
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