* [PATCH] xarray: fix index jumping backwards in xas_find()
@ 2026-09-03 13:48 syzbot
2026-09-03 18:22 ` Andrew Morton
0 siblings, 1 reply; 6+ messages in thread
From: syzbot @ 2026-09-03 13:48 UTC (permalink / raw)
To: syzkaller-bugs, Krystian Kaniewski, Andrew Morton, linux-fsdevel,
linux-mm, Matthew Wilcox
Cc: linux-kernel, syzbot
From: Krystian Kaniewski <krystianmkaniewski@gmail.com>
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().
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.
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>
---
diff --git a/lib/test_xarray.c b/lib/test_xarray.c
index 5ca0aefee..cda7245e4 100644
--- a/lib/test_xarray.c
+++ b/lib/test_xarray.c
@@ -1247,6 +1247,67 @@ static noinline void check_multi_find_3(struct xarray *xa)
}
}
+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 xarray *xa)
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 */
diff --git a/lib/xarray.c b/lib/xarray.c
index 9a8b49165..980324d68 100644
--- a/lib/xarray.c
+++ b/lib/xarray.c
@@ -1406,9 +1406,11 @@ void *xas_find(struct xa_state *xas, unsigned long max)
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);
base-commit: 8d3ae59288f1e7d58d76558a6ee96d533bc5019f
--
See https://goo.gle/syzbot-ai-patches for information about AI-generated patches.
The person who has signed off on the patch is responsible for
addressing comments.
syzbot engineers can be reached at syzkaller@googlegroups.com.
^ permalink raw reply related [flat|nested] 6+ messages in thread
* Re: [PATCH] xarray: fix index jumping backwards in xas_find()
2026-09-03 13:48 [PATCH] xarray: fix index jumping backwards in xas_find() syzbot
@ 2026-09-03 18:22 ` Andrew Morton
2026-09-04 8:21 ` Mike Rapoport
0 siblings, 1 reply; 6+ messages in thread
From: Andrew Morton @ 2026-09-03 18:22 UTC (permalink / raw)
To: syzbot
Cc: syzkaller-bugs, Krystian Kaniewski, linux-fsdevel, linux-mm,
Matthew Wilcox, linux-kernel, syzbot
On Thu, 3 Sep 2026 13:48:55 +0000 (UTC) "syzbot" <syzbot@kernel.org> wrote:
> From: Krystian Kaniewski <krystianmkaniewski@gmail.com>
>
> 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().
>
> 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.
Thanks. But the changelog omits vital information: a description of
the userspace-visible runtime effects of the bug.
> 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
I see from there that it was a use-after-free, detected by KASAN. I'll
update the changelog and I'll add cc:stable.
I'll queue the patch for testing while awaiting review input.
> Link: https://syzkaller.appspot.com/ai_job?id=a01c56bd-74d0-411c-afb4-ee6f0cb6cb61
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [PATCH] xarray: fix index jumping backwards in xas_find()
2026-09-03 18:22 ` Andrew Morton
@ 2026-09-04 8:21 ` Mike Rapoport
2026-09-04 8:41 ` Krystian Kaniewski
2026-09-04 12:12 ` [PATCH v2] " Krystian Kaniewski
0 siblings, 2 replies; 6+ messages in thread
From: Mike Rapoport @ 2026-09-04 8:21 UTC (permalink / raw)
To: Andrew Morton
Cc: syzbot, syzkaller-bugs, Krystian Kaniewski, linux-fsdevel,
linux-mm, Matthew Wilcox, linux-kernel, syzbot
On Thu, Sep 03, 2026 at 11:22:51AM -0700, Andrew Morton wrote:
> On Thu, 3 Sep 2026 13:48:55 +0000 (UTC) "syzbot" <syzbot@kernel.org> wrote:
>
> > From: Krystian Kaniewski <krystianmkaniewski@gmail.com>
> >
> > 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().
> >
> > 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.
>
> Thanks. But the changelog omits vital information: a description of
> the userspace-visible runtime effects of the bug.
>
> > 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
>
> I see from there that it was a use-after-free, detected by KASAN. I'll
> update the changelog and I'll add cc:stable.
>
> I'll queue the patch for testing while awaiting review input.
The testing shows there are issues with this patch:
https://github.com/linux-mm/linux-mm/actions/runs/33815757662/job/100847425973
And since it's in mm-unstable now it breaks all CI jobs :(
> > Link: https://syzkaller.appspot.com/ai_job?id=a01c56bd-74d0-411c-afb4-ee6f0cb6cb61
>
>
--
Sincerely yours,
Mike.
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [PATCH] xarray: fix index jumping backwards in xas_find()
2026-09-04 8:21 ` Mike Rapoport
@ 2026-09-04 8:41 ` Krystian Kaniewski
2026-09-04 12:16 ` Krystian Kaniewski
2026-09-04 12:12 ` [PATCH v2] " Krystian Kaniewski
1 sibling, 1 reply; 6+ messages in thread
From: Krystian Kaniewski @ 2026-09-04 8:41 UTC (permalink / raw)
To: Mike Rapoport
Cc: Andrew Morton, syzbot, syzkaller-bugs, linux-fsdevel, linux-mm,
Matthew Wilcox, linux-kernel, syzbot
[-- Attachment #1: Type: text/plain, Size: 3315 bytes --]
Hello,
Thanks for running tests on my patch. I've quickly looked into the test
result and the problem seems to be related to check_multi_find_4() test and
not the fix itself. I'll send a v2 patch once I get the test fixed.
pt., 4 wrz 2026 o 10:22 Mike Rapoport <rppt@kernel.org> napisał(a):
> On Thu, Sep 03, 2026 at 11:22:51AM -0700, Andrew Morton wrote:
> > On Thu, 3 Sep 2026 13:48:55 +0000 (UTC) "syzbot" <syzbot@kernel.org>
> wrote:
> >
> > > From: Krystian Kaniewski <krystianmkaniewski@gmail.com>
> > >
> > > 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().
> > >
> > > 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.
> >
> > Thanks. But the changelog omits vital information: a description of
> > the userspace-visible runtime effects of the bug.
> >
> > > 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
> >
> > I see from there that it was a use-after-free, detected by KASAN. I'll
> > update the changelog and I'll add cc:stable.
> >
> > I'll queue the patch for testing while awaiting review input.
>
> The testing shows there are issues with this patch:
>
>
> https://github.com/linux-mm/linux-mm/actions/runs/33815757662/job/100847425973
>
> And since it's in mm-unstable now it breaks all CI jobs :(
>
> > > Link:
> https://syzkaller.appspot.com/ai_job?id=a01c56bd-74d0-411c-afb4-ee6f0cb6cb61
> >
> >
>
> --
> Sincerely yours,
> Mike.
>
[-- Attachment #2: Type: text/html, Size: 4684 bytes --]
^ permalink raw reply [flat|nested] 6+ messages in thread
* [PATCH v2] xarray: fix index jumping backwards in xas_find()
2026-09-04 8:21 ` Mike Rapoport
2026-09-04 8:41 ` Krystian Kaniewski
@ 2026-09-04 12:12 ` Krystian Kaniewski
1 sibling, 0 replies; 6+ messages in thread
From: Krystian Kaniewski @ 2026-09-04 12:12 UTC (permalink / raw)
To: Andrew Morton
Cc: Matthew Wilcox, Mike Rapoport, linux-fsdevel, linux-mm,
linux-kernel, syzbot+b72767277f29b6407083, stable
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.
Fixes: b803b42823d0 ("xarray: Add XArray iterators")
Cc: <stable@vger.kernel.org>
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>
---
Changes in v2:
- Derive the regression-test geometry from XA_CHUNK_SHIFT so it works with
both the userspace harness (SHIFT=3) and the kernel (SHIFT=6).
- Check the xas_split_alloc() result before calling xas_split().
- Describe the userspace-triggered use-after-free and add the stable Cc.
v1: https://lore.kernel.org/all/2992424b-2120-489e-9010-f45f46ed52c8@mail.kernel.org/
Tests:
- Applied with git am to linux-mm commit b0266eddcb10 (the base of the
reported CI run).
- Ubuntu 24.04 x86_64 mm-ci host tests: memblock, VMA, Maple tree, XArray,
Radix tree and IDA, Multiorder XArray, and IDR all passed.
- Userspace XArray tests with ASan/UBSan passed for SHIFT=3 and SHIFT=6.
- The built-in XArray test suite passed under generic KASAN.
lib/test_xarray.c | 70 +++++++++++++++++++++++++++++++++++++++++++++++
lib/xarray.c | 8 ++++--
2 files changed, 75 insertions(+), 3 deletions(-)
diff --git a/lib/test_xarray.c b/lib/test_xarray.c
index 5ca0aefee9aa..615f7730a5bd 100644
--- a/lib/test_xarray.c
+++ b/lib/test_xarray.c
@@ -1247,6 +1247,75 @@ static noinline void check_multi_find_3(struct xarray *xa)
}
}
+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 xarray *xa)
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 */
diff --git a/lib/xarray.c b/lib/xarray.c
index 9a8b4916540c..980324d686bd 100644
--- a/lib/xarray.c
+++ b/lib/xarray.c
@@ -1406,9 +1406,11 @@ void *xas_find(struct xa_state *xas, unsigned long max)
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);
base-commit: 8d3ae59288f1e7d58d76558a6ee96d533bc5019f
--
2.53.0
^ permalink raw reply related [flat|nested] 6+ messages in thread
* Re: [PATCH] xarray: fix index jumping backwards in xas_find()
2026-09-04 8:41 ` Krystian Kaniewski
@ 2026-09-04 12:16 ` Krystian Kaniewski
0 siblings, 0 replies; 6+ messages in thread
From: Krystian Kaniewski @ 2026-09-04 12:16 UTC (permalink / raw)
To: Mike Rapoport
Cc: Andrew Morton, syzbot, syzkaller-bugs, linux-fsdevel, linux-mm,
Matthew Wilcox, linux-kernel, syzbot
I have fixed the test issue and submitted patch as v2. I'm sorry about
issue v1 caused, hopefully v2 is going to be fully correct now.
On 9/4/2026 10:41 AM, Krystian Kaniewski wrote:
> Hello,
>
> Thanks for running tests on my patch. I've quickly looked into the test
> result and the problem seems to be related to check_multi_find_4() test and
> not the fix itself. I'll send a v2 patch once I get the test fixed.
>
> pt., 4 wrz 2026 o 10:22 Mike Rapoport <rppt@kernel.org> napisał(a):
>
>> On Thu, Sep 03, 2026 at 11:22:51AM -0700, Andrew Morton wrote:
>>> On Thu, 3 Sep 2026 13:48:55 +0000 (UTC) "syzbot" <syzbot@kernel.org>
>> wrote:
>>>> From: Krystian Kaniewski <krystianmkaniewski@gmail.com>
>>>>
>>>> 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().
>>>>
>>>> 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.
>>> Thanks. But the changelog omits vital information: a description of
>>> the userspace-visible runtime effects of the bug.
>>>
>>>> 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
>>> I see from there that it was a use-after-free, detected by KASAN. I'll
>>> update the changelog and I'll add cc:stable.
>>>
>>> I'll queue the patch for testing while awaiting review input.
>> The testing shows there are issues with this patch:
>>
>>
>> https://github.com/linux-mm/linux-mm/actions/runs/33815757662/job/100847425973
>>
>> And since it's in mm-unstable now it breaks all CI jobs :(
>>
>>>> Link:
>> https://syzkaller.appspot.com/ai_job?id=a01c56bd-74d0-411c-afb4-ee6f0cb6cb61
>>>
>> --
>> Sincerely yours,
>> Mike.
>>
^ permalink raw reply [flat|nested] 6+ messages in thread
end of thread, other threads:[~2026-09-04 12:16 UTC | newest]
Thread overview: 6+ messages (download: mbox.gz follow: Atom feed
-- links below jump to the message on this page --
2026-09-03 13:48 [PATCH] xarray: fix index jumping backwards in xas_find() syzbot
2026-09-03 18:22 ` Andrew Morton
2026-09-04 8:21 ` Mike Rapoport
2026-09-04 8:41 ` Krystian Kaniewski
2026-09-04 12:16 ` Krystian Kaniewski
2026-09-04 12:12 ` [PATCH v2] " Krystian Kaniewski
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