[PATCH 2/2] maple_tree: reset mas->index and mas->last on write retries

From: Sidhartha Kumar
Date: Thu Aug 08 2024 - 12:30:24 EST


The following scenario can result in a race condition:

Consider a node with the following indices and values

a<------->b<----------->c<--------->d
0xA NULL 0xB

CPU 1 CPU 2
--------- ---------
mas_set_range(a,b)
mas_erase()
-> range is expanded (a,c) because of null expansion

mas_nomem()
mas_unlock()
mas_store_range(b,c,0xC)

The node now looks like:

a<------->b<----------->c<--------->d
0xA 0xC 0xB

mas_lock()
mas_erase() <------ range of erase is still (a,c)

The node is now NULL from (a,c) but the write from CPU 2 should have been
retained and range (b,c) should still have 0xC as its value. We can fix
this by re-intializing to the original index and last. This does not need
a cc: Stable as there are no users of the maple tree which use internal
locking and this condition is only possible with internal locking.

Signed-off-by: Sidhartha Kumar <sidhartha.kumar@xxxxxxxxxx>
---
lib/maple_tree.c | 15 ++++++++++++---
1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/lib/maple_tree.c b/lib/maple_tree.c
index 65fba37ef999..6ba95a278326 100644
--- a/lib/maple_tree.c
+++ b/lib/maple_tree.c
@@ -5451,14 +5451,21 @@ EXPORT_SYMBOL_GPL(mas_store);
*/
int mas_store_gfp(struct ma_state *mas, void *entry, gfp_t gfp)
{
+ unsigned long index = mas->index;
+ unsigned long last = mas->last;
MA_WR_STATE(wr_mas, mas, entry);

mas_wr_store_setup(&wr_mas);
trace_ma_write(__func__, mas, 0, entry);
retry:
mas_wr_store_entry(&wr_mas);
- if (unlikely(mas_nomem(mas, gfp)))
+ if (unlikely(mas_nomem(mas, gfp))) {
+ if (!entry) {
+ mas->index = index;
+ mas->last = last;
+ }
goto retry;
+ }

if (unlikely(mas_is_err(mas)))
return xa_err(mas->node);
@@ -6245,17 +6252,19 @@ EXPORT_SYMBOL_GPL(mas_find_range_rev);
void *mas_erase(struct ma_state *mas)
{
void *entry;
+ unsigned long index = mas->index;
MA_WR_STATE(wr_mas, mas, NULL);

if (!mas_is_active(mas) || !mas_is_start(mas))
mas->status = ma_start;

- /* Retry unnecessary when holding the write lock. */
+write_retry:
+ /* reset mas->index and mas->last in case range of entry changed */
+ mas->index = mas->last = index;
entry = mas_state_walk(mas);
if (!entry)
return NULL;

-write_retry:
/* Must reset to ensure spanning writes of last slot are detected */
mas_reset(mas);
mas_wr_store_setup(&wr_mas);
--
2.46.0