[PATCH 5.17 057/146] XArray: Disallow sibling entries of nodes

From: Greg Kroah-Hartman
Date: Tue Apr 26 2022 - 05:27:33 EST

From: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>

[ Upstream commit 63b1898fffcd8bd81905b95104ecc52b45a97e21 ]

There is a race between xas_split() and xas_load() which can result in
the wrong page being returned, and thus data corruption. Fortunately,
it's hard to hit (syzbot took three months to find it) and often guarded
with VM_BUG_ON().

The anatomy of this race is:

thread A thread B
order-9 page is stored at index 0x200
lookup of page at index 0x274
page split starts
load of sibling entry at offset 9
stores nodes at offsets 8-15
load of entry at offset 8

The entry at offset 8 turns out to be a node, and so we descend into it,
and load the page at index 0x234 instead of 0x274. This is hard to fix
on the split side; we could replace the entire node that contains the
order-9 page instead of replacing the eight entries. Fixing it on
the lookup side is easier; just disallow sibling entries that point
to nodes. This cannot ever be a useful thing as the descent would not
know the correct offset to use within the new node.

The test suite continues to pass, but I have not added a new test for
this bug.

Reported-by: syzbot+cf4cf13056f85dec2c40@xxxxxxxxxxxxxxxxxxxxxxxxx
Tested-by: syzbot+cf4cf13056f85dec2c40@xxxxxxxxxxxxxxxxxxxxxxxxx
Fixes: 6b24ca4a1a8d ("mm: Use multi-index entries in the page cache")
Signed-off-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
lib/xarray.c | 2 ++
1 file changed, 2 insertions(+)

diff --git a/lib/xarray.c b/lib/xarray.c
index 88ca87435e3d..32e1669d5b64 100644
--- a/lib/xarray.c
+++ b/lib/xarray.c
@@ -207,6 +207,8 @@ static void *xas_descend(struct xa_state *xas, struct xa_node *node)
if (xa_is_sibling(entry)) {
offset = xa_to_sibling(entry);
entry = xa_entry(xas->xa, node, offset);
+ if (node->shift && xa_is_node(entry))
+ entry = XA_RETRY_ENTRY;

xas->xa_offset = offset;