From: Matthew Wilcox (Oracle) willy@infradead.org
[ 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@syzkaller.appspotmail.com Tested-by: syzbot+cf4cf13056f85dec2c40@syzkaller.appspotmail.com Fixes: 6b24ca4a1a8d ("mm: Use multi-index entries in the page cache") Signed-off-by: Matthew Wilcox (Oracle) willy@infradead.org Signed-off-by: Sasha Levin sashal@kernel.org --- 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;