linux-kernel - [PATCH AUTOSEL 6.6 18/20] lib/generic-radix-tree.c: Fix rare race in __genradix_ptr

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Message-ID: <20241012112715.1763241-18-sashal@kernel.org>
Date: Sat, 12 Oct 2024 07:26:50 -0400
From: Sasha Levin <sashal@...nel.org>
To: linux-kernel@...r.kernel.org,
	stable@...r.kernel.org
Cc: Kent Overstreet <kent.overstreet@...ux.dev>,
	Sasha Levin <sashal@...nel.org>
Subject: [PATCH AUTOSEL 6.6 18/20] lib/generic-radix-tree.c: Fix rare race in __genradix_ptr_alloc()

From: Kent Overstreet <kent.overstreet@...ux.dev>

[ Upstream commit b2f11c6f3e1fc60742673b8675c95b78447f3dae ]

If we need to increase the tree depth, allocate a new node, and then
race with another thread that increased the tree depth before us, we'll
still have a preallocated node that might be used later.

If we then use that node for a new non-root node, it'll still have a
pointer to the old root instead of being zeroed - fix this by zeroing it
in the cmpxchg failure path.

Signed-off-by: Kent Overstreet <kent.overstreet@...ux.dev>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 lib/generic-radix-tree.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/lib/generic-radix-tree.c b/lib/generic-radix-tree.c
index 7dfa88282b006..78f081d695d0b 100644
--- a/lib/generic-radix-tree.c
+++ b/lib/generic-radix-tree.c
@@ -131,6 +131,8 @@ void *__genradix_ptr_alloc(struct __genradix *radix, size_t offset,
 		if ((v = cmpxchg_release(&radix->root, r, new_root)) == r) {
 			v = new_root;
 			new_node = NULL;
+		} else {
+			new_node->children[0] = NULL;
 		}
 	}
 
-- 
2.43.0