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From:   Zhang Zhiyu <zhiyuzhang999@gmail.com>
Date:   Mon, 7 Aug 2023 23:54:20 +0800
Message-ID: <CALf2hKtDJGqmsiSykbX8EEfbthwt6a4Bs98m60dUkS7URW-C8g@mail.gmail.com>
Subject: KASAN: slab-use-after-free Read in radix_tree_lookup in&after Linux
 Kernel 6.4-rc6
To:     linux-kernel@vger.kernel.org
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Hi,

I found a KASAN: slab-use-after-free Read in radix_tree_lookup while
fuzzing Linux kernel 6.4-rc6 with my modified syzkaller in 24 July.

The report, log, and config can be downloaded from:
https://drive.google.com/file/d/1KiZCUHEyp-_Mbq8wdXvjPLs6KU-12JwM/view?usp=sharing

Here is the bug-related key info:

BUG: KASAN: slab-use-after-free in radix_tree_descend
lib/radix-tree.c:87 [inline]
BUG: KASAN: slab-use-after-free in __radix_tree_lookup
lib/radix-tree.c:764 [inline]
BUG: KASAN: slab-use-after-free in radix_tree_lookup+0x12d/0x290
lib/radix-tree.c:817
Read of size 8 at addr ffff8880675ab1a8 by task kswapd0/88

I have preliminarily anlyzed the root cause. The suspected UAF Read is
located in the __radix_tree_lookup function, which is part of the
Radix Tree implementation in the Linux kernel. The condition check
radix_tree_is_internal_node(node) in the while loop is used to verify
if the node is an internal node. However, this check does not fully
guarantee that the passed pointer node is always valid. If an invalid
node pointer is passed to this function, it could lead to undefined
behavior, potentially including a Use-After-Free Read.

As this is a data race uaf, the syzkaller* cannot easily generate POC.
I am still trying to construct a POC. By comparing the code of
radix-tree.c, the vulnerability affects versions ranging from 6.4-rc6
to the latest mainline.

I recommend to patch it by adding invalid check of node in
__radix_tree_lookup, if it is validated as a bug.

Best. Have a good day!
Zhiyu Zhang