Re: [PATCH] nilfs2: use refcount_dec_and_lock() to fix potential UAF

[Ryusuke Konishi <konishi.ryusuke@gmail.com>]

Hi Andrew,

Please queue this patch additionally for the next merge window.

Thanks,
Ryusuke Konishi

On Wed, Aug 25, 2021 at 11:43 AM Ryusuke Konishi
<konishi.ryusuke@gmail.com> wrote:
>
> From: Zhen Lei <thunder.leizhen@huawei.com>
>
> When the refcount is decreased to 0, the resource reclamation branch is
> entered. Before CPU0 reaches the race point (1), CPU1 may obtain the
> spinlock and traverse the rbtree to find 'root', see nilfs_lookup_root().
> Although CPU1 will call refcount_inc() to increase the refcount, it is
> obviously too late. CPU0 will release 'root' directly, CPU1 then accesses
> 'root' and triggers UAF.
>
> Use refcount_dec_and_lock() to ensure that both the operations of decrease
> refcount to 0 and link deletion are lock protected eliminates this risk.
>
>      CPU0                      CPU1
> nilfs_put_root():
>                             <-------- (1)
> spin_lock(&nilfs->ns_cptree_lock);
> rb_erase(&root->rb_node, &nilfs->ns_cptree);
> spin_unlock(&nilfs->ns_cptree_lock);
>
> kfree(root);
>                             <-------- use-after-free
>
> ========================================================================
> refcount_t: underflow; use-after-free.
> WARNING: CPU: 2 PID: 9476 at lib/refcount.c:28 \
> refcount_warn_saturate+0x1cf/0x210 lib/refcount.c:28
> Modules linked in:
> CPU: 2 PID: 9476 Comm: syz-executor.0 Not tainted 5.10.45-rc1+ #3
> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), ...
> RIP: 0010:refcount_warn_saturate+0x1cf/0x210 lib/refcount.c:28
> ... ...
> Call Trace:
>  __refcount_sub_and_test include/linux/refcount.h:283 [inline]
>  __refcount_dec_and_test include/linux/refcount.h:315 [inline]
>  refcount_dec_and_test include/linux/refcount.h:333 [inline]
>  nilfs_put_root+0xc1/0xd0 fs/nilfs2/the_nilfs.c:795
>  nilfs_segctor_destroy fs/nilfs2/segment.c:2749 [inline]
>  nilfs_detach_log_writer+0x3fa/0x570 fs/nilfs2/segment.c:2812
>  nilfs_put_super+0x2f/0xf0 fs/nilfs2/super.c:467
>  generic_shutdown_super+0xcd/0x1f0 fs/super.c:464
>  kill_block_super+0x4a/0x90 fs/super.c:1446
>  deactivate_locked_super+0x6a/0xb0 fs/super.c:335
>  deactivate_super+0x85/0x90 fs/super.c:366
>  cleanup_mnt+0x277/0x2e0 fs/namespace.c:1118
>  __cleanup_mnt+0x15/0x20 fs/namespace.c:1125
>  task_work_run+0x8e/0x110 kernel/task_work.c:151
>  tracehook_notify_resume include/linux/tracehook.h:188 [inline]
>  exit_to_user_mode_loop kernel/entry/common.c:164 [inline]
>  exit_to_user_mode_prepare+0x13c/0x170 kernel/entry/common.c:191
>  syscall_exit_to_user_mode+0x16/0x30 kernel/entry/common.c:266
>  do_syscall_64+0x45/0x80 arch/x86/entry/common.c:56
>  entry_SYSCALL_64_after_hwframe+0x44/0xa9
>
> There is no reproduction program, and the above is only theoretical
> analysis.
>
> Fixes: ba65ae4729bf ("nilfs2: add checkpoint tree to nilfs object")
> Link: https://lkml.kernel.org/r/20210723012317.4146-1-thunder.leizhen@huawei.com
> Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
> Signed-off-by: Ryusuke Konishi <konishi.ryusuke@gmail.com>
> ---
>  fs/nilfs2/the_nilfs.c | 9 ++++-----
>  1 file changed, 4 insertions(+), 5 deletions(-)
>
> diff --git a/fs/nilfs2/the_nilfs.c b/fs/nilfs2/the_nilfs.c
> index 8b7b01a380ce..c8bfc01da5d7 100644
> --- a/fs/nilfs2/the_nilfs.c
> +++ b/fs/nilfs2/the_nilfs.c
> @@ -792,14 +792,13 @@ struct nilfs_root *
>
>  void nilfs_put_root(struct nilfs_root *root)
>  {
> -       if (refcount_dec_and_test(&root->count)) {
> -               struct the_nilfs *nilfs = root->nilfs;
> +       struct the_nilfs *nilfs = root->nilfs;
>
> -               nilfs_sysfs_delete_snapshot_group(root);
> -
> -               spin_lock(&nilfs->ns_cptree_lock);
> +       if (refcount_dec_and_lock(&root->count, &nilfs->ns_cptree_lock)) {
>                 rb_erase(&root->rb_node, &nilfs->ns_cptree);
>                 spin_unlock(&nilfs->ns_cptree_lock);
> +
> +               nilfs_sysfs_delete_snapshot_group(root);
>                 iput(root->ifile);
>
>                 kfree(root);
> --
> 1.8.3.1
>