On Wed, 2024-05-15 at 20:51 +0800, libaokun@xxxxxxxxxxxxxxx wrote:
> From: Baokun Li <libaokun1@xxxxxxxxxx>
>
> Reusing the msg_id after a maliciously completed reopen request may cause
> a read request to remain unprocessed and result in a hung, as shown below:
>
> t1 | t2 | t3
> -------------------------------------------------
> cachefiles_ondemand_select_req
> cachefiles_ondemand_object_is_close(A)
> cachefiles_ondemand_set_object_reopening(A)
> queue_work(fscache_object_wq, &info->work)
> ondemand_object_worker
> cachefiles_ondemand_init_object(A)
> cachefiles_ondemand_send_req(OPEN)
> // get msg_id 6
> wait_for_completion(&req_A->done)
> cachefiles_ondemand_daemon_read
> // read msg_id 6 req_A
> cachefiles_ondemand_get_fd
> copy_to_user
> // Malicious completion msg_id 6
> copen 6,-1
> cachefiles_ondemand_copen
> complete(&req_A->done)
> // will not set the object to close
> // because ondemand_id && fd is valid.
>
> // ondemand_object_worker() is done
> // but the object is still reopening.
>
> // new open req_B
> cachefiles_ondemand_init_object(B)
> cachefiles_ondemand_send_req(OPEN)
> // reuse msg_id 6
> process_open_req
> copen 6,A.size
> // The expected failed copen was executed successfully
>
> Expect copen to fail, and when it does, it closes fd, which sets the
> object to close, and then close triggers reopen again. However, due to
> msg_id reuse resulting in a successful copen, the anonymous fd is not
> closed until the daemon exits. Therefore read requests waiting for reopen
> to complete may trigger hung task.
>
> To avoid this issue, allocate the msg_id cyclically to avoid reusing the
> msg_id for a very short duration of time.
>
> Fixes: c8383054506c ("cachefiles: notify the user daemon when looking up cookie")
> Signed-off-by: Baokun Li <libaokun1@xxxxxxxxxx>
> ---
> fs/cachefiles/internal.h | 1 +
> fs/cachefiles/ondemand.c | 20 ++++++++++++++++----
> 2 files changed, 17 insertions(+), 4 deletions(-)
>
> diff --git a/fs/cachefiles/internal.h b/fs/cachefiles/internal.h
> index 8ecd296cc1c4..9200c00f3e98 100644
> --- a/fs/cachefiles/internal.h
> +++ b/fs/cachefiles/internal.h
> @@ -128,6 +128,7 @@ struct cachefiles_cache {
> unsigned long req_id_next;
> struct xarray ondemand_ids; /* xarray for ondemand_id allocation */
> u32 ondemand_id_next;
> + u32 msg_id_next;
> };
>
> static inline bool cachefiles_in_ondemand_mode(struct cachefiles_cache *cache)
> diff --git a/fs/cachefiles/ondemand.c b/fs/cachefiles/ondemand.c
> index f6440b3e7368..b10952f77472 100644
> --- a/fs/cachefiles/ondemand.c
> +++ b/fs/cachefiles/ondemand.c
> @@ -433,20 +433,32 @@ static int cachefiles_ondemand_send_req(struct cachefiles_object *object,
> smp_mb();
>
> if (opcode == CACHEFILES_OP_CLOSE &&
> - !cachefiles_ondemand_object_is_open(object)) {
> + !cachefiles_ondemand_object_is_open(object)) {
> WARN_ON_ONCE(object->ondemand->ondemand_id == 0);
> xas_unlock(&xas);
> ret = -EIO;
> goto out;
> }
>
> - xas.xa_index = 0;
> + /*
> + * Cyclically find a free xas to avoid msg_id reuse that would
> + * cause the daemon to successfully copen a stale msg_id.
> + */
> + xas.xa_index = cache->msg_id_next;
> xas_find_marked(&xas, UINT_MAX, XA_FREE_MARK);
> + if (xas.xa_node == XAS_RESTART) {
> + xas.xa_index = 0;
> + xas_find_marked(&xas, cache->msg_id_next - 1, XA_FREE_MARK);
> + }
> if (xas.xa_node == XAS_RESTART)
> xas_set_err(&xas, -EBUSY);
> +
> xas_store(&xas, req);
> - xas_clear_mark(&xas, XA_FREE_MARK);
> - xas_set_mark(&xas, CACHEFILES_REQ_NEW);
> + if (xas_valid(&xas)) {
> + cache->msg_id_next = xas.xa_index + 1;
If you have a long-standing stuck request, could this counter wrap
around and you still end up with reuse? Maybe this should be using
ida_alloc/free instead, which would prevent that too?
> + xas_clear_mark(&xas, XA_FREE_MARK);
> + xas_set_mark(&xas, CACHEFILES_REQ_NEW);
> + }
> xas_unlock(&xas);
> } while (xas_nomem(&xas, GFP_KERNEL));
>
--
Jeff Layton <jlayton@xxxxxxxxxx>
