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;
+ xas_clear_mark(&xas, XA_FREE_MARK);
+ xas_set_mark(&xas, CACHEFILES_REQ_NEW);
+ }
xas_unlock(&xas);
} while (xas_nomem(&xas, GFP_KERNEL));
--
2.39.2
