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cachefiles: cyclic allocation of msg_id to avoid reuse

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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: c838305450 ("cachefiles: notify the user daemon when looking up cookie")
Signed-off-by: Baokun Li <libaokun1@huawei.com>
Link: https://lore.kernel.org/r/20240628062930.2467993-9-libaokun@huaweicloud.com
Acked-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Reviewed-by: Jia Zhu <zhujia.zj@bytedance.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>
This commit is contained in:
Baokun Li 2024-06-28 14:29:29 +08:00 committed by Christian Brauner
parent 12e009d608
commit 19f4f39909
2 changed files with 17 additions and 4 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

1
fs/cachefiles/internal.h
View File

@ -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)

20
fs/cachefiles/ondemand.c
View File

@ -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));