Re: [PATCH v2 03/12] cachefiles: fix slab-use-after-free in cachefiles_ondemand_get_fd()

From: Jingbo Xu
Date: Mon May 20 2024 - 03:40:25 EST



On 5/15/24 4:45 PM, libaokun@xxxxxxxxxxxxxxx wrote:
> From: Baokun Li <libaokun1@xxxxxxxxxx>
>
> We got the following issue in a fuzz test of randomly issuing the restore
> command:
>
> ==================================================================
> BUG: KASAN: slab-use-after-free in cachefiles_ondemand_daemon_read+0x609/0xab0
> Write of size 4 at addr ffff888109164a80 by task ondemand-04-dae/4962
>
> CPU: 11 PID: 4962 Comm: ondemand-04-dae Not tainted 6.8.0-rc7-dirty #542
> Call Trace:
> kasan_report+0x94/0xc0
> cachefiles_ondemand_daemon_read+0x609/0xab0
> vfs_read+0x169/0xb50
> ksys_read+0xf5/0x1e0
>
> Allocated by task 626:
> __kmalloc+0x1df/0x4b0
> cachefiles_ondemand_send_req+0x24d/0x690
> cachefiles_create_tmpfile+0x249/0xb30
> cachefiles_create_file+0x6f/0x140
> cachefiles_look_up_object+0x29c/0xa60
> cachefiles_lookup_cookie+0x37d/0xca0
> fscache_cookie_state_machine+0x43c/0x1230
> [...]
>
> Freed by task 626:
> kfree+0xf1/0x2c0
> cachefiles_ondemand_send_req+0x568/0x690
> cachefiles_create_tmpfile+0x249/0xb30
> cachefiles_create_file+0x6f/0x140
> cachefiles_look_up_object+0x29c/0xa60
> cachefiles_lookup_cookie+0x37d/0xca0
> fscache_cookie_state_machine+0x43c/0x1230
> [...]
> ==================================================================
>
> Following is the process that triggers the issue:
>
> mount | daemon_thread1 | daemon_thread2
> ------------------------------------------------------------
> cachefiles_ondemand_init_object
> cachefiles_ondemand_send_req
> REQ_A = kzalloc(sizeof(*req) + data_len)
> wait_for_completion(&REQ_A->done)
>
> cachefiles_daemon_read
> cachefiles_ondemand_daemon_read
> REQ_A = cachefiles_ondemand_select_req
> cachefiles_ondemand_get_fd
> copy_to_user(_buffer, msg, n)
> process_open_req(REQ_A)
> ------ restore ------
> cachefiles_ondemand_restore
> xas_for_each(&xas, req, ULONG_MAX)
> xas_set_mark(&xas, CACHEFILES_REQ_NEW);
>
> cachefiles_daemon_read
> cachefiles_ondemand_daemon_read
> REQ_A = cachefiles_ondemand_select_req
>
> write(devfd, ("copen %u,%llu", msg->msg_id, size));
> cachefiles_ondemand_copen
> xa_erase(&cache->reqs, id)
> complete(&REQ_A->done)
> kfree(REQ_A)
> cachefiles_ondemand_get_fd(REQ_A)
> fd = get_unused_fd_flags
> file = anon_inode_getfile
> fd_install(fd, file)
> load = (void *)REQ_A->msg.data;
> load->fd = fd;
> // load UAF !!!
>
> This issue is caused by issuing a restore command when the daemon is still
> alive, which results in a request being processed multiple times thus
> triggering a UAF. So to avoid this problem, add an additional reference
> count to cachefiles_req, which is held while waiting and reading, and then
> released when the waiting and reading is over.
>


> Note that since there is only one reference count for waiting, we need to
> avoid the same request being completed multiple times, so we can only
> complete the request if it is successfully removed from the xarray.

Sorry the above description makes me confused. As the same request may
be got by different daemon threads multiple times, the introduced
refcount mechanism can't protect it from being completed multiple times
(which is expected). The refcount only protects it from being freed
multiple times.

>
> Fixes: e73fa11a356c ("cachefiles: add restore command to recover inflight ondemand read requests")
> Suggested-by: Hou Tao <houtao1@xxxxxxxxxx>
> Signed-off-by: Baokun Li <libaokun1@xxxxxxxxxx>
> Reviewed-by: Jia Zhu <zhujia.zj@xxxxxxxxxxxxx>
> ---
> fs/cachefiles/internal.h | 1 +
> fs/cachefiles/ondemand.c | 44 ++++++++++++++++++++++------------------
> 2 files changed, 25 insertions(+), 20 deletions(-)
>
> diff --git a/fs/cachefiles/internal.h b/fs/cachefiles/internal.h
> index d33169f0018b..7745b8abc3aa 100644
> --- a/fs/cachefiles/internal.h
> +++ b/fs/cachefiles/internal.h
> @@ -138,6 +138,7 @@ static inline bool cachefiles_in_ondemand_mode(struct cachefiles_cache *cache)
> struct cachefiles_req {
> struct cachefiles_object *object;
> struct completion done;
> + refcount_t ref;
> int error;
> struct cachefiles_msg msg;
> };
> diff --git a/fs/cachefiles/ondemand.c b/fs/cachefiles/ondemand.c
> index fd49728d8bae..56d12fe4bf73 100644
> --- a/fs/cachefiles/ondemand.c
> +++ b/fs/cachefiles/ondemand.c
> @@ -4,6 +4,12 @@
> #include <linux/uio.h>
> #include "internal.h"
>
> +static inline void cachefiles_req_put(struct cachefiles_req *req)
> +{
> + if (refcount_dec_and_test(&req->ref))
> + kfree(req);
> +}
> +
> static int cachefiles_ondemand_fd_release(struct inode *inode,
> struct file *file)
> {
> @@ -299,7 +305,6 @@ ssize_t cachefiles_ondemand_daemon_read(struct cachefiles_cache *cache,
> {
> struct cachefiles_req *req;
> struct cachefiles_msg *msg;
> - unsigned long id = 0;
> size_t n;
> int ret = 0;
> XA_STATE(xas, &cache->reqs, cache->req_id_next);
> @@ -330,41 +335,39 @@ ssize_t cachefiles_ondemand_daemon_read(struct cachefiles_cache *cache,
>
> xas_clear_mark(&xas, CACHEFILES_REQ_NEW);
> cache->req_id_next = xas.xa_index + 1;
> + refcount_inc(&req->ref);
> xa_unlock(&cache->reqs);
>
> - id = xas.xa_index;
> -
> if (msg->opcode == CACHEFILES_OP_OPEN) {
> ret = cachefiles_ondemand_get_fd(req);
> if (ret) {
> cachefiles_ondemand_set_object_close(req->object);
> - goto error;
> + goto out;
> }
> }
>
> - msg->msg_id = id;
> + msg->msg_id = xas.xa_index;
> msg->object_id = req->object->ondemand->ondemand_id;
>
> if (copy_to_user(_buffer, msg, n) != 0) {
> ret = -EFAULT;
> if (msg->opcode == CACHEFILES_OP_OPEN)
> close_fd(((struct cachefiles_open *)msg->data)->fd);
> - goto error;
> }
> -
> - /* CLOSE request has no reply */
> - if (msg->opcode == CACHEFILES_OP_CLOSE) {
> - xa_erase(&cache->reqs, id);
> - complete(&req->done);
> +out:
> + /* Remove error request and CLOSE request has no reply */
> + if (ret || msg->opcode == CACHEFILES_OP_CLOSE) {
> + xas_reset(&xas);
> + xas_lock(&xas);
> + if (xas_load(&xas) == req) {

Just out of curiosity... How could xas_load(&xas) doesn't equal to req?


> + req->error = ret;
> + complete(&req->done);
> + xas_store(&xas, NULL);
> + }
> + xas_unlock(&xas);
> }
> -
> - return n;
> -
> -error:
> - xa_erase(&cache->reqs, id);
> - req->error = ret;
> - complete(&req->done);
> - return ret;
> + cachefiles_req_put(req);
> + return ret ? ret : n;
> }

This is actually a combination of a fix and a cleanup which combines the
logic of removing error request and the CLOSE requests into one place.
Also it relies on the cleanup made in patch 2 ("cachefiles: remove
err_put_fd tag in cachefiles_ondemand_daemon_read()"), making it
difficult to be atomatically back ported to the stable (as patch 2 is
not marked as "Fixes").

Thus could we make the fix first, and then make the cleanup.

>
> typedef int (*init_req_fn)(struct cachefiles_req *req, void *private);
> @@ -394,6 +397,7 @@ static int cachefiles_ondemand_send_req(struct cachefiles_object *object,
> goto out;
> }
>
> + refcount_set(&req->ref, 1);
> req->object = object;
> init_completion(&req->done);
> req->msg.opcode = opcode;
> @@ -455,7 +459,7 @@ static int cachefiles_ondemand_send_req(struct cachefiles_object *object,
> wake_up_all(&cache->daemon_pollwq);
> wait_for_completion(&req->done);
> ret = req->error;
> - kfree(req);
> + cachefiles_req_put(req);
> return ret;
> out:
> /* Reset the object to close state in error handling path.


Don't we need to also convert "kfree(req)" to cachefiles_req_put(req)
for the error path of cachefiles_ondemand_send_req()?

```
out:
/* Reset the object to close state in error handling path.
* If error occurs after creating the anonymous fd,
* cachefiles_ondemand_fd_release() will set object to close.
*/
if (opcode == CACHEFILES_OP_OPEN)
cachefiles_ondemand_set_object_close(object);
kfree(req);
return ret;
```




--
Thanks,
Jingbo