Re: [PATCH v2 09/12] cachefiles: defer exposing anon_fd until after copy_to_user() succeeds

Jingbo Xu
2024-05-20T09:39:56Z

On 5/15/24 4:45 PM, libaokun@huaweicloud.com wrote:
> From: Baokun Li <libaokun1@huawei.com>
>
> After installing the anonymous fd, we can now see it in userland and close
> it. However, at this point we may not have gotten the reference count of
> the cache, but we will put it during colse fd, so this may cause a cache
> UAF.
>
> So grab the cache reference count before fd_install(). In addition, by
> kernel convention, fd is taken over by the user land after fd_install(),
> and the kernel should not call close_fd() after that, i.e., it should call
> fd_install() after everything is ready, thus fd_install() is called after
> copy_to_user() succeeds.
>
> Fixes: c8383054506c ("cachefiles: notify the user daemon when looking up cookie")
> Suggested-by: Hou Tao <houtao1@huawei.com>
> Signed-off-by: Baokun Li <libaokun1@huawei.com>
> ---
> fs/cachefiles/ondemand.c | 53 +++++++++++++++++++++++++---------------
> 1 file changed, 33 insertions(+), 20 deletions(-)
>
> diff --git a/fs/cachefiles/ondemand.c b/fs/cachefiles/ondemand.c
> index d2d4e27fca6f..3a36613e00a7 100644
> --- a/fs/cachefiles/ondemand.c
> +++ b/fs/cachefiles/ondemand.c
> @@ -4,6 +4,11 @@
> #include <linux/uio.h>
> #include "internal.h"
>
> +struct anon_file {
> + struct file *file;
> + int fd;
> +};
> +
> static inline void cachefiles_req_put(struct cachefiles_req *req)
> {
> if (refcount_dec_and_test(&req->ref))
> @@ -263,14 +268,14 @@ int cachefiles_ondemand_restore(struct cachefiles_cache *cache, char *args)
> return 0;
> }
>

> -static int cachefiles_ondemand_get_fd(struct cachefiles_req *req)
> +static int cachefiles_ondemand_get_fd(struct cachefiles_req *req,
> + struct anon_file *anon_file)

How about:

int cachefiles_ondemand_get_fd(struct cachefiles_req *req, int *fd,
struct file *file) ?

It isn't worth introducing a new structure as it is used only for
parameter passing.

> {
> struct cachefiles_object *object;
> struct cachefiles_cache *cache;
> struct cachefiles_open *load;
> - struct file *file;
> u32 object_id;
> - int ret, fd;
> + int ret;
>
> object = cachefiles_grab_object(req->object,
> cachefiles_obj_get_ondemand_fd);
> @@ -282,16 +287,16 @@ static int cachefiles_ondemand_get_fd(struct cachefiles_req *req)
> if (ret < 0)
> goto err;
>
> - fd = get_unused_fd_flags(O_WRONLY);
> - if (fd < 0) {
> - ret = fd;
> + anon_file->fd = get_unused_fd_flags(O_WRONLY);
> + if (anon_file->fd < 0) {
> + ret = anon_file->fd;
> goto err_free_id;
> }
>
> - file = anon_inode_getfile("[cachefiles]", &cachefiles_ondemand_fd_fops,
> - object, O_WRONLY);
> - if (IS_ERR(file)) {
> - ret = PTR_ERR(file);
> + anon_file->file = anon_inode_getfile("[cachefiles]",
> + &cachefiles_ondemand_fd_fops, object, O_WRONLY);
> + if (IS_ERR(anon_file->file)) {
> + ret = PTR_ERR(anon_file->file);
> goto err_put_fd;
> }
>
> @@ -299,16 +304,15 @@ static int cachefiles_ondemand_get_fd(struct cachefiles_req *req)
> if (object->ondemand->ondemand_id > 0) {
> spin_unlock(&object->ondemand->lock);
> /* Pair with check in cachefiles_ondemand_fd_release(). */
> - file->private_data = NULL;
> + anon_file->file->private_data = NULL;
> ret = -EEXIST;
> goto err_put_file;
> }
>
> - file->f_mode |= FMODE_PWRITE | FMODE_LSEEK;
> - fd_install(fd, file);
> + anon_file->file->f_mode |= FMODE_PWRITE | FMODE_LSEEK;
>
> load = (void *)req->msg.data;
> - load->fd = fd;
> + load->fd = anon_file->fd;
> object->ondemand->ondemand_id = object_id;
> spin_unlock(&object->ondemand->lock);
>
> @@ -317,9 +321,11 @@ static int cachefiles_ondemand_get_fd(struct cachefiles_req *req)
> return 0;
>
> err_put_file:
> - fput(file);
> + fput(anon_file->file);
> + anon_file->file = NULL;

When cachefiles_ondemand_get_fd() returns failure, anon_file->file is
not used, and thus I don't think it is worth resetting anon_file->file
to NULL. Or we could assign fd and struct file at the very end when all
succeed.

> err_put_fd:
> - put_unused_fd(fd);
> + put_unused_fd(anon_file->fd);
> + anon_file->fd = ret;

Ditto.

> err_free_id:
> xa_erase(&cache->ondemand_ids, object_id);
> err:
> @@ -376,6 +382,7 @@ ssize_t cachefiles_ondemand_daemon_read(struct cachefiles_cache *cache,
> struct cachefiles_msg *msg;
> size_t n;
> int ret = 0;
> + struct anon_file anon_file;
> XA_STATE(xas, &cache->reqs, cache->req_id_next);
>
> xa_lock(&cache->reqs);
> @@ -409,7 +416,7 @@ ssize_t cachefiles_ondemand_daemon_read(struct cachefiles_cache *cache,
> xa_unlock(&cache->reqs);
>
> if (msg->opcode == CACHEFILES_OP_OPEN) {
> - ret = cachefiles_ondemand_get_fd(req);
> + ret = cachefiles_ondemand_get_fd(req, &anon_file);
> if (ret)
> goto out;
> }
> @@ -417,10 +424,16 @@ ssize_t cachefiles_ondemand_daemon_read(struct cachefiles_cache *cache,
> msg->msg_id = xas.xa_index;
> msg->object_id = req->object->ondemand->ondemand_id;
>
> - if (copy_to_user(_buffer, msg, n) != 0) {
> + if (copy_to_user(_buffer, msg, n) != 0)
> ret = -EFAULT;
> - if (msg->opcode == CACHEFILES_OP_OPEN)
> - close_fd(((struct cachefiles_open *)msg->data)->fd);
> +
> + if (msg->opcode == CACHEFILES_OP_OPEN) {
> + if (ret < 0) {
> + fput(anon_file.file);
> + put_unused_fd(anon_file.fd);
> + goto out;
> + }
> + fd_install(anon_file.fd, anon_file.file);
> }
> out:
> cachefiles_put_object(req->object, cachefiles_obj_put_read_req);

--
Thanks,
Jingbo