Re: [PATCH] aio: fix async fsync creds

[Avi Kivity <avi@scylladb.com>]

On 11/22/19 12:34 PM, Miklos Szeredi wrote:
> From: Miklos Szeredi <mszeredi@redhat.com>
>
> Avi Kivity reports that on fuse filesystems running in a user namespace
> asyncronous fsync fails with EOVERFLOW.
>
> The reason is that f_ops->fsync() is called with the creds of the kthread
> performing aio work instead of the creds of the process originally
> submitting IOCB_CMD_FSYNC.
>
> Fuse sends the creds of the caller in the request header and it needs to
> translate the uid and gid into the server's user namespace.  Since the
> kthread is running in init_user_ns, the translation will fail and the
> operation returns an error.
>
> It can be argued that fsync doesn't actually need any creds, but just
> zeroing out those fields in the header (as with requests that currently
> don't take creds) is a backward compatibility risk.
>
> Instead of working around this issue in fuse, solve the core of the problem
> by calling the filesystem with the proper creds.
>
> Reported-by: Avi Kivity <avi@scylladb.com>
> Tested-by: Giuseppe Scrivano <gscrivan@redhat.com>
> Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>


May I suggest


     Cc: stable@vger.kernel.org  # 4.18+


?


> ---
>   fs/aio.c |    8 ++++++++
>   1 file changed, 8 insertions(+)
>
> --- a/fs/aio.c
> +++ b/fs/aio.c
> @@ -176,6 +176,7 @@ struct fsync_iocb {
>   	struct file		*file;
>   	struct work_struct	work;
>   	bool			datasync;
> +	struct cred		*creds;
>   };
>   
>   struct poll_iocb {
> @@ -1589,8 +1590,11 @@ static int aio_write(struct kiocb *req,
>   static void aio_fsync_work(struct work_struct *work)
>   {
>   	struct aio_kiocb *iocb = container_of(work, struct aio_kiocb, fsync.work);
> +	const struct cred *old_cred = override_creds(iocb->fsync.creds);
>   
>   	iocb->ki_res.res = vfs_fsync(iocb->fsync.file, iocb->fsync.datasync);
> +	revert_creds(old_cred);
> +	put_cred(iocb->fsync.creds);
>   	iocb_put(iocb);
>   }
>   
> @@ -1604,6 +1608,10 @@ static int aio_fsync(struct fsync_iocb *
>   	if (unlikely(!req->file->f_op->fsync))
>   		return -EINVAL;
>   
> +	req->creds = prepare_creds();
> +	if (!req->creds)
> +		return -ENOMEM;
> +
>   	req->datasync = datasync;
>   	INIT_WORK(&req->work, aio_fsync_work);
>   	schedule_work(&req->work);