Commit 530f32fc authored May 14, 2020 by Miklos Szeredi

aio: fix async fsync creds



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>
Fixes: c9582eb0 ("fuse: Fail all requests with invalid uids or gids")
Cc: stable@vger.kernel.org  # 4.18+
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>

parent a3c751a5

fs/aio.c

+8 −0

Original line number	Diff line number	Diff line
		@@ -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, const struct iocb iocb,
		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 req, const struct iocb 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);

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