[PATCH RESEND V12 7/8] fuse: Use daemon creds in passthrough mode

[Alessio Balsini <balsini@android.com>]

When using FUSE passthrough, read/write operations are directly
forwarded to the lower file system file through VFS, but there is no
guarantee that the process that is triggering the request has the right
permissions to access the lower file system. This would cause the
read/write access to fail.

In passthrough file systems, where the FUSE daemon is responsible for
the enforcement of the lower file system access policies, often happens
that the process dealing with the FUSE file system doesn't have access
to the lower file system.
Being the FUSE daemon in charge of implementing the FUSE file
operations, that in the case of read/write operations usually simply
results in the copy of memory buffers from/to the lower file system
respectively, these operations are executed with the FUSE daemon
privileges.

This patch adds a reference to the FUSE daemon credentials, referenced
at FUSE_DEV_IOC_PASSTHROUGH_OPEN ioctl() time so that they can be used
to temporarily raise the user credentials when accessing lower file
system files in passthrough.
The process accessing the FUSE file with passthrough enabled temporarily
receives the privileges of the FUSE daemon while performing read/write
operations. Similar behavior is implemented in overlayfs.
These privileges will be reverted as soon as the IO operation completes.
This feature does not provide any higher security privileges to those
processes accessing the FUSE file system with passthrough enabled. This
is because it is still the FUSE daemon responsible for enabling or not
the passthrough feature at file open time, and should enable the feature
only after appropriate access policy checks.

Signed-off-by: Alessio Balsini <balsini@android.com>
---
 fs/fuse/fuse_i.h      |  5 ++++-
 fs/fuse/passthrough.c | 11 +++++++++++
 2 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/fs/fuse/fuse_i.h b/fs/fuse/fuse_i.h
index c4730d893324..815af1845b16 100644
--- a/fs/fuse/fuse_i.h
+++ b/fs/fuse/fuse_i.h
@@ -182,10 +182,13 @@ struct fuse_release_args;
 
 /**
  * Reference to lower filesystem file for read/write operations handled in
- * passthrough mode
+ * passthrough mode.
+ * This struct also tracks the credentials to be used for handling read/write
+ * operations.
  */
 struct fuse_passthrough {
 	struct file *filp;
+	struct cred *cred;
 };
 
 /** FUSE specific file data */
diff --git a/fs/fuse/passthrough.c b/fs/fuse/passthrough.c
index c7fa1eeb7639..24866c5fe7e2 100644
--- a/fs/fuse/passthrough.c
+++ b/fs/fuse/passthrough.c
@@ -52,6 +52,7 @@ ssize_t fuse_passthrough_read_iter(struct kiocb *iocb_fuse,
 				   struct iov_iter *iter)
 {
 	ssize_t ret;
+	const struct cred *old_cred;
 	struct file *fuse_filp = iocb_fuse->ki_filp;
 	struct fuse_file *ff = fuse_filp->private_data;
 	struct file *passthrough_filp = ff->passthrough.filp;
@@ -59,6 +60,7 @@ ssize_t fuse_passthrough_read_iter(struct kiocb *iocb_fuse,
 	if (!iov_iter_count(iter))
 		return 0;
 
+	old_cred = override_creds(ff->passthrough.cred);
 	if (is_sync_kiocb(iocb_fuse)) {
 		ret = vfs_iter_read(passthrough_filp, iter, &iocb_fuse->ki_pos,
 				    iocb_to_rw_flags(iocb_fuse->ki_flags,
@@ -77,6 +79,7 @@ ssize_t fuse_passthrough_read_iter(struct kiocb *iocb_fuse,
 		if (ret != -EIOCBQUEUED)
 			fuse_aio_cleanup_handler(aio_req);
 	}
+	revert_creds(old_cred);
 
 	return ret;
 }
@@ -85,6 +88,7 @@ ssize_t fuse_passthrough_write_iter(struct kiocb *iocb_fuse,
 				    struct iov_iter *iter)
 {
 	ssize_t ret;
+	const struct cred *old_cred;
 	struct file *fuse_filp = iocb_fuse->ki_filp;
 	struct fuse_file *ff = fuse_filp->private_data;
 	struct inode *fuse_inode = file_inode(fuse_filp);
@@ -96,6 +100,7 @@ ssize_t fuse_passthrough_write_iter(struct kiocb *iocb_fuse,
 
 	inode_lock(fuse_inode);
 
+	old_cred = override_creds(ff->passthrough.cred);
 	if (is_sync_kiocb(iocb_fuse)) {
 		file_start_write(passthrough_filp);
 		ret = vfs_iter_write(passthrough_filp, iter, &iocb_fuse->ki_pos,
@@ -124,6 +129,7 @@ ssize_t fuse_passthrough_write_iter(struct kiocb *iocb_fuse,
 			fuse_aio_cleanup_handler(aio_req);
 	}
 out:
+	revert_creds(old_cred);
 	inode_unlock(fuse_inode);
 
 	return ret;
@@ -174,6 +180,7 @@ int fuse_passthrough_open(struct fuse_dev *fud,
 	}
 
 	passthrough->filp = passthrough_filp;
+	passthrough->cred = prepare_creds();
 
 	idr_preload(GFP_KERNEL);
 	spin_lock(&fc->passthrough_req_lock);
@@ -225,4 +232,8 @@ void fuse_passthrough_release(struct fuse_passthrough *passthrough)
 		fput(passthrough->filp);
 		passthrough->filp = NULL;
 	}
+	if (passthrough->cred) {
+		put_cred(passthrough->cred);
+		passthrough->cred = NULL;
+	}
 }
-- 
2.30.0.280.ga3ce27912f-goog