erofs: impersonate the opener's credentials when accessing backing file

Previously, file operations on a file-backed mount used the current
process' credentials to access the backing FD. Attempting to do so on
Android lead to SELinux denials, as ACL rules on the backing file (e.g.
/system/apex/foo.apex) is restricted to a small set of process.
Arguably, this error is redundant and leaking implementation details, as
access to files on a mount is already ACL'ed by path.

Instead, override to use the opener's cred when accessing the backing
file. This makes the behavior similar to a loop-backed mount, which
uses kworker cred when accessing the backing file and does not cause
SELinux denials.

Signed-off-by: Tatsuyuki Ishi <[email protected]>
Reviewed-by: Gao Xiang <[email protected]>
Reviewed-by: Hongbo Li <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Gao Xiang <[email protected]>

Author: ishitatsuyuki

fs/erofs/fileio.c

@@ -47,6 +47,7 @@ static void erofs_fileio_ki_complete(struct kiocb *iocb, long ret)
 
 static void erofs_fileio_rq_submit(struct erofs_fileio_rq *rq)
 {
+	const struct cred *old_cred;
 	struct iov_iter iter;
 	int ret;
 
@@ -60,7 +61,9 @@ static void erofs_fileio_rq_submit(struct erofs_fileio_rq *rq)
 		rq->iocb.ki_flags = IOCB_DIRECT;
 	iov_iter_bvec(&iter, ITER_DEST, rq->bvecs, rq->bio.bi_vcnt,
 		      rq->bio.bi_iter.bi_size);
+	old_cred = override_creds(rq->iocb.ki_filp->f_cred);
 	ret = vfs_iocb_iter_read(rq->iocb.ki_filp, &rq->iocb, &iter);
+	revert_creds(old_cred);
 	if (ret != -EIOCBQUEUED)
 		erofs_fileio_ki_complete(&rq->iocb, ret);
 }