The direct path is not supported on verity files. Attempts to use direct
I/O path on such files should fall back to buffered I/O path.

Add a fall back to buffered I/O at two place, in a common fast path and
latter when lock is acquired. The second check prevents TOCTOU issue
with reading fsverity_active() status and resetting IOCB_DIRECT flag.

If one thread saw fsverity_active() to be false, and then second thread
acquired XFS_IOLOCK_EXCL and enabled fsverity. The first thread will go
through the DIO path.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Andrey Albershteyn <aalbersh@kernel.org>
---
 fs/xfs/xfs_file.c | 59 +++++++++++++++++++++++++++++++----------------
 1 file changed, 39 insertions(+), 20 deletions(-)

diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
index e3f4353b3b78..cba194d678fc 100644
--- a/fs/xfs/xfs_file.c
+++ b/fs/xfs/xfs_file.c
@@ -246,6 +246,25 @@ static const struct iomap_dio_ops xfs_dio_read_bounce_ops = {
 	.bio_set	= &iomap_ioend_bioset,
 };
 
+STATIC ssize_t
+xfs_file_buffered_read(
+	struct kiocb		*iocb,
+	struct iov_iter		*to)
+{
+	struct xfs_inode	*ip = XFS_I(file_inode(iocb->ki_filp));
+	ssize_t			ret;
+
+	trace_xfs_file_buffered_read(iocb, to);
+
+	ret = xfs_ilock_iocb(iocb, XFS_IOLOCK_SHARED);
+	if (ret)
+		return ret;
+	ret = generic_file_read_iter(iocb, to);
+	xfs_iunlock(ip, XFS_IOLOCK_SHARED);
+
+	return ret;
+}
+
 STATIC ssize_t
 xfs_file_dio_read(
 	struct kiocb		*iocb,
@@ -266,6 +285,16 @@ xfs_file_dio_read(
 	ret = xfs_ilock_iocb(iocb, XFS_IOLOCK_SHARED);
 	if (ret)
 		return ret;
+	/*
+	 * Re-check verity status after acquiring lock. This prevents TOCTOU in
+	 * xfs_file_read_iter() while falling back from DIO to buffered I/O as
+	 * now we are holding a lock
+	 */
+	if (fsverity_active(VFS_I(ip))) {
+		xfs_iunlock(ip, XFS_IOLOCK_SHARED);
+		iocb->ki_flags &= ~IOCB_DIRECT;
+		return xfs_file_buffered_read(iocb, to);
+	}
 	if (mapping_stable_writes(iocb->ki_filp->f_mapping)) {
 		dio_ops = &xfs_dio_read_bounce_ops;
 		dio_flags |= IOMAP_DIO_BOUNCE;
@@ -282,7 +311,8 @@ xfs_file_dax_read(
 	struct kiocb		*iocb,
 	struct iov_iter		*to)
 {
-	struct xfs_inode	*ip = XFS_I(iocb->ki_filp->f_mapping->host);
+	struct inode		*inode = iocb->ki_filp->f_mapping->host;
+	struct xfs_inode	*ip = XFS_I(inode);
 	ssize_t			ret = 0;
 
 	trace_xfs_file_dax_read(iocb, to);
@@ -300,25 +330,6 @@ xfs_file_dax_read(
 	return ret;
 }
 
-STATIC ssize_t
-xfs_file_buffered_read(
-	struct kiocb		*iocb,
-	struct iov_iter		*to)
-{
-	struct xfs_inode	*ip = XFS_I(file_inode(iocb->ki_filp));
-	ssize_t			ret;
-
-	trace_xfs_file_buffered_read(iocb, to);
-
-	ret = xfs_ilock_iocb(iocb, XFS_IOLOCK_SHARED);
-	if (ret)
-		return ret;
-	ret = generic_file_read_iter(iocb, to);
-	xfs_iunlock(ip, XFS_IOLOCK_SHARED);
-
-	return ret;
-}
-
 STATIC ssize_t
 xfs_file_read_iter(
 	struct kiocb		*iocb,
@@ -333,6 +344,14 @@ xfs_file_read_iter(
 	if (xfs_is_shutdown(mp))
 		return -EIO;
 
+	/*
+	 * In case fs-verity is enabled, we also fallback to the buffered read
+	 * from the direct read path. Therefore, IOCB_DIRECT is set and need to
+	 * be cleared (see generic_file_read_iter())
+	 */
+	if (fsverity_active(inode))
+		iocb->ki_flags &= ~IOCB_DIRECT;
+
 	if (IS_DAX(inode))
 		ret = xfs_file_dax_read(iocb, to);
 	else if (iocb->ki_flags & IOCB_DIRECT)
-- 
2.54.0