[PATCH 6.12.y] exfat: advance valid_size to EOF for append writes

[Sasha Levin <sashal@kernel.org>]

From: Chi Zhiling <chizhiling@kylinos.cn>

[ Upstream commit b70ab407503706289163dd59f1ac3e942bacc37d ]

For an append write, the write position is recalculated by
generic_write_checks(). Therefore, the write position must be obtained
after generic_write_checks() returns.

Using the original write position to advance valid_size may cause the
valid_size update to be skipped, potentially exposing stale data.

Also move the truncate_pagecache() check below the recalculated
position, so that pos is never read before it is assigned.

Fixes: 11a347fb6cef ("exfat: change to get file size from DataLength")
Cc: stable@vger.kernel.org
Signed-off-by: Chi Zhiling <chizhiling@kylinos.cn>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
[ sashal: moved pos initialization before the existing direct I/O alignment checks ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 fs/exfat/file.c | 18 ++++++++----------
 1 file changed, 8 insertions(+), 10 deletions(-)

diff --git a/fs/exfat/file.c b/fs/exfat/file.c
index 033852efe5dc3..b624dc527f675 100644
--- a/fs/exfat/file.c
+++ b/fs/exfat/file.c
@@ -581,23 +581,18 @@ static ssize_t exfat_file_write_iter(struct kiocb *iocb, struct iov_iter *iter)
 	struct file *file = iocb->ki_filp;
 	struct inode *inode = file_inode(file);
 	struct exfat_inode_info *ei = EXFAT_I(inode);
-	loff_t pos = iocb->ki_pos;
-	loff_t valid_size;
+	loff_t pos, valid_size;
 
 	if (unlikely(exfat_forced_shutdown(inode->i_sb)))
 		return -EIO;
 
 	inode_lock(inode);
 
-	if (pos > i_size_read(inode))
-		truncate_pagecache(inode, i_size_read(inode));
-
-	valid_size = ei->valid_size;
-
 	ret = generic_write_checks(iocb, iter);
 	if (ret <= 0)
 		goto unlock;
 
+	pos = iocb->ki_pos;
 	if (iocb->ki_flags & IOCB_DIRECT) {
 		unsigned long align = pos | iov_iter_alignment(iter);
 
@@ -608,6 +603,10 @@ static ssize_t exfat_file_write_iter(struct kiocb *iocb, struct iov_iter *iter)
 		}
 	}
 
+	if (pos > i_size_read(inode))
+		truncate_pagecache(inode, i_size_read(inode));
+
+	valid_size = ei->valid_size;
 	if (pos > valid_size) {
 		ret = exfat_extend_valid_size(file, pos);
 		if (ret < 0 && ret != -ENOSPC) {
@@ -617,6 +616,8 @@ static ssize_t exfat_file_write_iter(struct kiocb *iocb, struct iov_iter *iter)
 		}
 		if (ret < 0)
 			goto unlock;
+
+		pos = valid_size;
 	}
 
 	ret = __generic_file_write_iter(iocb, iter);
@@ -625,9 +626,6 @@ static ssize_t exfat_file_write_iter(struct kiocb *iocb, struct iov_iter *iter)
 
 	inode_unlock(inode);
 
-	if (pos > valid_size)
-		pos = valid_size;
-
 	if (iocb->ki_pos > pos) {
 		ssize_t err = generic_write_sync(iocb, iocb->ki_pos - pos);
 		if (err < 0)
-- 
2.53.0