[PATCH 5.15.y] ksmbd: avoid restoring only in-memory EOF after allocation change

[Sasha Levin <sashal@kernel.org>]

From: DaeMyung Kang <charsyam@gmail.com>

[ Upstream commit 9455e920aebe41217cabff3f81e90da4c407fdb6 ]

When SetAllocation requests a size below the allocated block count but
above EOF, ksmbd truncates the file to that size and restores only the
in-memory i_size. The larger EOF remains on disk and reappears after a
reboot. A 3-byte file preallocated to 64 KiB can acquire a 4 KiB EOF
after reducing its allocation to 4 KiB. Extending EOF this way is not
a reliable way to reclaim the preallocation beyond the old EOF either.

Only use this truncate path when the requested allocation is at or below
the observed EOF. A truncate to the existing EOF still lets filesystems
release preallocated blocks beyond EOF. Remove the in-memory-only i_size
restoration. The size check still precedes vfs_truncate() taking the
inode lock, so a concurrent shrink can make the truncate extend EOF;
that race also existed in the original path.

Fixes: e2f34481b24d ("cifsd: add server-side procedures for SMB3")
Cc: stable@vger.kernel.org
Signed-off-by: DaeMyung Kang <charsyam@gmail.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 fs/ksmbd/smb2pdu.c | 15 ++++-----------
 1 file changed, 4 insertions(+), 11 deletions(-)

diff --git a/fs/ksmbd/smb2pdu.c b/fs/ksmbd/smb2pdu.c
index 4c42205fe5261..2b43c97a32859 100644
--- a/fs/ksmbd/smb2pdu.c
+++ b/fs/ksmbd/smb2pdu.c
@@ -5887,24 +5887,17 @@ static int set_file_allocation_info(struct ksmbd_work *work,
 			pr_err("vfs_fallocate is failed : %d\n", rc);
 			return rc;
 		}
-	} else if (alloc_blks < stat.blocks) {
-		loff_t size;
-
+	} else if (alloc_blks < stat.blocks &&
+		   alloc_blks * 512 <= i_size_read(inode)) {
 		/*
-		 * Allocation size could be smaller than original one
-		 * which means allocated blocks in file should be
-		 * deallocated. use truncate to cut out it, but inode
-		 * size is also updated with truncate offset.
-		 * inode size is retained by backup inode size.
+		 * Truncate at or below EOF to discard excess allocation.
+		 * Truncating above EOF would extend the file on disk.
 		 */
-		size = i_size_read(inode);
 		rc = ksmbd_vfs_truncate(work, fp, alloc_blks * 512);
 		if (rc) {
 			pr_err("truncate failed!, err %d\n", rc);
 			return rc;
 		}
-		if (size < alloc_blks * 512)
-			i_size_write(inode, size);
 	}
 	return 0;
 }
-- 
2.53.0