[PATCH] fs/ntfs3: prevent potential lcn remains uninitialized

[Edward Adam Davis <eadavis@qq.com>]

The target VCN being sought was not found within runs[0], causing
run_lookup() to return false. This causes run_lookup_entry() to return
false, which in turn results in a len value of 0, and the new parameter
passed to attr_data_get_block() is NULL. Collectively, these factors
ultimately cause attr_data_get_block_locked() to exit prematurely without
initializing lcn, thereby triggering [1].

To prevent [1], the clen check within ni_seek_data_or_hole() has been
moved to occur before the lcn check.

[1]
BUG: KMSAN: uninit-value in ni_seek_data_or_hole+0x24f/0x5f0 fs/ntfs3/frecord.c:2862
 ni_seek_data_or_hole+0x24f/0x5f0 fs/ntfs3/frecord.c:2862
 ntfs_llseek+0x22a/0x4a0 fs/ntfs3/file.c:1530
 vfs_llseek fs/read_write.c:391 [inline]

Fixes: c61326967728 ("fs/ntfs3: implement llseek SEEK_DATA/SEEK_HOLE by scanning data runs")
Reported-by: syzbot+c2cfe997245202e46f10@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=c2cfe997245202e46f10
Signed-off-by: Edward Adam Davis <eadavis@qq.com>
---
 fs/ntfs3/frecord.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/fs/ntfs3/frecord.c b/fs/ntfs3/frecord.c
index 7b035da63c12..d511b6a75ae8 100644
--- a/fs/ntfs3/frecord.c
+++ b/fs/ntfs3/frecord.c
@@ -2859,6 +2859,11 @@ loff_t ni_seek_data_or_hole(struct ntfs_inode *ni, loff_t offset, bool data)
 			return err;
 		}
 
+		if (!clen) {
+			/* Corrupted file. */
+			return -EINVAL;
+		}
+
 		if (lcn == RESIDENT_LCN) {
 			/* clen - resident size in bytes. clen == ni->vfs_inode.i_size */
 			if (offset >= clen) {
@@ -2909,10 +2914,6 @@ loff_t ni_seek_data_or_hole(struct ntfs_inode *ni, loff_t offset, bool data)
 			}
 		}
 
-		if (!clen) {
-			/* Corrupted file. */
-			return -EINVAL;
-		}
 	}
 }
 
-- 
2.43.0