[PATCH] fs/ntfs3: fix missing run load for vcn0 in attr_data_get_block_locked()

When a compressed or sparse attribute has its clusters frame-aligned,
vcn is rounded down to the frame start using cmask, which can result
in vcn != vcn0. In this case, vcn and vcn0 may reside in different
attribute segments.

The code already handles the case where vcn is in a different segment
by loading its runs before allocation. However, it fails to load runs
for vcn0 when vcn0 resides in a different segment than vcn. This causes
run_lookup_entry() to return SPARSE_LCN for vcn0 since its segment was
never loaded into the in-memory run list, triggering the WARN_ON(1).

Fix this by adding a missing check for vcn0 after the existing vcn
segment check. If vcn0 falls outside the current segment range
[svcn, evcn1), find and load the attribute segment containing vcn0
before performing the run lookup.

The following scenario triggers the bug:
  attr_data_get_block_locked()
    vcn = vcn0 & cmask        <- vcn != vcn0 after frame alignment
    load runs for vcn segment <- vcn0 segment not loaded!
    attr_allocate_clusters()  <- allocation succeeds
    run_lookup_entry(vcn0)    <- vcn0 not in run -> SPARSE_LCN
    WARN_ON(1)                <- bug fires here!

Reported-by: [email protected]
Closes: https://syzkaller.appspot.com/bug?extid=c1e9aedbd913fadad617
Fixes: c380b52f6c57 ("fs/ntfs3: Change new sparse cluster processing")
Signed-off-by: Deepanshu Kartikey <[email protected]>
---
 fs/ntfs3/attrib.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/fs/ntfs3/attrib.c b/fs/ntfs3/attrib.c
index 6cb9bc5d605c..6ef7db82a35d 100644
--- a/fs/ntfs3/attrib.c
+++ b/fs/ntfs3/attrib.c
@@ -1152,6 +1152,21 @@ int attr_data_get_block_locked(struct ntfs_inode *ni, CLST vcn, CLST clen,
 			if (err)
 				goto out;
 		}
+
+		if (vcn0 < svcn || evcn1 <= vcn0) {
+			struct ATTRIB *attr2;
+
+			attr2 = ni_find_attr(ni, attr_b, &le_b, ATTR_DATA, NULL,
+					       0, &vcn0, &mi);
+			if (!attr2) {
+				err = -EINVAL;
+				goto out;
+			}
+			err = attr_load_runs(attr2, ni, run, NULL);
+			if (err)
+				goto out;
+		}
+
 		da = false; /* no delalloc for compressed file. */
 	}
 
-- 
2.43.0