[PATCH] hfsplus: Prevent corruption in shrinking truncate

[Jouni Roivas <jouni.roivas@tuxera.com>]

I believe there are some issues introduced by
commit 31651c607151 ("hfsplus: avoid deadlock on file truncation")

HFS+ has extent records which always contains 8 extents. In case the
first extent record in catalog file gets full, new ones are allocated
from extents overflow file.

In case shrinking truncate happens to middle of an extent record which
locates in extents overflow file, the logic in hfsplus_file_truncate()
was changed so that call to hfs_brec_remove() is not guarded any more.

Right action would be just freeing the extents that exceed the new
size inside extent record by calling hfsplus_free_extents(), and then
check if the whole extent record should be removed. However since the
guard (blk_cnt > start) is now after the call to hfs_brec_remove(),
this has unfortunate effect that the last matching extent record is
removed unconditionally.

To reproduce this issue, create a file which has at least 10 extents,
and then perform shrinking truncate into middle of the last extent
record, so that the number of remaining extents is not under or
divisible by 8. This causes the last extent record (8 extents) to be
removed totally instead of truncating into middle of it. Thus this
causes corruption, and lost data.

Fix for this is simply checking if the new truncated end is below the
start of this extent record, making it safe to remove the full extent
record. However call to hfs_brec_remove() can't be moved to it's
previous place since we're dropping ->tree_lock and it can cause a race
condition and the cached info being invalidated possibly corrupting the
node data.

Another issue is related to this one. When entering into the block
(blk_cnt > start) we are not holding the ->tree_lock. We break out from
the loop not holding the lock, but hfs_find_exit() does unlock it. Not
sure if it's possible for someone else to take the lock under our feet,
but it can cause hard to debug errors and premature unlocking. Even if
there's no real risk of it, the locking should still always be kept in
balance. Thus taking the lock now just before the check.

Cc: <stable@vger.kernel.org>
Cc: <linux-fsdevel@vger.kernel.org>
Reviewed-by: Anton Altaparmakov <anton@tuxera.com>
Signed-off-by: Jouni Roivas <jouni.roivas@tuxera.com>
---
 fs/hfsplus/extents.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/fs/hfsplus/extents.c b/fs/hfsplus/extents.c
index a930ddd15681..7054a542689f 100644
--- a/fs/hfsplus/extents.c
+++ b/fs/hfsplus/extents.c
@@ -598,13 +598,15 @@ void hfsplus_file_truncate(struct inode *inode)
 		res = __hfsplus_ext_cache_extent(&fd, inode, alloc_cnt);
 		if (res)
 			break;
-		hfs_brec_remove(&fd);
 
-		mutex_unlock(&fd.tree->tree_lock);
 		start = hip->cached_start;
+		if (blk_cnt <= start)
+			hfs_brec_remove(&fd);
+		mutex_unlock(&fd.tree->tree_lock);
 		hfsplus_free_extents(sb, hip->cached_extents,
 				     alloc_cnt - start, alloc_cnt - blk_cnt);
 		hfsplus_dump_extent(hip->cached_extents);
+		mutex_lock(&fd.tree->tree_lock);
 		if (blk_cnt > start) {
 			hip->extent_state |= HFSPLUS_EXT_DIRTY;
 			break;
@@ -612,7 +614,6 @@ void hfsplus_file_truncate(struct inode *inode)
 		alloc_cnt = start;
 		hip->cached_start = hip->cached_blocks = 0;
 		hip->extent_state &= ~(HFSPLUS_EXT_DIRTY | HFSPLUS_EXT_NEW);
-		mutex_lock(&fd.tree->tree_lock);
 	}
 	hfs_find_exit(&fd);
 
-- 
2.25.1