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    Subject[PATCH 5.10 199/289] hfsplus: prevent corruption in shrinking truncate
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    From: Jouni Roivas <jouni.roivas@tuxera.com>

    commit c3187cf32216313fb316084efac4dab3a8459b1d upstream.

    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.

    Link: https://lkml.kernel.org/r/20210429165139.3082828-1-jouni.roivas@tuxera.com
    Fixes: 31651c607151f ("hfsplus: avoid deadlock on file truncation")
    Signed-off-by: Jouni Roivas <jouni.roivas@tuxera.com>
    Reviewed-by: Anton Altaparmakov <anton@tuxera.com>
    Cc: Anatoly Trosinenko <anatoly.trosinenko@gmail.com>
    Cc: Viacheslav Dubeyko <slava@dubeyko.com>
    Cc: <stable@vger.kernel.org>
    Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    fs/hfsplus/extents.c | 7 ++++---
    1 file changed, 4 insertions(+), 3 deletions(-)

    --- a/fs/hfsplus/extents.c
    +++ b/fs/hfsplus/extents.c
    @@ -598,13 +598,15 @@ void hfsplus_file_truncate(struct 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
    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);


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