Re: [PATCH v2] ntfs3: fix uninit memory after failed mi_read in mi_format_new

Konstantin Komarov
2025-11-20T08:46:48Z

On 10/12/25 22:16, Raphael Pinsonneault-Thibeault wrote:

> Fix a KMSAN un-init bug found by syzkaller.
>
> ntfs_get_bh() expects a buffer from sb_getblk(), that buffer may not be
> uptodate. We do not bring the buffer uptodate before setting it as
> uptodate. If the buffer were to not be uptodate, it could mean adding a
> buffer with un-init data to the mi record. Attempting to load that record
> will trigger KMSAN.
>
> Avoid this by setting the buffer as uptodate, if it’s not already, by
> overwriting it.
>
> Reported-by: syzbot+7a2ba6b7b66340cff225@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=7a2ba6b7b66340cff225
> Tested-by: syzbot+7a2ba6b7b66340cff225@syzkaller.appspotmail.com
> Fixes: 4342306f0f0d5 ("fs/ntfs3: Add file operations and implementation")
> Signed-off-by: Raphael Pinsonneault-Thibeault <rpthibeault@gmail.com>
> ---
>
> v1:
> https://lore.kernel.org/all/20250925203701.223744-2-rpthibeault@gmail.com/
> Differences from v1:
> In the previous version, I thought that mi_read() returning an error was
> a genuine reason to stop trying to format the record. That was wrong. If
> we find that mi_read() fails and that therefore we cannot reuse an old
> record, we should try to make a new one. It then follows that
> ntfs_get_bh() should provide an uptodate bh for the new record.
>
> On testing:
> I could not get xfstests-bld to work for ntfs3. Config nightmare. So I
> first tested with the syzkaller repro, then tested for regressions manually
> with a program that called all the syscall code paths touched by my change:
> mkdir, fallocate (FALLOC_FL_COLLAPSE_RANGE), and
> fallocate (FALLOC_FL_INSERT_RANGE).
>
> fs/ntfs3/fsntfs.c | 9 ++++++++-
> 1 file changed, 8 insertions(+), 1 deletion(-)
>
> diff --git a/fs/ntfs3/fsntfs.c b/fs/ntfs3/fsntfs.c
> index 938d351ebac7..0c07502fdb0f 100644
> --- a/fs/ntfs3/fsntfs.c
> +++ b/fs/ntfs3/fsntfs.c
> @@ -1373,7 +1373,14 @@ int ntfs_get_bh(struct ntfs_sb_info *sbi, const struct runs_tree *run, u64 vbo,
> }
> if (buffer_locked(bh))
> __wait_on_buffer(bh);
> - set_buffer_uptodate(bh);
> +
> + lock_buffer(bh);
> + if (!buffer_uptodate(bh))
> + {
> + memset(bh->b_data, 0, blocksize);
> + set_buffer_uptodate(bh);
> + }
> + unlock_buffer(bh);
> } else {
> bh = ntfs_bread(sb, block);
> if (!bh) {

Applied to my tree, thanks for the patch.

Regards,
Konstantin