fanotify: report names longer than NAME_MAX

fanotify keeps the length of a reported name in a u8, so it drops any
name longer than NAME_MAX and warns:

  WARNING: fs/notify/fanotify/fanotify.h:217 at fanotify_handle_event+0x2e82/0x39f0

Such names are legitimate on some filesystems.  vfat mounted with utf8
takes up to 255 UTF-16 characters, which are up to 765 bytes of UTF-8,
and FUSE accepts names up to PATH_MAX - 1.  readdir() and inotify report
them in full.  fanotify sends the event without the name, and for
FAN_RENAME it trips a second warning in copy_fid_info_to_user(), after
which read() fails with EFAULT and the event is lost.

When syzbot hit this, Jan suggested accommodating names up to PATH_MAX
[1], the limit readdir() already applies to a name
(verify_dirent_name()).  Store the name lengths as u16, which keeps
struct fanotify_info at 8 bytes, and drop without a warning only names
of PATH_MAX bytes or more, which no path can refer to.  An event with
such a name can exceed a page, so allocate name events with kvmalloc(),
as Jan also suggested.  Names up to NAME_MAX are reported exactly as
before, and the record format is unchanged: an event with a long name is
just longer.

Link: https://lore.kernel.org/all/20241112115059.ecvomkalee5m4i4i@quack3/ [1]
Fixes: 2d9374f09513 ("fanotify: use macros to get the offset to fanotify_info buffer")
Cc: stable@vger.kernel.org
Signed-off-by: Matthias Goergens <matthias.goergens@gmail.com>
Link: https://patch.msgid.link/20260926020851.2938961-1-matthias.goergens@gmail.com
Signed-off-by: Jan Kara <jack@suse.cz>

0 files changed, 0 insertions, 0 deletions