[RFC,0/5] Avoid MAP_FIXED gap exposure

Message

Liam R. Howlett May 31, 2024, 4:32 p.m. UTC

It is now possible to walk the vma tree using the rcu read locks and is
beneficial to do so to reduce lock contention.  Doing so while a
MAP_FIXED mapping is executing means that a reader may see a gap in the
vma tree that should never logically exist - and does not when using the
mmap lock in read mode.  The temporal gap exists because mmap_region()
calls munmap() prior to installing the new mapping.

This patch set stops rcu readers from seeing the temporal gap by
splitting up the munmap() function into two parts.  The first part
prepares the vma tree for modifications by doing the necessary splits
and tracks the vmas in a side tree.  The second part completes the
munmapping of the vmas after the vma tree has been overwritten (either
by a MAP_FIXED replacement vma or by a NULL in the munmap() case).

Please note that rcu walkers will still be able to see a temporary state
of split vmas that may be in the process of being removed, but the
temporal gap will not be exposed.  vma_start_write() are called on both
parts of the split vma, so this state is detectable.

I am sending this as an RFC as Andrii Nakryiko [1] and Suren
Baghdasaryan are both working on features that require the vma tree to
avoid exposing this temporal gap to rcu readers.

[1] https://lore.kernel.org/all/gkhzuurhqhtozk6u53ufkesbhtjse5ba6kovqm7mnzrqe3szma@3tpbspq7hxjl/

Liam R. Howlett (5):
  mm/mmap: Correctly position vma_iterator in __split_vma()
  mm/mmap: Split do_vmi_align_munmap() into a gather and complete
    operation
  mm/mmap: Introduce vma_munmap_struct for use in munmap operations
  mm/mmap: Change munmap to use vma_munmap_struct() for accounting and
    surrounding vmas
  mm/mmap: Use split munmap calls for MAP_FIXED

 mm/internal.h |  22 +++
 mm/mmap.c     | 382 +++++++++++++++++++++++++++++++-------------------
 2 files changed, 258 insertions(+), 146 deletions(-)