Re: [PATCH v3 16/40] mm/vma: only allow mmap to clear VMA_MAYWRITE_BIT if kernel-owned

On Thu Sep 17, 2026 at 12:22 PM EDT, Lorenzo Stoakes (ARM) wrote:

For ordinary files the only way the VMA_MAYWRITE_BIT flag is cleared is if
the underlying file is itself read-only.

This means that mprotect() cannot mark a shared mapping of a read-only file
as read/write, as doing so would violate the read only attribute, and
permit writes.

In general, we do not want file systems to be able to do this for
read/write files.

Doing so would violate fundamental user expectation of file attributes and
likely break userspace.

However, drivers pose a tricky problem here - the /dev/xxx file may be
read/write but provide access to a resource which is fundamentally
read-only.

Therefore we must allow drivers to be able to clear VMA_MAYWRITE_BIT.

To achieve both of these things, restrict this ability to kernel-owned
mappings as identified by vma_flags_is_kernel_owned().

This constrains this ability to drivers which own the mapping's contents,
whether memory-mapped I/O, kernel-allocated pages, or ordinary pages they
map themselves, and so define its semantics.

Every in-tree mmap hook which clears VMA_MAYWRITE_BIT, some twenty sites
across drivers, filesystems and bpf, establishes a kernel-owned mapping,
with usbmon and the ALSA PCM status page converted earlier in this series
to do so.

Note that drivers may, if they do not gate on VMA_SHARED_BIT, be able to
disable MAP_PRIVATE-file-backed mapping CoW semantics.

This is perhaps not always intended, but we retain this capacity to
maintain existing behaviour.

As all drivers which clear VMA_MAYWRITE_BIT establish kernel-owned
mappings, no functional change is intended.

Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
---
 mm/vma.c | 5 +++++
 1 file changed, 5 insertions(+)

Makes sense.

Acked-by: Zi Yan <ziy@nvidia.com>



-- 
Best Regards,
Yan, Zi