[PATCH v4 2/3] lsm: add backing_file LSM hooks

[Paul Moore]

On Fri, Apr 3, 2026 at 2:12 AM Amir Goldstein <amir73il at gmail.com> wrote:
> On Fri, Apr 3, 2026 at 5:09 AM Paul Moore <paul at paul-moore.com> wrote:
> >
> > Stacked filesystems such as overlayfs do not currently provide the
> > necessary mechanisms for LSMs to properly enforce access controls on the
> > mmap() and mprotect() operations.  In order to resolve this gap, a LSM
> > security blob is being added to the backing_file struct and the following
> > new LSM hooks are being created:
> >
> >  security_backing_file_alloc()
> >  security_backing_file_free()
> >  security_mmap_backing_file()
> >
> > The first two hooks are to manage the lifecycle of the LSM security blob
> > in the backing_file struct, while the third provides a new mmap() access
> > control point for the underlying backing file.  It is also expected that
> > LSMs will likely want to update their security_file_mprotect() callback
> > to address issues with their mprotect() controls, but that does not
> > require a change to the security_file_mprotect() LSM hook.
> >
> > There are a three other small changes to support these new LSM hooks:
> > * Pass the user file associated with a backing file down to
> > alloc_empty_backing_file() so it can be included in the
> > security_backing_file_alloc() hook.
> > * Add getter and setter functions for the backing_file struct LSM blob
> > as the backing_file struct remains private to fs/file_table.c.
> > * Constify the file struct field in the LSM common_audit_data struct to
> > better support LSMs that need to pass a const file struct pointer into
> > the common LSM audit code.
> >
> > Thanks to Arnd Bergmann for identifying the missing EXPORT_SYMBOL_GPL()
> > and supplying a fixup.
> >
> > Cc: stable at vger.kernel.org
> > Cc: linux-fsdevel at vger.kernel.org
> > Cc: linux-unionfs at vger.kernel.org
> > Cc: linux-erofs at lists.ozlabs.org
> > Signed-off-by: Paul Moore <paul at paul-moore.com>
>
> That looks nicer.
>
> Reviewed-by: Amir Goldstein <amir73il at gmail.com>
>
> Thanks,
> Amir.

Thanks for refreshing your review.  Since we are at the end of -rc6,
it probably doesn't make much sense to put this in lsm/stable-7.0; I'm
going to merge this into lsm/dev which should give us at least one
week in linux-next before the v7.1 merge window opens.  If others want
to add their ACKs/Reviewed-by during that time, I'll update the
branch.

-- 
paul-moore.com