Re: [PATCH] mm/gup: disallow GUP writing to file-backed mappings by default

[Simon Horman <horms@kernel.org>]

On Sun, Apr 23, 2023 at 08:40:53PM +0100, Lorenzo Stoakes wrote:
> On Sun, Apr 23, 2023 at 09:32:01PM +0200, Simon Horman wrote:
> > On Sat, Apr 22, 2023 at 02:37:05PM +0100, Lorenzo Stoakes wrote:
> > > It isn't safe to write to file-backed mappings as GUP does not ensure that
> > > the semantics associated with such a write are performed correctly, for
> > > instance filesystems which rely upon write-notify will not be correctly
> > > notified.
> > >
> > > There are exceptions to this - shmem and hugetlb mappings are (in effect)
> > > anonymous mappings by other names so we do permit this operation in these
> > > cases.
> > >
> > > In addition, if no pinning takes place (neither FOLL_GET nor FOLL_PIN is
> > > specified and neither flags gets implicitly set) then no writing can occur
> > > so we do not perform the check in this instance.
> > >
> > > This is an important exception, as populate_vma_page_range() invokes
> > > __get_user_pages() in this way (and thus so does __mm_populate(), used by
> > > MAP_POPULATE mmap() and mlock() invocations).
> > >
> > > There are GUP users within the kernel that do nevertheless rely upon this
> > > behaviour, so we introduce the FOLL_ALLOW_BROKEN_FILE_MAPPING flag to
> > > explicitly permit this kind of GUP access.
> > >
> > > This is required in order to not break userspace in instances where the
> > > uAPI might permit file-mapped addresses - a number of RDMA users require
> > > this for instance, as do the process_vm_[read/write]v() system calls,
> > > /proc/$pid/mem, ptrace and SDT uprobes. Each of these callers have been
> > > updated to use this flag.
> > >
> > > Making this change is an important step towards a more reliable GUP, and
> > > explicitly indicates which callers might encouter issues moving forward.
> >
> > nit: s/encouter/encounter/
> >
> 
> Ack, I always seem to leave at least one or two easter egg spelling
> mistakes in :)

:)

> Will fix up on next respin (in unlikely event of no further review,
> hopefully Andrew would pick up!)
>