Re: [PATCH v3 2/5] mm/mseal: update madvise() logic

[Lorenzo Stoakes]

On Fri, Jul 25, 2025 at 09:38:14AM +0200, David Hildenbrand wrote:
> > >> ... assuming we have:
> > >
> > > 1. A MAP_PRIVATE R/O file-backed mapping.
> > > 2. The mapping is mseal()'d.
> > >
> > > We only really have anon folios in there with things like (a) uprobe (b)
> > > debugger access (c) similarly weird FOLL_FORCE stuff.
> > >
> > > Now, most executables/libraries are mapped that way. If someone would rely
> > > on MADV_DONTNEED to zap pages in there (to free up memory), that would get
> > > rejected.
> >
> > Right, yes.
> >
> > This is odd behaviour to me. But I guess this is what Jeff meant by 'detecting
> > this' in android.
>
> It's rather weird usage of MADV_DONTNEED, but maybe, for some R/O buffers
> ...

Yeah, curious if anybody is actually doing this.

> > >
> > > Checking for anon_vma in addition, ad mentioned in the other thread, would
> > > be a "cheap" check to rule out that there are currently anon vmas in there.
> > >
> > > Well, not 100% reliable, because MADV_DONTNEED can race with page faults ...
> >
> > But hang on, it's read-only so we shouldn't get racing faults... right?
>
> You mean, ones that populate anon folios.

Right, but these are the only ones we care about right? file-backed mappings
won't change vma->anon_vma.

Changes to that field from NULL use mmap read lock and... page_table_lock :P
fun.

>
> Well, there is long-term pinning that can break COW and other weird stuff
> like FOLL_FORCE. Most of the latter probably holds the mmap lock in write
> mode. Probably.

Well GUP uses read lock.

FOLL_FORCE won't override anything as we have this check in check_vma_flags():

	if (write) {
		if (!vma_anon &&
		    !writable_file_mapping_allowed(vma, gup_flags))
			return -EFAULT;

		if (!(vm_flags & VM_WRITE) || (vm_flags & VM_SHADOW_STACK)) {
			if (!(gup_flags & FOLL_FORCE))
				return -EFAULT;
			/*
			 * We used to let the write,force case do COW in a
			 * VM_MAYWRITE VM_SHARED !VM_WRITE vma, so ptrace could
			 * set a breakpoint in a read-only mapping of an
			 * executable, without corrupting the file (yet only
			 * when that file had been opened for writing!).
			 * Anon pages in shared mappings are surprising: now
			 * just reject it.
			 */
			if (!is_cow_mapping(vm_flags))
				return -EFAULT;
		}
	}

With:

static inline bool is_cow_mapping(vm_flags_t flags)
{
	return (flags & (VM_SHARED | VM_MAYWRITE)) == VM_MAYWRITE;
}

So - we explicitly disallow FOLL_FORCE write override for CoW file-backed
mappings.

Obviously if FOLL_FORCE is not set, then we're ALSO not allowed to get past a
FOLL_WRITE and !VM_WRITE situation.

>
> >
> > Hmm maybe I'll soften on this anon_vma idea then. Maybe it is a 'cheap fix' to
> > rule out the _usual_ cases.
>
> Yeah, something to evaluate.

I'm thinking more and more we're probably actually safe with !vma->anon_vma ||
!(vma->vm_flags & VM_MAYWRITE).

>
> --
> Cheers,
>
> David / dhildenb
>

Cheers, Lorenzo