Re: [PATCH] buildid: validate page-backed file before parsing build ID

[David Hildenbrand (Red Hat)]

On 1/5/26 23:52, Andrii Nakryiko wrote:
> On Tue, Dec 30, 2025 at 2:11 PM David Hildenbrand (Red Hat)
> <david@xxxxxxxxxx> wrote:
> > On 12/23/25 18:29, Andrew Morton wrote:
> > > On Tue, 23 Dec 2025 18:32:07 +0800 Jinchao Wang <wangjinchao600@xxxxxxxxx> wrote:
> > >
> > > > __build_id_parse() only works on page-backed storage.  Its helper paths
> > > > eventually call mapping->a_ops->read_folio(), so explicitly reject VMAs
> > > > that do not map a regular file or lack valid address_space operations.
> > > >
> > > > Reported-by: syzbot+e008db2ac01e282550ee@xxxxxxxxxxxxxxxxxxxxxxxxx
> > > > Signed-off-by: Jinchao Wang <wangjinchao600@xxxxxxxxx>
> > > >
> > > > ...
> > > >
> > > > --- a/lib/buildid.c
> > > > +++ b/lib/buildid.c
> > > > @@ -280,7 +280,10 @@ static int __build_id_parse(struct vm_area_struct *vma, unsigned char *build_id,
> > > >       int ret;
> > > >
> > > >       /* only works for page backed storage  */
> > > > -    if (!vma->vm_file)
> > > > +    if (!vma->vm_file ||
> > > > +        !S_ISREG(file_inode(vma->vm_file)->i_mode) ||
> > > > +        !vma->vm_file->f_mapping->a_ops ||
> > > > +        !vma->vm_file->f_mapping->a_ops->read_folio)
> > > >               return -EINVAL;
> >
> > Just wondering, we are fine with MAP_PRIVATE files, right? I guess it's
> > not about the actual content in the VMA (which might be different for a
> > MAP_PRIVATE VMA), but only about the content of the mapped file.
>
> Yep, this code is fetching contents of a file that backs given VMA.

Good!

> > LGTM, although I wonder whether some of these these checks should be
> > exposed as part of the read_cache_folio()/do_read_cache_folio() API.
> >
> > Like, having a helper function that tells us whether we can use
> > do_read_cache_folio() against a given mapping+file.
>
> I agree, this seems to be leaking a lot of internal mm details into
> higher-level caller (__build_id_parse). Right now we try to fetch
> folio with filemap_get_folio() and if that succeeds, then we do
> read_cache_folio. Would it be possible for filemap_get_folio() to
> return error if the folio cannot be read using read_cache_folio()? Or
> maybe have a variant of filemap_get_folio() that would have this
> semantic?

Good question. But really, for files that always have everything in the pagecache,
there would not be a problem, right? I'm thinking about hugetlb, for example.

There, we never expect to fallback to do_read_cache_folio().

So maybe we could just teach do_read_cache_folio() to fail properly?

diff --git a/mm/filemap.c b/mm/filemap.c
index ebd75684cb0a7..3f81b8481af4c 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -4051,8 +4051,11 @@ static struct folio *do_read_cache_folio(struct address_space *mapping,
        struct folio *folio;
        int err;
 
-       if (!filler)
+       if (!filler) {
+               if (!mapping->a_ops || !mapping->a_ops->read_folio)
+                       return ERR_PTR(-EINVAL);
                filler = mapping->a_ops->read_folio;
+       }
 repeat:
        folio = filemap_get_folio(mapping, index);
        if (IS_ERR(folio)) {

Then __build_id_parse() would only check for the existence of vma->vm_file and maybe
the !S_ISREG(file_inode(vma->vm_file)->i_mode).


--
Cheers

David