On Mon, 24 Apr 2023 at 15:21, Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
>
> On Mon, Apr 24, 2023 at 09:38:43AM +0200, Dmitry Vyukov wrote:
> > On Mon, 24 Apr 2023 at 09:19, syzbot
> > <syzbot+606f94dfeaaa45124c90@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
> > If I am reading this correctly, it can lead to NULL derefs in
> > folio_mapping() if folio->mapping is read twice. I think
> > folio->mapping reads/writes need to use READ/WRITE_ONCE if racy.
>
> You aren't reading it correctly.
>
> mapping = folio->mapping;
> if ((unsigned long)mapping & PAGE_MAPPING_FLAGS)
> return NULL;
>
> return mapping;
>
> The racing write is storing NULL. So it might return NULL or it might
> return the old mapping, or it might return NULL. Either way, the caller
> has to be prepared for NULL to be returned.
>
> It's a false posiive, but probably worth silencing with a READ_ONCE().
Yes, but the end of the function does not limit effects of races. I
think this can still crash on NULL deref.
The simplest example would be to compile this:
struct address_space *folio_mapping(struct folio *folio)
{
...
mapping = folio->mapping;
if ((unsigned long)mapping & PAGE_MAPPING_FLAGS)
return NULL;
return mapping;
}
ret = !mapping_unevictable(folio_mapping(folio)) &&
!folio_test_mlocked(folio);
static inline bool mapping_unevictable(struct address_space *mapping)
{
return mapping && test_bit(AS_UNEVICTABLE, &mapping->flags);
}
to this:
if (!((unsigned long)folio->mapping & PAGE_MAPPING_FLAGS) && folio->mapping)
if (test_bit(AS_UNEVICTABLE, &folio->mapping->flags))
which does crash.
