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Date:   Thu, 27 Jul 2023 19:46:39 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     Suren Baghdasaryan <surenb@google.com>
Cc:     Jann Horn <jannh@google.com>,
        "Liam R. Howlett" <Liam.Howlett@oracle.com>,
        akpm@linux-foundation.org, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, syzkaller-bugs@googlegroups.com,
        syzbot <syzbot+8645fe63c4d22c8d27b8@syzkaller.appspotmail.com>
Subject: Re: [syzbot] [mm?] WARNING: suspicious RCU usage in mas_walk (2)
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On Thu, Jul 27, 2023 at 11:31:01AM -0700, Suren Baghdasaryan wrote:
> A comment for __anon_vma_prepare() says "This must be called with the
> mmap_lock held for reading."
> I think adding an explicit mmap_assert_locked() in this function would
> help catch such issues.

Would it catch them faster than syzbot?

> > +++ b/mm/memory.c
> > @@ -3197,6 +3197,12 @@ static vm_fault_t wp_page_copy(struct vm_fault *vmf)
> >         struct mmu_notifier_range range;
> >         int ret;
> >
> > +       if (!vma->anon_vma) {
> > +               // check if there are other things to undo here
> > +               vma_end_read(vmf->vma);
> > +               return VM_FAULT_RETRY;
> > +       }
> > +
> 
> This one bails out later but if the path is not taken too often I
> think it's cleaner.

OK, then I think the right place to put this check is slightly before
this function is called, to avoid bouncing the folio refcount:

+++ b/mm/memory.c
@@ -3567,6 +3567,12 @@ static vm_fault_t do_wp_page(struct vm_fault *vmf)
                return 0;
        }
 copy:
+       if ((vmf->flags & FAULT_FLAG_VMA_LOCK) && !vma->anon_vma) {
+               pte_unmap_unlock(vmf->pte, vmf->ptl);
+               vma_end_read(vmf->vma);
+               return VM_FAULT_RETRY;
+       }
+
        /*
         * Ok, we need to copy. Oh, well..
         */

... it compiles.  Guess I should figure out the syntax to ask syzbot to
test it.