Re: [syzbot] [mm?] WARNING: suspicious RCU usage in mas_walk (2)

[Matthew Wilcox <willy@infradead.org>]

On Thu, Jul 27, 2023 at 07:59:33PM +0200, Jann Horn wrote:
> On Thu, Jul 27, 2023 at 7:22 PM Suren Baghdasaryan <surenb@google.com> wrote:
> > Hmm. lock_vma_under_rcu() specifically checks for vma->anon_vma==NULL
> > condition (see [1]) to avoid going into find_mergeable_anon_vma() (a
> > check inside anon_vma_prepare() should prevent that). So, it should
> > fall back to mmap_lock'ing.
> 
> This syzkaller report applies to a tree with Willy's in-progress patch
> series, where lock_vma_under_rcu() only checks for vma->anon_vma if
> vma_is_anonymous() is true - it permits private non-anonymous VMAs
> (which require an anon_vma for handling write faults)  even if they
> don't have an anon_vma.
> 
> The commit bisected by syzkaller
> (https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/commit/?id=a52f58b34afe095ebc5823684eb264404dad6f7b)
> removes the vma_is_anonymous() check in handle_pte_fault(), so it lets
> us reach do_wp_page() with a non-anonymous private VMA without
> anon_vma, even though that requires allocation of an anon_vma.
> 
> So I think this is pretty clearly an issue with Willy's in-progress
> patch series that syzkaller blamed correctly.

Agreed.  What do we think the right solution is?

Option 1:

+++ 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;
+       }
+
        delayacct_wpcopy_start();

Option 2:

@@ -5581,7 +5587,8 @@ struct vm_area_struct *lock_vma_under_rcu(struct mm_struct *mm,
                goto inval;

        /* find_mergeable_anon_vma uses adjacent vmas which are not locked */
-       if (vma_is_anonymous(vma) && !vma->anon_vma)
+       if ((vma_is_anonymous(vma) ||
+            vma->vm_flags & (VM_SHARED | VM_MAYSHARE)) && !vma->anon_vma)
                goto inval;

The problem with option 2 is that we don't know whether this is a write
fault or not, so we'll handle read faults on private file
mappings under the mmap_lock UNTIL somebody writes to the mapping, which
might be never.  That seems like a bad idea.

We could pass FAULT_FLAG_WRITE into lock_vma_under_rcu(), but that also
seems like a bad idea.  I dunno.  Three bad ideas.  Anyone think of a
good one?