Re: [PATCH] mm/mmap/vma_merge: always check invariants

[Lorenzo Stoakes]

On Wed, May 10, 2023 at 05:17:49PM +0100, Mark Rutland wrote:
> On Wed, May 10, 2023 at 09:04:44AM -0700, Lorenzo Stoakes wrote:
> > On Wed, May 10, 2023 at 03:15:51PM +0100, Mark Rutland wrote:
> > > Hi,
> > >
> > > On Sun, Apr 30, 2023 at 09:19:17PM +0100, Lorenzo Stoakes wrote:
> > > > We may still have inconsistent input parameters even if we choose not to
> > > > merge and the vma_merge() invariant checks are useful for checking this
> > > > with no production runtime cost (these are only relevant when
> > > > CONFIG_DEBUG_VM is specified).
> > > >
> > > > Therefore, perform these checks regardless of whether we merge.
> > > >
> > > > This is relevant, as a recent issue (addressed in commit "mm/mempolicy:
> > > > Correctly update prev when policy is equal on mbind") in the mbind logic
> > > > was only picked up in the 6.2.y stable branch where these assertions are
> > > > performed prior to determining mergeability.
> > > >
> > > > Had this remained the same in mainline this issue may have been picked up
> > > > faster, so moving forward let's always check them.
> > > >
> > > > Signed-off-by: Lorenzo Stoakes <lstoakes@xxxxxxxxx>
> > > > ---
> > > >  mm/mmap.c | 10 +++++-----
> > > >  1 file changed, 5 insertions(+), 5 deletions(-)
> > > >
> > > > diff --git a/mm/mmap.c b/mm/mmap.c
> > > > index 5522130ae606..13678edaa22c 100644
> > > > --- a/mm/mmap.c
> > > > +++ b/mm/mmap.c
> > > > @@ -960,17 +960,17 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
> > > >  		merge_next = true;
> > > >  	}
> > > >
> > > > +	/* Verify some invariant that must be enforced by the caller. */
> > > > +	VM_WARN_ON(prev && addr <= prev->vm_start);
> > > > +	VM_WARN_ON(curr && (addr != curr->vm_start || end > curr->vm_end));
> > > > +	VM_WARN_ON(addr >= end);
> > > > +
> > >
> > > I'm seeing this fire a lot when fuzzing v6.4-rc1 on arm64 using Syzkaller.
> > >
> >
> > Thanks, from the line I suspect addr != curr->vm_start, but need to look
> > into the repro, at lsf/mm so a bit time lagged :)
>
> No problem; FWIW I can confirm your theory, the reproducer is causing:
>
> 	addr > curr->vm_start
>
> ... confirmed the the following hack, log below.

Awesome thanks for that! Just been firing up qemu to do this.

Cases 5-8 should really have addr == curr->vm_start, I wonder if it's
another case but curr is being set incorrectly, it should in theory not be
the case.

(See [1] for a visualisation of merge cases as a handy reference)

Of course userfaultfd might be the offender here and might be relying on no
merge case arising but passing dodgy parameters.

[1]:https://ljs.io/merge_cases.png

>
> | diff --git a/mm/mmap.c b/mm/mmap.c
> | index 13678edaa22c..2cdebba15719 100644
> | --- a/mm/mmap.c
> | +++ b/mm/mmap.c
> | @@ -961,9 +961,21 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
> |         }
> |
> |         /* Verify some invariant that must be enforced by the caller. */
> | -       VM_WARN_ON(prev && addr <= prev->vm_start);
> | -       VM_WARN_ON(curr && (addr != curr->vm_start || end > curr->vm_end));
> | -       VM_WARN_ON(addr >= end);
> | +       VM_WARN(prev && addr <= prev->vm_start,
> | +               "addr = 0x%016lx, prev->vm_start = 0x%016lx\n",
> | +               addr, prev->vm_start);
> | +
> | +       VM_WARN(curr && addr != curr->vm_start,
> | +               "addr = 0x%016lx, curr->vm_start = 0x%016lx\n",
> | +               addr, curr->vm_start);
> | +
> | +       VM_WARN(curr && addr > curr->vm_end,
> | +               "addr = 0x%016lx, curr->vm_end = 0x%016lx\n",
> | +               addr, curr->vm_end);
> | +
> | +       VM_WARN(addr >= end,
> | +               "addr = 0x%016lx, end = 0x%016lx\n",
> | +               addr, end);
> |
> |         if (!merge_prev && !merge_next)
> |                 return NULL; /* Not mergeable. */
>
> ... with that applied, running the reproducer results in:
>
> | addr = 0x0000ffff99dc2000, curr->vm_start = 0x0000ffff99db2000
> | WARNING: CPU: 0 PID: 163 at mm/mmap.c:968 vma_merge+0x3d4/0x1260
>
> ... i.e. addr > curr->vm_start
>
> Thanks,
> Mark.