Re: [PATCH 1/1] mm/madvise: initialize prev pointer in madvise_walk_vmas

[Lance Yang]

On 2025/6/17 13:19, Barry Song wrote:
> On Tue, Jun 17, 2025 at 4:57 PM Lance Yang <lance.yang@xxxxxxxxx> wrote:
> > On 2025/6/17 10:24, Barry Song wrote:
> > > On Tue, Jun 17, 2025 at 2:05 PM Lance Yang <ioworker0@xxxxxxxxx> wrote:
> > > > From: Lance Yang <lance.yang@xxxxxxxxx>
> > > >
> > > > The prev pointer was uninitialized, which could lead to undefined behavior
> > > > where its address is taken and passed to the visit() callback without being
> > > > assigned a value.
> > > >
> > > > Initializing it to NULL makes the code safer and prevents potential bugs
> > > > if a future callback function attempts to read from it.
> > >
> > > Is there any read-before-write case here? I haven't found one.
> >
> >
> > It appears that the following is a call chain showing the read-before-write
> > of prev:
> >
> > -> madvise_vma_anon_name(..., struct vm_area_struct **prev, ...)
> >           Receives the address of madvise_walk_vmas's prev.
> >           Passes this pointer directly to madvise_update_vma.
> >           Note that prev is not updated before visit() is called
> >           if !(start > vma->vm_start) in the slow path.
> >
> >           -> madvise_update_vma(..., struct vm_area_struct **prev, ...)
> >                   It calls the next function with *prev.
> >
> >                   -> vma_modify_flags_name(..., *prev, ...)
> >                           Stores the value of madvise_walk_vmas's prev in
> > vmg.prev
> >                           using the VMG_VMA_STATE macro.
> >
> >                           -> vma_modify(struct vma_merge_struct *vmg)
> >                                   Receives the vmg struct.
> >                                   Passes vmg to vma_merge_existing_range.
> >
> >                                   -> vma_merge_existing_range(struct
> > vma_merge_struct *vmg)
> >                                           Retrieves the value: struct
> > vm_area_struct *prev = vmg->prev;
> >                                           The value is now used in a
> > conditional check:
> >                                           VM_WARN_ON_VMG(prev && start <=
> > prev->vm_start, vmg)
> >                                           If prev was uninitialized, this
> > would cause a crash.
>
> Thanks!
>
> Do you have a reproducer? I'd like to try.

Not yet ;)

It was found during code review to prevent potential bugs, and the
initialization itself is harmless.

Thanks,
Lance

> > Thanks,
> > Lance
> >
> > > It also looks like we're assuming that *prev == NULL implies
> > > a specific condition:
> > >
> > > *prev = NULL;   /* tell sys_madvise we drop mmap_lock */
> > >
> > > *prev = NULL; /* mmap_lock has been dropped, prev is stale */
> > >
> > > > Signed-off-by: Lance Yang <lance.yang@xxxxxxxxx>
> > > > ---
> > > >    mm/madvise.c | 4 ++--
> > > >    1 file changed, 2 insertions(+), 2 deletions(-)
> > > >
> > > > diff --git a/mm/madvise.c b/mm/madvise.c
> > > > index 267d8e4adf31..c87325000303 100644
> > > > --- a/mm/madvise.c
> > > > +++ b/mm/madvise.c
> > > > @@ -1536,10 +1536,10 @@ int madvise_walk_vmas(struct mm_struct *mm, unsigned long start,
> > > >                                      struct vm_area_struct **prev, unsigned long start,
> > > >                                      unsigned long end, void *arg))
> > > >    {
> > > > +       struct vm_area_struct *prev = NULL;
> > > >           struct vm_area_struct *vma;
> > > > -       struct vm_area_struct *prev;
> > > > -       unsigned long tmp;
> > > >           int unmapped_error = 0;
> > > > +       unsigned long tmp;
> > > >           int error;
> > > >
> > > >           /*
> > > > --
> > > > 2.49.0
>
> Thanks
> Barry