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

[Lance Yang]

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,
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