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

[Barry Song <21cnbao@gmail.com>]

On Tue, Jun 17, 2025 at 4:57 PM Lance Yang <lance.yang@linux.dev> wrote:
>
>
>
> On 2025/6/17 10:24, Barry Song wrote:
> > On Tue, Jun 17, 2025 at 2:05 PM Lance Yang <ioworker0@gmail.com> wrote:
> >>
> >> From: Lance Yang <lance.yang@linux.dev>
> >>
> >> 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.

>
> 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@linux.dev>
> >> ---
> >>   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