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

[Lance Yang <lance.yang@linux.dev>]

On 2025/6/17 13:19, Barry Song wrote:
> 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.

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@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