Re: [PATCH] fix mmap return value when vma is merged after call_mmap()

[David Hildenbrand <david@redhat.com>]

On 01.12.20 14:27, Liu Zixian wrote:
> On success, mmap should return the begin address of newly mapped area,
> but patch "mm: mmap: merge vma after call_mmap() if possible" 
> set vm_start of newly merged vma to return value addr.
> Users of mmap will get wrong address if vma is merged after call_mmap().
> We fix this by moving the assignment to addr before merging vma.
> 
> Fixes: d70cec898324 ("mm: mmap: merge vma after call_mmap() if possible")
> 
> Signed-off-by: Liu Zixian <liuzixian4@huawei.com>
> ---
>  mm/mmap.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/mmap.c b/mm/mmap.c
> index d91ecb00d38c..9199b5e8cc1e 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -1820,12 +1820,12 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
>  				 * and cause general protection fault ultimately.
>  				 */
>  				fput(vma->vm_file);
> -				vm_area_free(vma);
> -				vma = merge;
>  				/* Update vm_flags and possible addr to pick up the change. We don't
>  				 * warn here if addr changed as the vma is not linked by vma_link().
>  				 */
>  				addr = vma->vm_start;
> +				vm_area_free(vma);
> +				vma = merge;
>  				vm_flags = vma->vm_flags;
>  				goto unmap_writable;
>  			}
> 

I assume this is quite hard to trigger, right (having vm_flags change)?

"The vm_flags may be changed after call_mmap() because drivers may set
some flags for their own purpose."

Because how you describe it (returning wrong mmap address) this will
completely mess up userspace.

-- 
Thanks,

David / dhildenb