Re: [PATCH 2/2] mm/hmm: hmm_range_fault() infinite loop

[Christoph Hellwig]

On Fri, Aug 23, 2019 at 03:17:53PM -0700, Ralph Campbell wrote:
> Normally, callers to handle_mm_fault() are supposed to check the
> vma->vm_flags first. hmm_range_fault() checks for VM_READ but doesn't
> check for VM_WRITE if the caller requests a page to be faulted in
> with write permission (via the hmm_range.pfns[] value).
> If the vma is write protected, this can result in an infinite loop:
>   hmm_range_fault()
>     walk_page_range()
>       ...
>       hmm_vma_walk_hole()
>         hmm_vma_walk_hole_()
>           hmm_vma_do_fault()
>             handle_mm_fault(FAULT_FLAG_WRITE)
>             /* returns VM_FAULT_WRITE */
>           /* returns -EBUSY */
>         /* returns -EBUSY */
>       /* returns -EBUSY */
>     /* loops on -EBUSY and range->valid */
> Prevent this by checking for vma->vm_flags & VM_WRITE before calling
> handle_mm_fault().
> 
> Signed-off-by: Ralph Campbell <rcampbell@xxxxxxxxxx>

Looks good,

Reviewed-by: Christoph Hellwig <hch@xxxxxx>