Re: [PATCH] mm: don't clobber partially overlapping VMA with MAP_FIXED_NOREPLACE

[Khalid Aziz <khalid.aziz@oracle.com>]

On 10/10/2018 09:27 AM, Jann Horn wrote:
> Daniel Micay reports that attempting to use MAP_FIXED_NOREPLACE in an
> application causes that application to randomly crash. The existing check
> for handling MAP_FIXED_NOREPLACE looks up the first VMA that either
> overlaps or follows the requested region, and then bails out if that VMA
> overlaps *the start* of the requested region. It does not bail out if the
> VMA only overlaps another part of the requested region.
> 
> Fix it by checking that the found VMA only starts at or after the end of
> the requested region, in which case there is no overlap.
> 
> Reported-by: Daniel Micay <danielmicay@gmail.com>
> Fixes: a4ff8e8620d3 ("mm: introduce MAP_FIXED_NOREPLACE")
> Cc: stable@vger.kernel.org
> Signed-off-by: Jann Horn <jannh@google.com>
> ---
>   mm/mmap.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 5f2b2b184c60..f7cd9cb966c0 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -1410,7 +1410,7 @@ unsigned long do_mmap(struct file *file, unsigned long addr,
>   	if (flags & MAP_FIXED_NOREPLACE) {
>   		struct vm_area_struct *vma = find_vma(mm, addr);
>   
> -		if (vma && vma->vm_start <= addr)
> +		if (vma && vma->vm_start < addr + len)
>   			return -EEXIST;
>   	}
>   
> 

Makes sense.

Reviewed-by: Khalid Aziz <khalid.aziz@oracle.com>

--
Khalid