Re: [PATCH v2] userfaultfd: allow registration of ranges below mmap_min_addr

[Mike Rapoport <rppt@kernel.org>]

On Thu, Apr 09, 2026 at 01:33:45PM +0300, Denis M. Karpov wrote:
> The current implementation of validate_range() in fs/userfaultfd.c
> performs a hard check against mmap_min_addr. This is redundant because
> UFFDIO_REGISTER operates on memory ranges that must already be backed
> by a VMA.
> 
> Enforcing mmap_min_addr or capability checks again in userfaultfd is
> unnecessary and prevents applications like binary compilers from
> using UFFD for valid memory regions mapped by application.
> 
> Remove the redundant check for mmap_min_addr.
> 
> Fixes: 86039bd3b4e6 ("userfaultfd: add new syscall to provide memory externalization")
> Signed-off-by: Denis M. Karpov <komlomal@gmail.com>

Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>

> ---
> v2:
> - Remove the check entirely rather than replacing it, as suggested by
>   Harry Yoo and Lorenzo Stoakes.
> - Added Fixes tag.
> - Link to v1: https://lore.kernel.org/r/20260407081442.6256-1-komlomal@gmail.com
> ---
>  fs/userfaultfd.c | 2 --
>  1 file changed, 2 deletions(-)
> 
> diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
> index bdc84e521..4b53dc4a3 100644
> --- a/fs/userfaultfd.c
> +++ b/fs/userfaultfd.c
> @@ -1238,8 +1238,6 @@ static __always_inline int validate_unaligned_range(
>  		return -EINVAL;
>  	if (!len)
>  		return -EINVAL;
> -	if (start < mmap_min_addr)
> -		return -EINVAL;
>  	if (start >= task_size)
>  		return -EINVAL;
>  	if (len > task_size - start)
> -- 
> 2.43.0
> 

-- 
Sincerely yours,
Mike.