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

[Pedro Falcato]

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@xxxxxxxxx>

Reviewed-by: Pedro Falcato <pfalcato@xxxxxxx>

This looks relatively safe. However, I'm not sure if we want this in stable.
This has been broken for 11 years now, with no complaints.

-- 
Pedro