Re: [PATCH] arm64: pageattr: Explicitly bail out when changing permissions for vmalloc_huge mappings

[Mike Rapoport <rppt@kernel.org>]

On Sat, Mar 29, 2025 at 09:46:56AM +0000, Ryan Roberts wrote:
> On 28/03/2025 18:50, Mike Rapoport wrote:
> > On Fri, Mar 28, 2025 at 11:51:03AM +0530, Dev Jain wrote:
> >> arm64 uses apply_to_page_range to change permissions for kernel VA mappings,
> > 
> >                                                      for vmalloc mappings ^
> > 
> > arm64 does not allow changing permissions to any VA address right now.
> > 
> >> which does not support changing permissions for leaf mappings. This function
> >> will change permissions until it encounters a leaf mapping, and will bail
> >> out. To avoid this partial change, explicitly disallow changing permissions
> >> for VM_ALLOW_HUGE_VMAP mappings.
> >>
> >> Signed-off-by: Dev Jain <dev.jain@arm.com>
> 
> I wonder if we want a Fixes: tag here? It's certainly a *latent* bug.

We have only a few places that use vmalloc_huge() or VM_ALLOW_HUGE_VMAP and
if there was a code that plays permission games on these allocations, x86
set_memory would blow up immediately, so I don't think Fixes: is needed
here.
 
> >> ---
> >>  arch/arm64/mm/pageattr.c | 4 ++--
> >>  1 file changed, 2 insertions(+), 2 deletions(-)
> >>
> >> diff --git a/arch/arm64/mm/pageattr.c b/arch/arm64/mm/pageattr.c
> >> index 39fd1f7ff02a..8337c88eec69 100644
> >> --- a/arch/arm64/mm/pageattr.c
> >> +++ b/arch/arm64/mm/pageattr.c
> >> @@ -96,7 +96,7 @@ static int change_memory_common(unsigned long addr, int numpages,
> >>  	 * we are operating on does not result in such splitting.
> >>  	 *
> >>  	 * Let's restrict ourselves to mappings created by vmalloc (or vmap).
> >> -	 * Those are guaranteed to consist entirely of page mappings, and
> >> +	 * Disallow VM_ALLOW_HUGE_VMAP vmalloc mappings so that
> > 
> > I'd keep mention of page mappings in the comment, e.g
> > 
> > 	* Disallow VM_ALLOW_HUGE_VMAP mappings to guarantee that only page
> > 	* mappings are updated and splitting is never needed.
> > 
> > With this and changelog updates Ryan asked for
> > 
> > Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
> > 
> > 
> >>  	 * splitting is never needed.
> >>  	 *
> >>  	 * So check whether the [addr, addr + size) interval is entirely
> >> @@ -105,7 +105,7 @@ static int change_memory_common(unsigned long addr, int numpages,
> >>  	area = find_vm_area((void *)addr);
> >>  	if (!area ||
> >>  	    end > (unsigned long)kasan_reset_tag(area->addr) + area->size ||
> >> -	    !(area->flags & VM_ALLOC))
> >> +	    ((area->flags & (VM_ALLOC | VM_ALLOW_HUGE_VMAP)) != VM_ALLOC))
> >>  		return -EINVAL;
> >>  
> >>  	if (!numpages)
> >> -- 
> >> 2.30.2
> >>
> > 
> 

-- 
Sincerely yours,
Mike.