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

[Dev Jain]

On 29/03/25 3:16 pm, 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@xxxxxxx>
>
> I wonder if we want a Fixes: tag here? It's certainly a *latent* bug.

I am struggling to find the commit till which we want to backport. 
Should it be e920722 (arm64: support huge vmalloc mappings)?

> > > ---
> > >   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@xxxxxxxxxx>
> >
> >
> > >   	 * 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