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

[Mike Rapoport <rppt@kernel.org>]

On Sun, Mar 30, 2025 at 01:53:57PM +0530, Dev Jain wrote:
> 
> 
> On 30/03/25 1:02 pm, Mike Rapoport wrote:
> > 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.
> 
> But I think x86 can handle this (split_large_page() in __change_page_attr())
> ?

Yes, but it also updates corresponding direct map entries when vmalloc
permissions change and the direct map update presumes physical contiguity
of the range. 

> > > > > ---
> > > > >   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.