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

[Mike Rapoport <rppt@kernel.org>]

Hi Ryan,

On Tue, Apr 01, 2025 at 10:43:01AM +0100, Ryan Roberts wrote:
> On 30/03/2025 03:32, 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.
> 
> Hi Mike,
> 
> I think I may have misunderstood your comments when we spoke at LSF/MM the other
> day, as this statement seems to contradict. I thought you said that on x86 BPF
> allocates memory using vmalloc_huge()/VM_ALLOW_HUGE_VMAP and then it's
> sub-allocator will set_memory_*() on a sub-region of that allocation? (And we
> then agreed that it would be good for arm64 to eventually support this with BBML2).

I misremembered :)
They do allocate several PMD_SIZE chunks at once, but they don't use
vmalloc_huge(), so everything there is mapped with base pages.

And now they are using execmem rather than vmalloc directly, and execmem
doesn't use VM_ALLOW_HUGE_VMAP anywhere except modules on x86.
 
> Anyway, regardless, I think this change is useful first step to improving
> vmalloc as it makes us more defensive against any future attempt to change
> permissions on a huge allocation. In the long term I'd like to get to the point
> where arm64 (with BBML2) can map with VM_ALLOW_HUGE_VMAP by default.
> 
> Thanks,
> Ryan

-- 
Sincerely yours,
Mike.