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

[Dev Jain]

On 28/03/25 8:09 pm, Ryan Roberts wrote:
> On 28/03/2025 02:21, Dev Jain wrote:
> > arm64 uses apply_to_page_range to change permissions for kernel VA mappings,
> > which does not support changing permissions for leaf mappings. This function
>
> I think you mean "block" mappings here? A leaf mapping refers to a page table
> entry that maps a piece of memory at any level in the pgtable (i.e. a present
> entry that does not map a table).
>
> A block mapping is an Arm ARM term used to mean a leaf mapping at a level other
> than the last level (e.g. pmd, pud). A page mapping is an Arm ARM term used to
> mean a leaf mapping at the last level (e.g. pte).
>
> > will change permissions until it encounters a leaf mapping, and will bail
>
> block mapping
>
> > out. To avoid this partial change, explicitly disallow changing permissions
> > for VM_ALLOW_HUGE_VMAP mappings.
>
> It will also emit a warning. Since there are no reports of this triggering, it
> implies that there are currently no cases of code doing a vmalloc_huge()
> followed by partial permission change, at least on arm64 (I'm told BPF does do
> this on x86 though). But this is a footgun waiting to go off, so let's detect it
> early and avoid the possibility of permissions in an intermediate state. (It
> might be worth wordsmithing this into the commit log).

Thanks, I will add this.

> > Signed-off-by: Dev Jain <dev.jain@xxxxxxx>
>
> With the commit log fixed up:
>
> Reviewed-by: Ryan Roberts <ryan.roberts@xxxxxxx>

Thanks!

> > ---
> >   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
> >   	 * 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)