Re: [PATCH v2] arm64/mm: avoid fixmap race condition when create pud mapping

Will Deacon
2021-12-13T13:45:12Z

On Mon, Dec 13, 2021 at 11:35:16AM +0100, Ard Biesheuvel wrote:
> On Mon, 13 Dec 2021 at 11:16, Anshuman Khandual
> <anshuman.khandual@arm.com> wrote:
> > On 12/10/21 3:24 PM, Jianyong Wu wrote:
> > > diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> > > index acfae9b41cc8..98ac09ae9588 100644
> > > --- a/arch/arm64/mm/mmu.c
> > > +++ b/arch/arm64/mm/mmu.c
> > > @@ -63,6 +63,7 @@ static pmd_t bm_pmd[PTRS_PER_PMD] __page_aligned_bss __maybe_unused;
> > > static pud_t bm_pud[PTRS_PER_PUD] __page_aligned_bss __maybe_unused;
> > >
> > > static DEFINE_SPINLOCK(swapper_pgdir_lock);
> > > +static DEFINE_SPINLOCK(fixmap_lock);
> > >
> > > void set_swapper_pgd(pgd_t *pgdp, pgd_t pgd)
> > > {
> > > @@ -329,6 +330,11 @@ static void alloc_init_pud(pgd_t *pgdp, unsigned long addr, unsigned long end,
> > > }
> > > BUG_ON(p4d_bad(p4d));
> > >
> > > + /*
> > > + * fixmap is global resource, thus it needs to be protected by a lock
> > > + * in case of race condition.
> > > + */
> >
> > Small nit, format and align this comment block. I guess
> > could also be done while merging this patch as well.
> >
> > > + spin_lock(&fixmap_lock);
> > > pudp = pud_set_fixmap_offset(p4dp, addr);
> > > do {
> > > pud_t old_pud = READ_ONCE(*pudp);
> > > @@ -359,6 +365,7 @@ static void alloc_init_pud(pgd_t *pgdp, unsigned long addr, unsigned long end,
> > > } while (pudp++, addr = next, addr != end);
> > >
> > > pud_clear_fixmap();
> > > + spin_unlock(&fixmap_lock);
> > > }
> > >
> > > static void __create_pgd_mapping(pgd_t *pgdir, phys_addr_t phys,
> > >
> >
> > Reviewed-by: Anshuman Khandual <anshuman.khandual@arm.com>
>
> We have different fixmap slots for different page table levels, so
> 'fixmap_lock' is not the right name.
>
> But we already have swapper_pgdir_lock as well, which serializes the
> use of the pgdir level fixmap slot. And we have no spinlocks
> protecting the other levels.
>
> So should we perhaps clean this up more comprehensively? Wouldn't it
> be better to add a mutex to __create_pgd_mapping(), for instance?

That does sound like a better way to do things, but the simplicity of this
patch is quite attractive for backporting. Would you object to me queuing
it as-is, on the premise that I'm more than happy to take consolidation
changes on top?

Will