On Thu, May 20, 2021 at 12:55:21PM +0100, Steven Price wrote:
> On 19/05/2021 19:06, Catalin Marinas wrote:
> > On Mon, May 17, 2021 at 01:32:34PM +0100, Steven Price wrote:
> >> A KVM guest could store tags in a page even if the VMM hasn't mapped
> >> the page with PROT_MTE. So when restoring pages from swap we will
> >> need to check to see if there are any saved tags even if !pte_tagged().
> >>
> >> However don't check pages for which pte_access_permitted() returns false
> >> as these will not have been swapped out.
> >>
> >> Signed-off-by: Steven Price <steven.price@xxxxxxx>
> >> ---
> >> arch/arm64/include/asm/pgtable.h | 9 +++++++--
> >> arch/arm64/kernel/mte.c | 16 ++++++++++++++--
> >> 2 files changed, 21 insertions(+), 4 deletions(-)
> >>
> >> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> >> index 0b10204e72fc..275178a810c1 100644
> >> --- a/arch/arm64/include/asm/pgtable.h
> >> +++ b/arch/arm64/include/asm/pgtable.h
> >> @@ -314,8 +314,13 @@ static inline void set_pte_at(struct mm_struct *mm, unsigned long addr,
> >> if (pte_present(pte) && pte_user_exec(pte) && !pte_special(pte))
> >> __sync_icache_dcache(pte);
> >>
> >> - if (system_supports_mte() &&
> >> - pte_present(pte) && pte_tagged(pte) && !pte_special(pte))
> >> + /*
> >> + * If the PTE would provide user space access to the tags associated
> >> + * with it then ensure that the MTE tags are synchronised. Exec-only
> >> + * mappings don't expose tags (instruction fetches don't check tags).
> >> + */
> >> + if (system_supports_mte() && pte_present(pte) &&
> >> + pte_access_permitted(pte, false) && !pte_special(pte))
> >> mte_sync_tags(ptep, pte);
> >
> > Looking at the mte_sync_page_tags() logic, we bail out early if it's the
> > old pte is not a swap one and the new pte is not tagged. So we only need
> > to call mte_sync_tags() if it's a tagged new pte or the old one is swap.
> > What about changing the set_pte_at() test to:
> >
> > if (system_supports_mte() && pte_present(pte) && !pte_special(pte) &&
> > (pte_tagged(pte) || is_swap_pte(READ_ONCE(*ptep))))
> > mte_sync_tags(ptep, pte);
> >
> > We can even change mte_sync_tags() to take the old pte directly:
> >
> > if (system_supports_mte() && pte_present(pte) && !pte_special(pte)) {
> > pte_t old_pte = READ_ONCE(*ptep);
> > if (pte_tagged(pte) || is_swap_pte(old_pte))
> > mte_sync_tags(old_pte, pte);
> > }
> >
> > It would save a function call in most cases where the page is not
> > tagged.
>
> Yes that looks like a good optimisation - although you've missed the
> pte_access_permitted() part of the check ;)
I was actually wondering if we could remove it. I don't think it buys us
much as we have a pte_present() check already, so we know it is pointing
to a valid page. Currently we'd only get a tagged pte on user mappings,
same with swap entries.
When vmalloc kasan_hw will be added, I think we have a set_pte_at() with
a tagged pte but init_mm and high address (we might as well add a
warning if addr > TASK_SIZE_64 on the mte_sync_tags path so that we
don't forget).
> The problem I hit is one of include dependencies:
>
> is_swap_pte() is defined (as a static inline) in
> include/linux/swapops.h. However the definition depends on
> pte_none()/pte_present() which are defined in pgtable.h - so there's a
> circular dependency.
>
> Open coding is_swap_pte() in set_pte_at() works, but it's a bit ugly.
> Any ideas on how to improve on the below?
>
> if (system_supports_mte() && pte_present(pte) &&
> pte_access_permitted(pte, false) && !pte_special(pte)) {
> pte_t old_pte = READ_ONCE(*ptep);
> /*
> * We only need to synchronise if the new PTE has tags enabled
> * or if swapping in (in which case another mapping may have
> * set tags in the past even if this PTE isn't tagged).
> * (!pte_none() && !pte_present()) is an open coded version of
> * is_swap_pte()
> */
> if (pte_tagged(pte) || (!pte_none(pte) && !pte_present(pte)))
> mte_sync_tags(old_pte, pte);
> }
That's why I avoided testing my suggestion ;). I think we should just
add !pte_none() in there with a comment that it may be a swap pte and
use the is_swap_pte() again on the mte_sync_tags() path. We already have
the pte_present() check.
--
Catalin
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