2021-03-12 12:18:22

by Marco Elver

Subject: [PATCH mm] kfence: zero guard page after out-of-bounds access

After an out-of-bounds accesses, zero the guard page before
re-protecting in kfence_guarded_free(). On one hand this helps make the
failure mode of subsequent out-of-bounds accesses more deterministic,
but could also prevent certain information leaks.

Signed-off-by: Marco Elver <[email protected]>
---
mm/kfence/core.c | 1 +
1 file changed, 1 insertion(+)

diff --git a/mm/kfence/core.c b/mm/kfence/core.c
index 3b8ec938470a..f7106f28443d 100644
--- a/mm/kfence/core.c
+++ b/mm/kfence/core.c
@@ -371,6 +371,7 @@ static void kfence_guarded_free(void *addr, struct kfence_metadata *meta, bool z

/* Restore page protection if there was an OOB access. */
if (meta->unprotected_page) {
+ memzero_explicit((void *)ALIGN_DOWN(meta->unprotected_page, PAGE_SIZE), PAGE_SIZE);
kfence_protect(meta->unprotected_page);
meta->unprotected_page = 0;
}
--
2.31.0.rc2.261.g7f71774620-goog

2021-03-12 13:13:31

by Alexander Potapenko

Subject: Re: [PATCH mm] kfence: zero guard page after out-of-bounds access

On Fri, Mar 12, 2021 at 1:16 PM Marco Elver <[email protected]> wrote:
>
> After an out-of-bounds accesses, zero the guard page before
> re-protecting in kfence_guarded_free(). On one hand this helps make the
> failure mode of subsequent out-of-bounds accesses more deterministic,
> but could also prevent certain information leaks.
>
> Signed-off-by: Marco Elver <[email protected]>
Acked-by: Alexander Potapenko <[email protected]>

> ---
> mm/kfence/core.c | 1 +
> 1 file changed, 1 insertion(+)
>
> diff --git a/mm/kfence/core.c b/mm/kfence/core.c
> index 3b8ec938470a..f7106f28443d 100644
> --- a/mm/kfence/core.c
> +++ b/mm/kfence/core.c
> @@ -371,6 +371,7 @@ static void kfence_guarded_free(void *addr, struct kfence_metadata *meta, bool z
>
> /* Restore page protection if there was an OOB access. */
> if (meta->unprotected_page) {
> + memzero_explicit((void *)ALIGN_DOWN(meta->unprotected_page, PAGE_SIZE), PAGE_SIZE);
> kfence_protect(meta->unprotected_page);
> meta->unprotected_page = 0;
> }
> --
> 2.31.0.rc2.261.g7f71774620-goog
>


--
Alexander Potapenko
Software Engineer

Google Germany GmbH
Erika-Mann-Straße, 33
80636 München

Geschäftsführer: Paul Manicle, Halimah DeLaine Prado
Registergericht und -nummer: Hamburg, HRB 86891
Sitz der Gesellschaft: Hamburg