2023-11-21 22:04:15

by Ilya Leoshkevich

Subject: [PATCH v2 17/33] mm: kfence: Disable KMSAN when checking the canary

KMSAN warns about check_canary() accessing the canary.

The reason is that, even though set_canary() is properly instrumented
and sets shadow, slub explicitly poisons the canary's address range
afterwards.

Unpoisoning the canary is not the right thing to do: only
check_canary() is supposed to ever touch it. Instead, disable KMSAN
checks around canary read accesses.

Signed-off-by: Ilya Leoshkevich <[email protected]>
---
mm/kfence/core.c | 5 +++--
1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/mm/kfence/core.c b/mm/kfence/core.c
index 3872528d0963..a2ea8e5a1ad9 100644
--- a/mm/kfence/core.c
+++ b/mm/kfence/core.c
@@ -306,7 +306,7 @@ metadata_update_state(struct kfence_metadata *meta, enum kfence_object_state nex
}

/* Check canary byte at @addr. */
-static inline bool check_canary_byte(u8 *addr)
+__no_kmsan_checks static inline bool check_canary_byte(u8 *addr)
{
struct kfence_metadata *meta;
unsigned long flags;
@@ -341,7 +341,8 @@ static inline void set_canary(const struct kfence_metadata *meta)
*((u64 *)addr) = KFENCE_CANARY_PATTERN_U64;
}

-static inline void check_canary(const struct kfence_metadata *meta)
+__no_kmsan_checks static inline void
+check_canary(const struct kfence_metadata *meta)
{
const unsigned long pageaddr = ALIGN_DOWN(meta->addr, PAGE_SIZE);
unsigned long addr = pageaddr;
--
2.41.0

2023-12-08 12:54:39

by Alexander Potapenko

Subject: Re: [PATCH v2 17/33] mm: kfence: Disable KMSAN when checking the canary

On Tue, Nov 21, 2023 at 11:02 PM Ilya Leoshkevich <[email protected]> wrote:
>
> KMSAN warns about check_canary() accessing the canary.
>
> The reason is that, even though set_canary() is properly instrumented
> and sets shadow, slub explicitly poisons the canary's address range
> afterwards.
>
> Unpoisoning the canary is not the right thing to do: only
> check_canary() is supposed to ever touch it. Instead, disable KMSAN
> checks around canary read accesses.
>
> Signed-off-by: Ilya Leoshkevich <[email protected]>
Reviewed-by: Alexander Potapenko <[email protected]>

2023-12-08 13:56:45

by Alexander Potapenko

Subject: Re: [PATCH v2 17/33] mm: kfence: Disable KMSAN when checking the canary

On Fri, Dec 8, 2023 at 1:53 PM Alexander Potapenko <[email protected]> wrote:
>
> On Tue, Nov 21, 2023 at 11:02 PM Ilya Leoshkevich <[email protected]> wrote:
> >
> > KMSAN warns about check_canary() accessing the canary.
> >
> > The reason is that, even though set_canary() is properly instrumented
> > and sets shadow, slub explicitly poisons the canary's address range
> > afterwards.
> >
> > Unpoisoning the canary is not the right thing to do: only
> > check_canary() is supposed to ever touch it. Instead, disable KMSAN
> > checks around canary read accesses.
> >
> > Signed-off-by: Ilya Leoshkevich <[email protected]>
> Reviewed-by: Alexander Potapenko <[email protected]>

and even

Tested-by: Alexander Potapenko <[email protected]>