* [PATCH] kasan: fix kasan_byte_accessible() to be consistent with actual checks
@ 2021-04-05 21:43 Peter Collingbourne
2021-04-05 21:52 ` Andrey Konovalov
0 siblings, 1 reply; 3+ messages in thread
From: Peter Collingbourne @ 2021-04-05 21:43 UTC (permalink / raw)
To: Marco Elver, Dmitry Vyukov, Alexander Potapenko
Cc: Peter Collingbourne, Evgenii Stepanov, Andrey Konovalov, linux-mm,
linux-kernel
We can sometimes end up with kasan_byte_accessible() being called
on non-slab memory. For example ksize() and krealloc() may end up
calling it on KFENCE allocated memory. In this case the memory will
be tagged with KASAN_SHADOW_INIT, which a subsequent patch ("kasan:
initialize shadow to TAG_INVALID for SW_TAGS") will set to the same
value as KASAN_TAG_INVALID, causing kasan_byte_accessible() to fail
when called on non-slab memory.
This highlighted the fact that the check in kasan_byte_accessible()
was inconsistent with checks as implemented for loads and stores
(kasan_check_range() in SW tags mode and hardware-implemented
checks in HW tags mode). kasan_check_range() does not have a
check for KASAN_TAG_INVALID, and instead has a comparison against
KASAN_SHADOW_START. In HW tags mode, we do not have either, but we
do set TCR_EL1.TCMA which corresponds with the comparison against
KASAN_TAG_KERNEL.
Therefore, update kasan_byte_accessible() for both SW and HW tags
modes to correspond with the respective checks on loads and stores.
Link: https://linux-review.googlesource.com/id/Ic6d40803c57dcc6331bd97fbb9a60b0d38a65a36
Signed-off-by: Peter Collingbourne <pcc@google.com>
---
mm/kasan/kasan.h | 3 +--
mm/kasan/sw_tags.c | 8 +++++---
2 files changed, 6 insertions(+), 5 deletions(-)
diff --git a/mm/kasan/kasan.h b/mm/kasan/kasan.h
index 8c55634d6edd..e18e8da35255 100644
--- a/mm/kasan/kasan.h
+++ b/mm/kasan/kasan.h
@@ -368,8 +368,7 @@ static inline bool kasan_byte_accessible(const void *addr)
u8 ptr_tag = get_tag(addr);
u8 mem_tag = hw_get_mem_tag((void *)addr);
- return (mem_tag != KASAN_TAG_INVALID) &&
- (ptr_tag == KASAN_TAG_KERNEL || ptr_tag == mem_tag);
+ return ptr_tag == KASAN_TAG_KERNEL || ptr_tag == mem_tag;
}
#else /* CONFIG_KASAN_HW_TAGS */
diff --git a/mm/kasan/sw_tags.c b/mm/kasan/sw_tags.c
index 94c2d33be333..914225eeda99 100644
--- a/mm/kasan/sw_tags.c
+++ b/mm/kasan/sw_tags.c
@@ -121,10 +121,12 @@ bool kasan_check_range(unsigned long addr, size_t size, bool write,
bool kasan_byte_accessible(const void *addr)
{
u8 tag = get_tag(addr);
- u8 shadow_byte = READ_ONCE(*(u8 *)kasan_mem_to_shadow(kasan_reset_tag(addr)));
+ void *untagged_addr = kasan_reset_tag(addr);
+ u8 shadow_byte = READ_ONCE(*(u8 *)kasan_mem_to_shadow(untagged_addr));
- return (shadow_byte != KASAN_TAG_INVALID) &&
- (tag == KASAN_TAG_KERNEL || tag == shadow_byte);
+ return untagged_addr >=
+ kasan_shadow_to_mem((void *)KASAN_SHADOW_START) &&
+ (tag == KASAN_TAG_KERNEL || tag == shadow_byte);
}
#define DEFINE_HWASAN_LOAD_STORE(size) \
--
2.31.0.208.g409f899ff0-goog
^ permalink raw reply related [flat|nested] 3+ messages in thread
* Re: [PATCH] kasan: fix kasan_byte_accessible() to be consistent with actual checks
2021-04-05 21:43 [PATCH] kasan: fix kasan_byte_accessible() to be consistent with actual checks Peter Collingbourne
@ 2021-04-05 21:52 ` Andrey Konovalov
2021-04-05 22:07 ` Peter Collingbourne
0 siblings, 1 reply; 3+ messages in thread
From: Andrey Konovalov @ 2021-04-05 21:52 UTC (permalink / raw)
To: Peter Collingbourne
Cc: Marco Elver, Dmitry Vyukov, Alexander Potapenko, Evgenii Stepanov,
Linux Memory Management List, LKML
On Mon, Apr 5, 2021 at 11:43 PM Peter Collingbourne <pcc@google.com> wrote:
>
> We can sometimes end up with kasan_byte_accessible() being called
> on non-slab memory. For example ksize() and krealloc() may end up
> calling it on KFENCE allocated memory. In this case the memory will
> be tagged with KASAN_SHADOW_INIT, which a subsequent patch ("kasan:
> initialize shadow to TAG_INVALID for SW_TAGS") will set to the same
> value as KASAN_TAG_INVALID, causing kasan_byte_accessible() to fail
> when called on non-slab memory.
>
> This highlighted the fact that the check in kasan_byte_accessible()
> was inconsistent with checks as implemented for loads and stores
> (kasan_check_range() in SW tags mode and hardware-implemented
> checks in HW tags mode). kasan_check_range() does not have a
> check for KASAN_TAG_INVALID, and instead has a comparison against
> KASAN_SHADOW_START. In HW tags mode, we do not have either, but we
> do set TCR_EL1.TCMA which corresponds with the comparison against
> KASAN_TAG_KERNEL.
>
> Therefore, update kasan_byte_accessible() for both SW and HW tags
> modes to correspond with the respective checks on loads and stores.
>
> Link: https://linux-review.googlesource.com/id/Ic6d40803c57dcc6331bd97fbb9a60b0d38a65a36
> Signed-off-by: Peter Collingbourne <pcc@google.com>
> ---
> mm/kasan/kasan.h | 3 +--
> mm/kasan/sw_tags.c | 8 +++++---
> 2 files changed, 6 insertions(+), 5 deletions(-)
>
> diff --git a/mm/kasan/kasan.h b/mm/kasan/kasan.h
> index 8c55634d6edd..e18e8da35255 100644
> --- a/mm/kasan/kasan.h
> +++ b/mm/kasan/kasan.h
> @@ -368,8 +368,7 @@ static inline bool kasan_byte_accessible(const void *addr)
> u8 ptr_tag = get_tag(addr);
> u8 mem_tag = hw_get_mem_tag((void *)addr);
>
> - return (mem_tag != KASAN_TAG_INVALID) &&
> - (ptr_tag == KASAN_TAG_KERNEL || ptr_tag == mem_tag);
> + return ptr_tag == KASAN_TAG_KERNEL || ptr_tag == mem_tag;
> }
>
> #else /* CONFIG_KASAN_HW_TAGS */
> diff --git a/mm/kasan/sw_tags.c b/mm/kasan/sw_tags.c
> index 94c2d33be333..914225eeda99 100644
> --- a/mm/kasan/sw_tags.c
> +++ b/mm/kasan/sw_tags.c
> @@ -121,10 +121,12 @@ bool kasan_check_range(unsigned long addr, size_t size, bool write,
> bool kasan_byte_accessible(const void *addr)
> {
> u8 tag = get_tag(addr);
> - u8 shadow_byte = READ_ONCE(*(u8 *)kasan_mem_to_shadow(kasan_reset_tag(addr)));
> + void *untagged_addr = kasan_reset_tag(addr);
> + u8 shadow_byte = READ_ONCE(*(u8 *)kasan_mem_to_shadow(untagged_addr));
Hi Peter,
Let's move dereferencing shadow memory past the KASAN_SHADOW_START
check. Otherwise, in case the check is to fail, accessing shadow will
likely crash the kernel.
Thanks!
>
> - return (shadow_byte != KASAN_TAG_INVALID) &&
> - (tag == KASAN_TAG_KERNEL || tag == shadow_byte);
> + return untagged_addr >=
> + kasan_shadow_to_mem((void *)KASAN_SHADOW_START) &&
> + (tag == KASAN_TAG_KERNEL || tag == shadow_byte);
> }
>
> #define DEFINE_HWASAN_LOAD_STORE(size) \
> --
> 2.31.0.208.g409f899ff0-goog
>
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: [PATCH] kasan: fix kasan_byte_accessible() to be consistent with actual checks
2021-04-05 21:52 ` Andrey Konovalov
@ 2021-04-05 22:07 ` Peter Collingbourne
0 siblings, 0 replies; 3+ messages in thread
From: Peter Collingbourne @ 2021-04-05 22:07 UTC (permalink / raw)
To: Andrey Konovalov
Cc: Marco Elver, Dmitry Vyukov, Alexander Potapenko, Evgenii Stepanov,
Linux Memory Management List, LKML
On Mon, Apr 5, 2021 at 2:53 PM Andrey Konovalov <andreyknvl@gmail.com> wrote:
>
> On Mon, Apr 5, 2021 at 11:43 PM Peter Collingbourne <pcc@google.com> wrote:
> >
> > We can sometimes end up with kasan_byte_accessible() being called
> > on non-slab memory. For example ksize() and krealloc() may end up
> > calling it on KFENCE allocated memory. In this case the memory will
> > be tagged with KASAN_SHADOW_INIT, which a subsequent patch ("kasan:
> > initialize shadow to TAG_INVALID for SW_TAGS") will set to the same
> > value as KASAN_TAG_INVALID, causing kasan_byte_accessible() to fail
> > when called on non-slab memory.
> >
> > This highlighted the fact that the check in kasan_byte_accessible()
> > was inconsistent with checks as implemented for loads and stores
> > (kasan_check_range() in SW tags mode and hardware-implemented
> > checks in HW tags mode). kasan_check_range() does not have a
> > check for KASAN_TAG_INVALID, and instead has a comparison against
> > KASAN_SHADOW_START. In HW tags mode, we do not have either, but we
> > do set TCR_EL1.TCMA which corresponds with the comparison against
> > KASAN_TAG_KERNEL.
> >
> > Therefore, update kasan_byte_accessible() for both SW and HW tags
> > modes to correspond with the respective checks on loads and stores.
> >
> > Link: https://linux-review.googlesource.com/id/Ic6d40803c57dcc6331bd97fbb9a60b0d38a65a36
> > Signed-off-by: Peter Collingbourne <pcc@google.com>
> > ---
> > mm/kasan/kasan.h | 3 +--
> > mm/kasan/sw_tags.c | 8 +++++---
> > 2 files changed, 6 insertions(+), 5 deletions(-)
> >
> > diff --git a/mm/kasan/kasan.h b/mm/kasan/kasan.h
> > index 8c55634d6edd..e18e8da35255 100644
> > --- a/mm/kasan/kasan.h
> > +++ b/mm/kasan/kasan.h
> > @@ -368,8 +368,7 @@ static inline bool kasan_byte_accessible(const void *addr)
> > u8 ptr_tag = get_tag(addr);
> > u8 mem_tag = hw_get_mem_tag((void *)addr);
> >
> > - return (mem_tag != KASAN_TAG_INVALID) &&
> > - (ptr_tag == KASAN_TAG_KERNEL || ptr_tag == mem_tag);
> > + return ptr_tag == KASAN_TAG_KERNEL || ptr_tag == mem_tag;
> > }
> >
> > #else /* CONFIG_KASAN_HW_TAGS */
> > diff --git a/mm/kasan/sw_tags.c b/mm/kasan/sw_tags.c
> > index 94c2d33be333..914225eeda99 100644
> > --- a/mm/kasan/sw_tags.c
> > +++ b/mm/kasan/sw_tags.c
> > @@ -121,10 +121,12 @@ bool kasan_check_range(unsigned long addr, size_t size, bool write,
> > bool kasan_byte_accessible(const void *addr)
> > {
> > u8 tag = get_tag(addr);
> > - u8 shadow_byte = READ_ONCE(*(u8 *)kasan_mem_to_shadow(kasan_reset_tag(addr)));
> > + void *untagged_addr = kasan_reset_tag(addr);
> > + u8 shadow_byte = READ_ONCE(*(u8 *)kasan_mem_to_shadow(untagged_addr));
>
> Hi Peter,
>
> Let's move dereferencing shadow memory past the KASAN_SHADOW_START
> check. Otherwise, in case the check is to fail, accessing shadow will
> likely crash the kernel.
>
> Thanks!
Makes sense, fixed in v2.
Peter
^ permalink raw reply [flat|nested] 3+ messages in thread
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2021-04-05 21:43 [PATCH] kasan: fix kasan_byte_accessible() to be consistent with actual checks Peter Collingbourne
2021-04-05 21:52 ` Andrey Konovalov
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