| Message ID | 20210405214321.1953019-1-pcc@google.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | kasan: fix kasan_byte_accessible() to be consistent with actual checks | expand |
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 >
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
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) \
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(-)