Re: [PATCH 1/1] kasan: fix shadow_size calculation error in kasan_module_alloc

[Andrey Ryabinin]

On 06/22/2018 12:27 PM, Zhen Lei wrote:
> There is a special case that the size is "(N << KASAN_SHADOW_SCALE_SHIFT)
> Pages plus X", the value of X is [1, KASAN_SHADOW_SCALE_SIZE-1]. The
> operation "size >> KASAN_SHADOW_SCALE_SHIFT" will drop X, and the roundup
> operation can not retrieve the missed one page. For example: size=0x28006,
> PAGE_SIZE=0x1000, KASAN_SHADOW_SCALE_SHIFT=3, we will get
> shadow_size=0x5000, but actually we need 6 pages.
> 
> shadow_size = round_up(size >> KASAN_SHADOW_SCALE_SHIFT, PAGE_SIZE);
> 
> This can lead kernel to be crashed, when kasan is enabled and the value
> of mod->core_layout.size or mod->init_layout.size is like above. Because
> the shadow memory of X has not been allocated and mapped.
> 
> move_module:
> ptr = module_alloc(mod->core_layout.size);
> ...
> memset(ptr, 0, mod->core_layout.size);//crashed
> 
> Unable to handle kernel paging request at virtual address 0f97b000
> ..
> Call trace:
> [] __asan_storeN+0x174/0x1a8
> [] memset+0x24/0x48
> [] layout_and_allocate+0xcd8/0x1800
> [] load_module+0x190/0x23e8
> [] SyS_finit_module+0x148/0x180
> 
> Signed-off-by: Zhen Lei 
> ---

Acked-by: Andrey Ryabinin 


>  mm/kasan/kasan.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/kasan/kasan.c b/mm/kasan/kasan.c
> index 81a2f45..f5ac4ac 100644
> --- a/mm/kasan/kasan.c
> +++ b/mm/kasan/kasan.c
> @@ -427,12 +427,13 @@ void kasan_kfree_large(const void *ptr)
>  int kasan_module_alloc(void *addr, size_t size)
>  {
>   void *ret;
> + size_t scaled_size;
>   size_t shadow_size;
>   unsigned long shadow_start;
> 
>   shadow_start = (unsigned long)kasan_mem_to_shadow(addr);
> - shadow_size = round_up(size >> KASAN_SHADOW_SCALE_SHIFT,
> - PAGE_SIZE);
> + scaled_size = (size + KASAN_SHADOW_MASK) >> KASAN_SHADOW_SCALE_SHIFT;
> + shadow_size = round_up(scaled_size, PAGE_SIZE);
> 
>   if (WARN_ON(!PAGE_ALIGNED(shadow_start)))
>   return -EINVAL;
> --
> 1.8.3
> 
> 

Re: [PATCH 1/1] kasan: fix shadow_size calculation error in kasan_module_alloc

[Dmitry Vyukov]

On Fri, Jun 22, 2018 at 11:27 AM, Zhen Lei  wrote:
> There is a special case that the size is "(N << KASAN_SHADOW_SCALE_SHIFT)
> Pages plus X", the value of X is [1, KASAN_SHADOW_SCALE_SIZE-1]. The
> operation "size >> KASAN_SHADOW_SCALE_SHIFT" will drop X, and the roundup
> operation can not retrieve the missed one page. For example: size=0x28006,
> PAGE_SIZE=0x1000, KASAN_SHADOW_SCALE_SHIFT=3, we will get
> shadow_size=0x5000, but actually we need 6 pages.
>
> shadow_size = round_up(size >> KASAN_SHADOW_SCALE_SHIFT, PAGE_SIZE);
>
> This can lead kernel to be crashed, when kasan is enabled and the value
> of mod->core_layout.size or mod->init_layout.size is like above. Because
> the shadow memory of X has not been allocated and mapped.
>
> move_module:
> ptr = module_alloc(mod->core_layout.size);
> ...
> memset(ptr, 0, mod->core_layout.size);  //crashed
>
> Unable to handle kernel paging request at virtual address 0f97b000
> ..
> Call trace:
> [] __asan_storeN+0x174/0x1a8
> [] memset+0x24/0x48
> [] layout_and_allocate+0xcd8/0x1800
> [] load_module+0x190/0x23e8
> [] SyS_finit_module+0x148/0x180
>
> Signed-off-by: Zhen Lei 
> ---
>  mm/kasan/kasan.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/mm/kasan/kasan.c b/mm/kasan/kasan.c
> index 81a2f45..f5ac4ac 100644
> --- a/mm/kasan/kasan.c
> +++ b/mm/kasan/kasan.c
> @@ -427,12 +427,13 @@ void kasan_kfree_large(const void *ptr)
>  int kasan_module_alloc(void *addr, size_t size)
>  {
> void *ret;
> +   size_t scaled_size;
> size_t shadow_size;
> unsigned long shadow_start;
>
> shadow_start = (unsigned long)kasan_mem_to_shadow(addr);
> -   shadow_size = round_up(size >> KASAN_SHADOW_SCALE_SHIFT,
> -   PAGE_SIZE);
> +   scaled_size = (size + KASAN_SHADOW_MASK) >> KASAN_SHADOW_SCALE_SHIFT;
> +   shadow_size = round_up(scaled_size, PAGE_SIZE);
>
> if (WARN_ON(!PAGE_ALIGNED(shadow_start)))
> return -EINVAL;


Hi Zhen,

Yes, this is a bug. Thanks for fixing it!

Reviewed-by: Dmitriy Vyukov 

[PATCH 1/1] kasan: fix shadow_size calculation error in kasan_module_alloc

[Zhen Lei]

There is a special case that the size is "(N << KASAN_SHADOW_SCALE_SHIFT)
Pages plus X", the value of X is [1, KASAN_SHADOW_SCALE_SIZE-1]. The
operation "size >> KASAN_SHADOW_SCALE_SHIFT" will drop X, and the roundup
operation can not retrieve the missed one page. For example: size=0x28006,
PAGE_SIZE=0x1000, KASAN_SHADOW_SCALE_SHIFT=3, we will get
shadow_size=0x5000, but actually we need 6 pages.

shadow_size = round_up(size >> KASAN_SHADOW_SCALE_SHIFT, PAGE_SIZE);

This can lead kernel to be crashed, when kasan is enabled and the value
of mod->core_layout.size or mod->init_layout.size is like above. Because
the shadow memory of X has not been allocated and mapped.

move_module:
ptr = module_alloc(mod->core_layout.size);
...
memset(ptr, 0, mod->core_layout.size);  //crashed

Unable to handle kernel paging request at virtual address 0f97b000
..
Call trace:
[] __asan_storeN+0x174/0x1a8
[] memset+0x24/0x48
[] layout_and_allocate+0xcd8/0x1800
[] load_module+0x190/0x23e8
[] SyS_finit_module+0x148/0x180

Signed-off-by: Zhen Lei 
---
 mm/kasan/kasan.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/mm/kasan/kasan.c b/mm/kasan/kasan.c
index 81a2f45..f5ac4ac 100644
--- a/mm/kasan/kasan.c
+++ b/mm/kasan/kasan.c
@@ -427,12 +427,13 @@ void kasan_kfree_large(const void *ptr)
 int kasan_module_alloc(void *addr, size_t size)
 {
void *ret;
+   size_t scaled_size;
size_t shadow_size;
unsigned long shadow_start;

shadow_start = (unsigned long)kasan_mem_to_shadow(addr);
-   shadow_size = round_up(size >> KASAN_SHADOW_SCALE_SHIFT,
-   PAGE_SIZE);
+   scaled_size = (size + KASAN_SHADOW_MASK) >> KASAN_SHADOW_SCALE_SHIFT;
+   shadow_size = round_up(scaled_size, PAGE_SIZE);

if (WARN_ON(!PAGE_ALIGNED(shadow_start)))
return -EINVAL;
--
1.8.3