Re: [PATCH 2/3] fork: support VMAP_STACK with KASAN_VMALLOC

[Dmitry Vyukov <dvyukov@google.com>]

On Thu, Jul 25, 2019 at 7:55 AM Daniel Axtens <dja@axtens.net> wrote:
>
> Supporting VMAP_STACK with KASAN_VMALLOC is straightforward:
>
>  - clear the shadow region of vmapped stacks when swapping them in
>  - tweak Kconfig to allow VMAP_STACK to be turned on with KASAN
>
> Signed-off-by: Daniel Axtens <dja@axtens.net>

Reviewed-by: Dmitry Vyukov <dvyukov@google.com>

> ---
>  arch/Kconfig  | 9 +++++----
>  kernel/fork.c | 4 ++++
>  2 files changed, 9 insertions(+), 4 deletions(-)
>
> diff --git a/arch/Kconfig b/arch/Kconfig
> index a7b57dd42c26..e791196005e1 100644
> --- a/arch/Kconfig
> +++ b/arch/Kconfig
> @@ -825,16 +825,17 @@ config HAVE_ARCH_VMAP_STACK
>  config VMAP_STACK
>         default y
>         bool "Use a virtually-mapped stack"
> -       depends on HAVE_ARCH_VMAP_STACK && !KASAN
> +       depends on HAVE_ARCH_VMAP_STACK
> +       depends on !KASAN || KASAN_VMALLOC
>         ---help---
>           Enable this if you want the use virtually-mapped kernel stacks
>           with guard pages.  This causes kernel stack overflows to be
>           caught immediately rather than causing difficult-to-diagnose
>           corruption.
>
> -         This is presently incompatible with KASAN because KASAN expects
> -         the stack to map directly to the KASAN shadow map using a formula
> -         that is incorrect if the stack is in vmalloc space.
> +         To use this with KASAN, the architecture must support backing
> +         virtual mappings with real shadow memory, and KASAN_VMALLOC must
> +         be enabled.
>
>  config ARCH_OPTIONAL_KERNEL_RWX
>         def_bool n
> diff --git a/kernel/fork.c b/kernel/fork.c
> index d8ae0f1b4148..ce3150fe8ff2 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -94,6 +94,7 @@
>  #include <linux/livepatch.h>
>  #include <linux/thread_info.h>
>  #include <linux/stackleak.h>
> +#include <linux/kasan.h>
>
>  #include <asm/pgtable.h>
>  #include <asm/pgalloc.h>
> @@ -215,6 +216,9 @@ static unsigned long *alloc_thread_stack_node(struct task_struct *tsk, int node)
>                 if (!s)
>                         continue;
>
> +               /* Clear the KASAN shadow of the stack. */
> +               kasan_unpoison_shadow(s->addr, THREAD_SIZE);
> +
>                 /* Clear stale pointers from reused stack. */
>                 memset(s->addr, 0, THREAD_SIZE);
>
> --
> 2.20.1
>
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