Re: [PATCH] kasan, scs: collect stack traces from shadow stack

[Marco Elver <elver@google.com>]

On Sat, 12 Mar 2022 at 21:14, <andrey.konovalov@linux.dev> wrote:
>
> From: Andrey Konovalov <andreyknvl@google.com>
>
> Currently, KASAN always uses the normal stack trace collection routines,
> which rely on the unwinder, when saving alloc and free stack traces.
>
> Instead of invoking the unwinder, collect the stack trace by copying
> frames from the Shadow Call Stack whenever it is enabled. This reduces
> boot time by 30% for all KASAN modes when Shadow Call Stack is enabled.

This is impressive.

> To avoid potentially leaking PAC pointer tags, strip them when saving
> the stack trace.
>
> Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
>
> ---
>
> Things to consider:
>
> We could integrate shadow stack trace collection into kernel/stacktrace.c
> as e.g. stack_trace_save_shadow(). However, using stack_trace_consume_fn
> leads to invoking a callback on each saved from, which is undesirable.
> The plain copy loop is faster.

Why is stack_trace_consume_fn required? This is an internal detail of
arch_stack_walk(), but to implement stack_trace_save_shadow() that's
not used at all.

I think having stack_trace_save_shadow() as you have implemented in
kernel/stacktrace.c or simply in kernel/scs.c itself would be
appropriate.

> We could add a command line flag to switch between stack trace collection
> modes. I noticed that Shadow Call Stack might be missing certain frames
> in stacks originating from a fault that happens in the middle of a
> function. I am not sure if this case is important to handle though.

I think SCS should just work - and if it doesn't, can we fix it? It is
unclear to me what would be a deciding factor to choose between stack
trace collection modes, since it is hard to quantify when and if SCS
doesn't work as intended. So I fear it'd just be an option that's
never used because we don't understand when it's required to be used.

> Looking forward to thoughts and comments.
>
> Thanks!
>
> ---
>  mm/kasan/common.c | 36 +++++++++++++++++++++++++++++++++++-
>  1 file changed, 35 insertions(+), 1 deletion(-)
>
> diff --git a/mm/kasan/common.c b/mm/kasan/common.c
> index d9079ec11f31..65a0723370c7 100644
> --- a/mm/kasan/common.c
> +++ b/mm/kasan/common.c
> @@ -9,6 +9,7 @@
>   *        Andrey Konovalov <andreyknvl@gmail.com>
>   */
>
> +#include <linux/bits.h>
>  #include <linux/export.h>
>  #include <linux/init.h>
>  #include <linux/kasan.h>
> @@ -21,6 +22,7 @@
>  #include <linux/printk.h>
>  #include <linux/sched.h>
>  #include <linux/sched/task_stack.h>
> +#include <linux/scs.h>
>  #include <linux/slab.h>
>  #include <linux/stacktrace.h>
>  #include <linux/string.h>
> @@ -30,12 +32,44 @@
>  #include "kasan.h"
>  #include "../slab.h"
>
> +#ifdef CONFIG_SHADOW_CALL_STACK
> +
> +#ifdef CONFIG_ARM64_PTR_AUTH
> +#define PAC_TAG_RESET(x) (x | GENMASK(63, CONFIG_ARM64_VA_BITS))

This should go into arch/arm64/include/asm/kasan.h, and here it should
then just do

#ifndef PAC_TAG_RESET
#define ...


> +#else
> +#define PAC_TAG_RESET(x) (x)
> +#endif

But perhaps there's a better, more generic location for this macro?

> +static unsigned int save_shadow_stack(unsigned long *entries,
> +                                     unsigned int nr_entries)
> +{
> +       unsigned long *scs_sp = task_scs_sp(current);
> +       unsigned long *scs_base = task_scs(current);
> +       unsigned long *frame;
> +       unsigned int i = 0;
> +
> +       for (frame = scs_sp - 1; frame >= scs_base; frame--) {
> +               entries[i++] = PAC_TAG_RESET(*frame);
> +               if (i >= nr_entries)
> +                       break;
> +       }
> +
> +       return i;
> +}
> +#else /* CONFIG_SHADOW_CALL_STACK */
> +static inline unsigned int save_shadow_stack(unsigned long *entries,
> +                                       unsigned int nr_entries) { return 0; }
> +#endif /* CONFIG_SHADOW_CALL_STACK */
> +
>  depot_stack_handle_t kasan_save_stack(gfp_t flags, bool can_alloc)
>  {
>         unsigned long entries[KASAN_STACK_DEPTH];
>         unsigned int nr_entries;
>
> -       nr_entries = stack_trace_save(entries, ARRAY_SIZE(entries), 0);
> +       if (IS_ENABLED(CONFIG_SHADOW_CALL_STACK))
> +               nr_entries = save_shadow_stack(entries, ARRAY_SIZE(entries));
> +       else
> +               nr_entries = stack_trace_save(entries, ARRAY_SIZE(entries), 0);
>         return __stack_depot_save(entries, nr_entries, flags, can_alloc);
>  }
>
> --
> 2.25.1
>