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From:   Marco Elver <elver@google.com>
Date:   Mon, 14 Mar 2022 09:57:15 +0100
Message-ID: <CANpmjNN-UPGOwkYWiOWX5DeSBWnYcobWb+M1ZyWMuSbzJQcFsg@mail.gmail.com>
Subject: Re: [PATCH] kasan, scs: collect stack traces from shadow stack
To:     Andrey Konovalov <andreyknvl@gmail.com>
Cc:     andrey.konovalov@linux.dev,
        Alexander Potapenko <glider@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Dmitry Vyukov <dvyukov@google.com>,
        Andrey Ryabinin <ryabinin.a.a@gmail.com>,
        kasan-dev <kasan-dev@googlegroups.com>,
        Vincenzo Frascino <vincenzo.frascino@arm.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Sami Tolvanen <samitolvanen@google.com>,
        Peter Collingbourne <pcc@google.com>,
        Evgenii Stepanov <eugenis@google.com>,
        Linux Memory Management List <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Andrey Konovalov <andreyknvl@google.com>,
        Florian Mayer <fmayer@google.com>
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On Mon, 14 Mar 2022 at 00:44, Andrey Konovalov <andreyknvl@gmail.com> wrote:
>
> On Sat, Mar 12, 2022 at 9:14 PM <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.
> >
> > 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.
> >
> > 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.
> >
> > 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))
> > +#else
> > +#define PAC_TAG_RESET(x) (x)
> > +#endif
> > +
> > +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);
>
> Another option here is to instruct stack depot to get the stack from
> the Shadow Call Stack. This would avoid copying the frames twice.

Yes, I think a stack_depot_save_shadow() would be appropriate if it
saves a copy.