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

[andrey.konovalov@linux.dev]

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);
 }
 
-- 
2.25.1