[PATCH] mm/kasan: Print frame description for stack bugs

[Marco Elver]

This adds support for printing stack frame description on invalid stack
accesses. The frame description is embedded by the compiler, which is
parsed and then pretty-printed.

Currently, we can only print the stack frame info for accesses to the
task's own stack, but not accesses to other tasks' stacks.

Example of what it looks like:

[   17.924050] page dumped because: kasan: bad access detected
[   17.924908]
[   17.925153] addr ffff8880673ef98a is located in stack of task insmod/2008 at offset 106 in frame:
[   17.926542]  kasan_stack_oob+0x0/0xf5 [test_kasan]
[   17.927932]
[   17.928206] this frame has 2 objects:
[   17.928783]  [32, 36) 'i'
[   17.928784]  [96, 106) 'stack_array'
[   17.929216]
[   17.930031] Memory state around the buggy address:

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=198435
Signed-off-by: Marco Elver <elver@xxxxxxxxxx>
---
Change-Id: I4836cde103052991ac8871796a45b4c977c9e2e7
---
 mm/kasan/kasan.h  |   5 ++
 mm/kasan/report.c | 160 ++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 165 insertions(+)

diff --git a/mm/kasan/kasan.h b/mm/kasan/kasan.h
index 3ce956efa0cb..1979db4763e2 100644
--- a/mm/kasan/kasan.h
+++ b/mm/kasan/kasan.h
@@ -43,6 +43,11 @@
 
 #define KASAN_ALLOCA_REDZONE_SIZE	32
 
+/*
+ * Stack frame marker (compiler ABI).
+ */
+#define KASAN_CURRENT_STACK_FRAME_MAGIC 0x41B58AB3
+
 /* Don't break randconfig/all*config builds */
 #ifndef KASAN_ABI_VERSION
 #define KASAN_ABI_VERSION 1
diff --git a/mm/kasan/report.c b/mm/kasan/report.c
index 03a443579386..c6ad8462c0dc 100644
--- a/mm/kasan/report.c
+++ b/mm/kasan/report.c
@@ -28,6 +28,7 @@
 #include <linux/types.h>
 #include <linux/kasan.h>
 #include <linux/module.h>
+#include <linux/sched/task_stack.h>
 
 #include <asm/sections.h>
 
@@ -181,6 +182,163 @@ static inline bool init_task_stack_addr(const void *addr)
 			sizeof(init_thread_union.stack));
 }
 
+static bool __must_check tokenize_frame_descr(const char **frame_descr,
+					      char *token, size_t max_tok_len,
+					      unsigned long *value)
+{
+	const char *sep = strchr(*frame_descr, ' ');
+	const ptrdiff_t tok_len = sep - *frame_descr;
+
+	if (sep == NULL)
+		sep = *frame_descr + strlen(*frame_descr);
+
+	if (token != NULL) {
+		if (tok_len + 1 > max_tok_len) {
+			pr_err("KASAN internal error: frame description too long: %s\n",
+			       *frame_descr);
+			return false;
+		}
+		/* Copy token (+ 1 byte for '\0'). */
+		strlcpy(token, *frame_descr, tok_len + 1);
+	}
+	/* Advance frame_descr past separator. */
+	*frame_descr = sep + 1;
+
+	if (value != NULL && kstrtoul(token, 10, value)) {
+		pr_err("KASAN internal error: not a valid number: %s\n", token);
+		return false;
+	}
+
+	return true;
+}
+
+static void print_decoded_frame_descr(const char *frame_descr)
+{
+	/*
+	 * We need to parse the following string:
+	 *    "n alloc_1 alloc_2 ... alloc_n"
+	 * where alloc_i looks like
+	 *    "offset size len name"
+	 * or "offset size len name:line".
+	 */
+
+	char token[64];
+	unsigned long num_objects;
+
+	if (!tokenize_frame_descr(&frame_descr, token, sizeof(token),
+				  &num_objects))
+		return;
+
+	pr_err("\n");
+	pr_err("this frame has %zu %s:\n", num_objects,
+	       num_objects == 1 ? "object" : "objects");
+
+	while (num_objects--) {
+		unsigned long offset;
+		unsigned long size;
+
+		/* access offset */
+		if (!tokenize_frame_descr(&frame_descr, token, sizeof(token),
+					  &offset))
+			return;
+		/* access size */
+		if (!tokenize_frame_descr(&frame_descr, token, sizeof(token),
+					  &size))
+			return;
+		/* name length (unused) */
+		if (!tokenize_frame_descr(&frame_descr, NULL, 0, NULL))
+			return;
+		/* object name */
+		if (!tokenize_frame_descr(&frame_descr, token, sizeof(token),
+					  NULL))
+			return;
+
+		/* Strip line number, if it exists. */
+		strreplace(token, ':', '\0');
+
+		/* Finally, print object information. */
+		pr_err(" [%zu, %zu) '%s'", offset, offset + size, token);
+	}
+}
+
+static bool __must_check get_address_stack_frame_info(const void *addr,
+						      size_t *offset,
+						      const char **frame_descr,
+						      const void **frame_pc)
+{
+	size_t aligned_addr;
+	size_t mem_ptr;
+	const u8 *shadow_bottom;
+	const u8 *shadow_ptr;
+	const size_t *frame;
+
+	/*
+	 * NOTE: We currently only support printing frame information for
+	 * accesses to the task's own stack.
+	 */
+	if (!object_is_on_stack(addr))
+		return false;
+
+	aligned_addr = round_down((size_t)addr, sizeof(long));
+	mem_ptr = round_down(aligned_addr, KASAN_SHADOW_SCALE_SIZE);
+	shadow_ptr = kasan_mem_to_shadow((void *)aligned_addr);
+	shadow_bottom = kasan_mem_to_shadow(end_of_stack(current));
+
+	while (shadow_ptr >= shadow_bottom && *shadow_ptr != KASAN_STACK_LEFT) {
+		shadow_ptr--;
+		mem_ptr -= KASAN_SHADOW_SCALE_SIZE;
+	}
+
+	while (shadow_ptr >= shadow_bottom && *shadow_ptr == KASAN_STACK_LEFT) {
+		shadow_ptr--;
+		mem_ptr -= KASAN_SHADOW_SCALE_SIZE;
+	}
+
+	if (shadow_ptr < shadow_bottom)
+		return false;
+
+	frame = (const size_t *)(mem_ptr + KASAN_SHADOW_SCALE_SIZE);
+	if (frame[0] != KASAN_CURRENT_STACK_FRAME_MAGIC) {
+		pr_err("KASAN internal error: frame info validation failed; invalid marker: %zu\n",
+		       frame[0]);
+		return false;
+	}
+
+	*offset = (size_t)addr - (size_t)frame;
+	*frame_descr = (const char *)frame[1];
+	*frame_pc = (void *)frame[2];
+
+	return true;
+}
+
+static void print_address_stack_frame(const void *addr)
+{
+	size_t offset;
+	const char *frame_descr;
+	const void *frame_pc;
+
+	if (IS_ENABLED(CONFIG_KASAN_SW_TAGS))
+		return;
+
+	if (!get_address_stack_frame_info(addr, &offset, &frame_descr,
+					  &frame_pc))
+		return;
+
+	/*
+	 * get_address_stack_frame_info only returns true if the given addr is
+	 * on the current task's stack.
+	 */
+	pr_err("\n");
+	pr_err("addr %px is located in stack of task %s/%d at offset %zu in frame:\n",
+	       addr, current->comm, task_pid_nr(current), offset);
+	pr_err(" %pS\n", frame_pc);
+
+	if (!frame_descr)
+		return;
+
+	print_decoded_frame_descr(frame_descr);
+}
+
 static void print_address_description(void *addr)
 {
 	struct page *page = addr_to_page(addr);
@@ -204,6 +362,8 @@ static void print_address_description(void *addr)
 		pr_err("The buggy address belongs to the page:\n");
 		dump_page(page, "kasan: bad access detected");
 	}
+
+	print_address_stack_frame(addr);
 }
 
 static bool row_is_guilty(const void *row, const void *guilty)
-- 
2.21.0.1020.gf2820cf01a-goog