fortify: Improve buffer overflow reporting

Improve the reporting of buffer overflows under CONFIG_FORTIFY_SOURCE to
help accelerate debugging efforts. The calculations are all just sitting
in registers anyway, so pass them along to the function to be reported.

For example, before:

  detected buffer overflow in memcpy

and after:

  memcpy: detected buffer overflow: 4096 byte read of buffer size 1

Link: https://lore.kernel.org/r/20230407192717.636137-10-keescook@chromium.org
Signed-off-by: Kees Cook <keescook@chromium.org>

2 files changed, 7 insertions, 6 deletions

diff --git a/lib/fortify_kunit.c b/lib/fortify_kunit.c
index f0accebeca02..493ec02dd5b3 100644
--- a/lib/fortify_kunit.c
+++ b/lib/fortify_kunit.c
@@ -17,8 +17,8 @@
 
 /* Redefine fortify_panic() to track failures. */
 void fortify_add_kunit_error(int write);
-#define fortify_panic(func, write, retfail) do {			\
-	__fortify_report(FORTIFY_REASON(func, write));			\
+#define fortify_panic(func, write, avail, size, retfail) do {		\
+	__fortify_report(FORTIFY_REASON(func, write), avail, size);	\
 	fortify_add_kunit_error(write);					\
 	return (retfail);						\
 } while (0)
diff --git a/lib/string_helpers.c b/lib/string_helpers.c
index 5e53d42e32bb..6bbafd6a10d9 100644
--- a/lib/string_helpers.c
+++ b/lib/string_helpers.c
@@ -1016,20 +1016,21 @@ static const char * const fortify_func_name[] = {
 #undef  MAKE_FORTIFY_FUNC_NAME
 };
 
-void __fortify_report(const u8 reason)
+void __fortify_report(const u8 reason, const size_t avail, const size_t size)
 {
 	const u8 func = FORTIFY_REASON_FUNC(reason);
 	const bool write = FORTIFY_REASON_DIR(reason);
 	const char *name;
 
 	name = fortify_func_name[umin(func, FORTIFY_FUNC_UNKNOWN)];
-	WARN(1, "%s: detected buffer %s overflow\n", name, str_read_write(!write));
+	WARN(1, "%s: detected buffer overflow: %zu byte %s of buffer size %zu\n",
+		 name, size, str_read_write(!write), avail);
 }
 EXPORT_SYMBOL(__fortify_report);
 
-void __fortify_panic(const u8 reason)
+void __fortify_panic(const u8 reason, const size_t avail, const size_t size)
 {
-	__fortify_report(reason);
+	__fortify_report(reason, avail, size);
 	BUG();
 }
 EXPORT_SYMBOL(__fortify_panic);