tracing: Add back FORTIFY_SOURCE logic to kernel_stack event structure

For backward compatibility, older tooling expects to see the kernel_stack
event with a "caller" field that is a fixed size array of 8 addresses. The
code now supports more than 8 with an added "size" field that states the
real number of entries. But the "caller" field still just looks like a
fixed size to user space.

Since the tracing macros that create the user space format files also
creates the structures that those files represent, the kernel_stack event
structure had its "caller" field a fixed size of 8, but in reality, when
it is allocated on the ring buffer, it can hold more if the stack trace is
bigger that 8 functions. The copying of these entries was simply done with
a memcpy():

  size = nr_entries * sizeof(unsigned long);
  memcpy(entry->caller, fstack->calls, size);

The FORTIFY_SOURCE logic noticed at runtime that when the nr_entries was
larger than 8, that the memcpy() was writing more than what the structure
stated it can hold and it complained about it. This is because the
FORTIFY_SOURCE code is unaware that the amount allocated is actually
enough to hold the size. It does not expect that a fixed size field will
hold more than the fixed size.

This was originally solved by hiding the caller assignment with some
pointer arithmetic.

  ptr = ring_buffer_data();
  entry = ptr;

  ptr += offsetof(typeof(*entry), caller);
  memcpy(ptr, fstack->calls, size);

But it is considered bad form to hide from kernel hardening. Instead, make
it work nicely with FORTIFY_SOURCE by adding a new __stack_array() macro
that is specific for this one special use case. The macro will take 4
arguments: type, item, len, field (whereas the __array() macro takes just
the first three). This macro will act just like the __array() macro when
creating the code to deal with the format file that is exposed to user
space. But for the kernel, it will turn the caller field into:

  type item[] __counted_by(field);

or for this instance:

  unsigned long caller[] __counted_by(size);

Now the kernel code can expose the assignment of the caller to the
FORTIFY_SOURCE and everyone is happy!

Link: https://lore.kernel.org/linux-trace-kernel/20230712105235.5fc441aa@gandalf.local.home/
Link: https://lore.kernel.org/linux-trace-kernel/20230713092605.2ddb9788@rorschach.local.home

Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Sven Schnelle <svens@linux.ibm.com>
Suggested-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Reviewed-by: Kees Cook <keescook@chromium.org>

1 files changed, 9 insertions, 0 deletions

diff --git a/kernel/trace/trace_export.c b/kernel/trace/trace_export.c
index 58f3946081e2..1698fc22afa0 100644
--- a/kernel/trace/trace_export.c
+++ b/kernel/trace/trace_export.c
@@ -51,6 +51,9 @@ static int ftrace_event_register(struct trace_event_call *call,
 #undef __array
 #define __array(type, item, size)			type item[size];
 
+#undef __stack_array
+#define __stack_array(type, item, size, field)		__array(type, item, size)
+
 #undef __array_desc
 #define __array_desc(type, container, item, size)	type item[size];
 
@@ -114,6 +117,9 @@ static void __always_unused ____ftrace_check_##name(void)		\
 	is_signed_type(_type), .filter_type = FILTER_OTHER,			\
 	.len = _len },
 
+#undef __stack_array
+#define __stack_array(_type, _item, _len, _field) __array(_type, _item, _len)
+
 #undef __array_desc
 #define __array_desc(_type, _container, _item, _len) __array(_type, _item, _len)
 
@@ -149,6 +155,9 @@ static struct trace_event_fields ftrace_event_fields_##name[] = {	\
 #undef __array
 #define __array(type, item, len)
 
+#undef __stack_array
+#define __stack_array(type, item, len, field)
+
 #undef __array_desc
 #define __array_desc(type, container, item, len)