On Mon, Apr 30, 2018 at 8:50 AM, Anna-Maria Gleixner
<[email protected]> wrote:
> Hi,
>
> I stumbled over an issue with hashed pointers and tracing.
>
> I'm using trace points for examination and on error the trace buffers
> are dumped. The error occurs when entropy has not been set up, so the
> pointers are not hashed and only (ptrval) is printed instead. The
> pointers are required to distinguish the different objects in the
> trace.
>
> Beside workarounds like patching lib/vsprintf.c helpers before testing
> or dumping trace buffers later (given that kernel comes up properly
> and entropy is set up), is there a possible generic solution for this
> issue? A commandline option for disabling the pointer obfuscation
> would be a pretty handy tool.
I (or other folks?) had proposed this before, but, AIUI, Linus remains
opposed. I still think something like this would be useful:
diff --git a/lib/vsprintf.c b/lib/vsprintf.c
index 30c0cb8cc9bc..22bf631395d1 100644
--- a/lib/vsprintf.c
+++ b/lib/vsprintf.c
@@ -1703,12 +1703,27 @@ static int __init initialize_ptr_random(void)
}
early_initcall(initialize_ptr_random);
+static bool bypass_pointer_hashing __ro_after_init;
+
+static int __init early_bypass_pointer_hashing_param(char *buf)
+{
+ if (!buf)
+ return -EINVAL;
+ return strtobool(buf, &bypass_pointer_hashing);
+}
+early_param("bypass_pointer_hashing", early_bypass_pointer_hashing_param);
+
/* Maps a pointer to a 32 bit unique identifier. */
static char *ptr_to_id(char *buf, char *end, void *ptr, struct
printf_spec spec)
{
unsigned long hashval;
const int default_width = 2 * sizeof(ptr);
+ if (unlikely(bypass_pointer_hashing)) {
+ hashval = (uintptr_t)ptr;
+ goto bypass;
+ }
+
if (unlikely(!have_filled_random_ptr_key)) {
spec.field_width = default_width;
/* string length must be less than default_width */
@@ -1726,6 +1741,7 @@ static char *ptr_to_id(char *buf, char *end,
void *ptr, struct printf_spec spec)
hashval = (unsigned long)siphash_1u32((u32)ptr, &ptr_key);
#endif
+bypass:
spec.flags |= SMALL;
if (spec.field_width == -1) {
spec.field_width = default_width;
(apologies for gmail-induced whitespace damage...)
--
Kees Cook
Pixel Security
