On Tue, Apr 03, 2018 at 05:06:12PM -0400, Steven Rostedt wrote: > On Tue, 3 Apr 2018 13:07:58 -0700 > Kees Cook <keesc...@chromium.org> wrote: > > > On Tue, Apr 3, 2018 at 12:41 PM, Steven Rostedt <rost...@goodmis.org> wrote: > > > Both trace_debug is set and kptr_restrict is set to zero in the same > > > code that produces the above banner. This will allow trace_printk() to > > > not be affected by security code, as trace_printk() should never be run > > > on a machine that needs security of this kind. > > > > While I think it'd be nice to have a boot-time knob for this (a debate > > that was unsuccessful in earlier threads), I remain skeptical of > > having a _runtime_ knob for this, as then it becomes a target (and > > yes, there are plenty of targets, but why add another). > > > > If this was __ro_after_init, maybe that'd be nicer. CONFIG_TRACING=y > > Well, then of course this would need a check to keep modules from > setting it. But I think I know of a nice alternative. > > > > is used everywhere, so this is really just the whole knob debate over > > again. Instead, I've been following Linus's distillation of %p usage > > in the kernel: > > > > http://lkml.kernel.org/r/ca+55afwqed_d40g4mucssvrzzrfpujt74vc6pppb675hynx...@mail.gmail.com > > Remember, this isn't a printk() that hangs around for production. I was > debugging code that modified pointers, and I wanted to make sure that > the pointer arithmetic was correct (it wasn't), and randomizing the > output made my prints useless. > > If you are concerned about attack surface, I could make it a bit more > difficult to tweak by malicious software. What about the patch below? > It would be much more difficult to modify this knob from an attack > vector. > > -- Steve > > diff --git a/include/linux/printk.h b/include/linux/printk.h > index e9b603ee9953..b624493b3991 100644 > --- a/include/linux/printk.h > +++ b/include/linux/printk.h > @@ -278,6 +278,7 @@ static inline void printk_safe_flush_on_panic(void) > #endif > > extern int kptr_restrict; > +extern struct static_key trace_debug; > > extern asmlinkage void dump_stack(void) __cold; > > diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c > index 0f47e653ffd8..6c151d00848b 100644 > --- a/kernel/trace/trace.c > +++ b/kernel/trace/trace.c > @@ -2864,6 +2864,10 @@ void trace_printk_init_buffers(void) > > buffers_allocated = 1; > > + /* This is a debug kernel, allow pointers to be shown */ > + static_key_enable(&trace_debug); > + kptr_restrict = 0; > + > /* > * trace_printk_init_buffers() can be called by modules. > * If that happens, then we need to start cmdline recording > diff --git a/lib/vsprintf.c b/lib/vsprintf.c > index 89f8a4a4b770..c3d8eafecb39 100644 > --- a/lib/vsprintf.c > +++ b/lib/vsprintf.c > @@ -1345,6 +1345,7 @@ char *uuid_string(char *buf, char *end, const u8 *addr, > } > > int kptr_restrict __read_mostly; > +struct static_key trace_debug = STATIC_KEY_INIT_FALSE; > > static noinline_for_stack > char *restricted_pointer(char *buf, char *end, const void *ptr, > @@ -1962,6 +1963,10 @@ char *pointer(const char *fmt, char *buf, char *end, > void *ptr, > return pointer_string(buf, end, ptr, spec); > } > > + /* When the kernel is in debugging mode, show all pointers */ > + if (static_key_false(&trace_debug)) > + return restricted_pointer(buf, end, ptr, spec); > + > /* default is to _not_ leak addresses, hash before printing */ > return ptr_to_id(buf, end, ptr, spec); > }
This uses the deprecated API Steve (I only know because I went to read Documentation/static-keys.txt after seeing this patch). Hope this helps, Tobin.
