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); }
