Re: [PATCH v3] scripts: add leaking_addresses.pl

[Kees Cook]

On Mon, Nov 6, 2017 at 9:27 AM, Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Sun, Nov 5, 2017 at 9:19 PM, Tobin C. Harding <me@xxxxxxxx> wrote:
>> Currently we are leaking addresses from the kernel to user space. This
>> script is an attempt to find some of those leakages. Script parses
>> `dmesg` output and /proc and /sys files for hex strings that look like
>> kernel addresses.
>
> Lovely. This is great. It shows just how much totally pointless stuff
> we leak, and to normal users that really shouldn't need it.
>
> I had planned to wait for 4.15 to look at the printk hashing stuff
> etc, but this part I think I could/should merge early just because I
> think a lot of kernel developers will go "Why the f*ck would we expose
> that kernel address there?"
>
> The module sections stuff etc should likely be obviously root-only,
> although maybe I'm missing some tool that ends up using it and is
> useful to normal developers.
>
> And I'm thinking we could make kallsyms smarter too, and instead of
> depending on kptr_restrict that screws over things with much too big a
> hammer, we could make it take 'perf_event_paranoid' into account. I
> suspect that's the main user of kallsyms that would still be relevant
> to non-root.

Linus, what do you have in mind for the root-only "yes we really need
the actual address output" exceptions?

For example, right now /sys/kernel/debug/kernel_page_tables
(CONFIG_X86_PTDUMP=y) needs actual address and currently uses %x.

Looking other places that stand out, it seems like
/proc/lockdep_chains and /proc/lockdep (CONFIG_LOCKDEP=y) has a ton of
%p usage. It's unclear to me if a hash is sufficient for meaningful
debugging there?

Seems like these three from dmesg could be removed?

[    0.000000] Base memory trampoline at [ffffa3fc40099000] 99000 size 24576
arch/x86/realmode/init.c

[    0.000000] percpu: Embedded 38 pages/cpu @ffffa4007fc00000 s116944
r8192 d30512 u524288
mm/percpu.c

[    0.456395] software IO TLB [mem 0xbbfdf000-0xbffdf000] (64MB)
mapped at [ffffa3fcfbfdf000-ffffa3fcfffdefff]
lib/swiotlb.c

Tobin, some other feedback on v4...

I find the output hard to parse. Instead of:

[27527 lockdep_chains] [ffffffffb226c628] cgroup_mutex

Could we have:

27527 /proc/lockdep_chains: [ffffffffb226c628] cgroup_mutex

At the very least, getting the full file path is needed or might not
be clear where something lives.

And for my kernels, I needed to exclude usbmon or the script would
hang (perhaps add a read timeout to the script to detect stalling
files?)

diff --git a/scripts/leaking_addresses.pl b/scripts/leaking_addresses.pl
index 282c0cc2bdea..a9b729c0a052 100644
--- a/scripts/leaking_addresses.pl
+++ b/scripts/leaking_addresses.pl
@@ -70,6 +70,7 @@ my @skip_walk_dirs_any = ('self',
                          'thread-self',
                          'cwd',
                          'fd',
+                         'usbmon',
                          'stderr',
                          'stdin',
                          'stdout');


-Kees

-- 
Kees Cook
Pixel Security