Re: [PATCH V11 3/5] printk: hash addresses printed with %p

Andrew Morton
2017-11-29T23:21:12Z

On Wed, 29 Nov 2017 13:05:03 +1100 "Tobin C. Harding" <me@tobin.cc> wrote:

> Currently there exist approximately 14 000 places in the kernel where
> addresses are being printed using an unadorned %p. This potentially
> leaks sensitive information regarding the Kernel layout in memory. Many
> of these calls are stale, instead of fixing every call lets hash the
> address by default before printing. This will of course break some
> users, forcing code printing needed addresses to be updated.
>
> Code that _really_ needs the address will soon be able to use the new
> printk specifier %px to print the address.
>
> For what it's worth, usage of unadorned %p can be broken down as
> follows (thanks to Joe Perches).
>
> $ git grep -E '%p[^A-Za-z0-9]' | cut -f1 -d"/" | sort | uniq -c
> 1084 arch
> 20 block
> 10 crypto
> 32 Documentation
> 8121 drivers
> 1221 fs
> 143 include
> 101 kernel
> 69 lib
> 100 mm
> 1510 net
> 40 samples
> 7 scripts
> 11 security
> 166 sound
> 152 tools
> 2 virt
>
> Add function ptr_to_id() to map an address to a 32 bit unique
> identifier. Hash any unadorned usage of specifier %p and any malformed
> specifiers.
>
> ...
>
> @@ -1644,6 +1646,73 @@ char *device_node_string(char *buf, char *end, struct device_node *dn,
> return widen_string(buf, buf - buf_start, end, spec);
> }
>
> +static bool have_filled_random_ptr_key __read_mostly;
> +static siphash_key_t ptr_key __read_mostly;
> +
> +static void fill_random_ptr_key(struct random_ready_callback *unused)
> +{
> + get_random_bytes(&ptr_key, sizeof(ptr_key));
> + /*
> + * have_filled_random_ptr_key==true is dependent on get_random_bytes().
> + * ptr_to_id() needs to see have_filled_random_ptr_key==true
> + * after get_random_bytes() returns.
> + */
> + smp_mb();
> + WRITE_ONCE(have_filled_random_ptr_key, true);
> +}

I don't think I'm seeing anything which prevents two CPUs from
initializing ptr_key at the same time. Probably doesn't matter much...