[kernel-hardening] Re: [PATCH V9] printk: hash addresses printed with %p

[Petr Mladek <pmladek@suse.com>]

On Mon 2017-10-30 09:59:16, Tobin C. Harding wrote:
> Currently there are many places in the kernel where addresses are being
> printed using an unadorned %p. Kernel pointers should be printed using
> %pK allowing some control via the kptr_restrict sysctl. Exposing addresses
> gives attackers sensitive information about the kernel layout in memory.
> 
> We can reduce the attack surface by hashing all addresses printed with
> %p. This will of course break some users, forcing code printing needed
> addresses to be updated.

I am sorry for my ignorance but what is the right update, please?
I expect that there are several possibilities:

  + remove the pointer at all

  + replace it with %pK so that it honors kptr_restrict setting

  + any other option?

Is kptr_restrict considered a safe mechanism?

Also kptr_restrict seems to be primary for the messages that are available
via /proc and /sys. Is it good enough for the messages logged by
printk()?

Will there be a debug option that would allow to see the original
pointers? Or what is the preferred way for debug messages?


> 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

It is evident that it will hit many people. I guess that they will
be suprised and might have similar questions. It might make sense
to decribe this in Documentation/printk-formats.txt.

Best Regards,
Petr