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

[Kees Cook]

On Sun, Oct 29, 2017 at 3:59 PM, Tobin C. Harding <me@xxxxxxxx> 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.
>
> 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.
>
> Signed-off-by: Tobin C. Harding <me@xxxxxxxx>
>
> ---
>
> It seems we don't have consensus on a couple of things
>
> 1. The size of the hashed address on 64 bit architectures.
> 2. The use of '0x' pre-fix for hashed addresses.
>
> In regards to (1), we are agreed that we only need 32 bits of
> information. There is some questions however that outputting _only_ 32
> bits may break userland.
>
> In regards to (2), irrespective of the arguments for and against, if
> point 1 is correct and changing the format will break userland then we
> can't add the '0x' suffix for the same reason.
>
> Therefore this patch masks off the first 32 bits, retaining
> only 32 bits of information. We do not add a '0x' suffix. All in all,
> that results in _no_ change to the format of output only the content of
> the output.
>
> The leading 0's also make explicit that we have messed with the address,
> maybe this will save some debugging time by doing so. Although this
> would probably already be obvious since there is no leading 'ffff'.
>
> We hash malformed specifiers also. Malformed specifiers include
> incomplete (e.g %pi) and also non-existent specifiers. checkpatch should
> warn for non-existent specifiers but AFAICT won't warn for incomplete
> specifiers.
>
> Here is the behaviour that this patch implements.
>
> For kpt_restrict==0
>
> Randomness not ready:
>   printed with %p:              (pointer value)     # NOTE: with padding
> Valid pointer:
>   printed with %pK:             deadbeefdeadbeef
>   printed with %p:              00000000deadbeef
>   malformed specifier (eg %i):  00000000deadbeef
> NULL pointer:
>   printed with %pK:             0000000000000000
>   printed with %p:                       (null)     # NOTE: with padding
>   malformed specifier (eg %i):           (null)
>
> For kpt_restrict==2
>
> Valid pointer:
>   printed with %pK:             0000000000000000
>
> All other output as for kptr_restrict==0
>
> V9:
>  - Drop the initial patch from V8, leaving null pointer handling as is.
>  - Print the hashed ID _without_ a '0x' suffix.
>  - Mask the first 32 bits of the hashed ID to all zeros on 64 bit
>    architectures.

Oops, I had missed v9. This addresses my concerns. I think the leading
zeros are a good way to identify the "this is clearly not a kernel
address" issue (though the 32-bit folks may remain confused, but we
can fix that later, IMO).

-Kees

-- 
Kees Cook
Pixel Security