Re: [kernel-hardening] [RFC PATCH 1/2] SROP Mitigation: Architecture independent code for signal cookies

[Scotty Bauer <sbauer@eng.utah.edu>]

On 01/28/2016 04:25 PM, Kees Cook wrote:
> On Sat, Jan 23, 2016 at 11:59 PM, Scott Bauer <sbauer@eng.utah.edu> wrote:
>> This patch adds a per-process secret to the task struct which
>> will be used during signal delivery and during a sigreturn.
>> Also, logic is added in signal.c to generate, place, extract,
>> clear and verify the signal cookie.
>>
>> Signed-off-by: Scott Bauer <sbauer@eng.utah.edu>
>> ---
>>  fs/exec.c              |  3 +++
>>  include/linux/sched.h  |  7 +++++++
>>  include/linux/signal.h |  2 ++
>>  kernel/signal.c        | 53 ++++++++++++++++++++++++++++++++++++++++++++++++++
>>  4 files changed, 65 insertions(+)
>>
>> diff --git a/fs/exec.c b/fs/exec.c
>> index 828ec5f..1b1b27b 100644
>> --- a/fs/exec.c
>> +++ b/fs/exec.c
>> @@ -56,6 +56,7 @@
>>  #include <linux/pipe_fs_i.h>
>>  #include <linux/oom.h>
>>  #include <linux/compat.h>
>> +#include <linux/random.h>
>>
>>  #include <asm/uaccess.h>
>>  #include <asm/mmu_context.h>
>> @@ -1135,6 +1136,8 @@ void setup_new_exec(struct linux_binprm * bprm)
>>         /* This is the point of no return */
>>         current->sas_ss_sp = current->sas_ss_size = 0;
>>
>> +       get_random_bytes(&current->sig_cookie, sizeof(current->sig_cookie));
>> +
>>         if (uid_eq(current_euid(), current_uid()) && gid_eq(current_egid(), current_gid()))
>>                 set_dumpable(current->mm, SUID_DUMP_USER);
>>         else
>> diff --git a/include/linux/sched.h b/include/linux/sched.h
>> index 61aa9bb..8d8f5ae 100644
>> --- a/include/linux/sched.h
>> +++ b/include/linux/sched.h
>> @@ -1496,6 +1496,13 @@ struct task_struct {
>>         unsigned long stack_canary;
>>  #endif
>>         /*
>> +        * Canary value for signal frames placed on user stack.
>> +        * This helps mitigate "Signal Return oriented program"
>> +        * exploits in userland.
>> +        */
>> +       unsigned long sig_cookie;
>> +
>> +       /*
>>          * pointers to (original) parent process, youngest child, younger sibling,
>>          * older sibling, respectively.  (p->father can be replaced with
>>          * p->real_parent->pid)
>> diff --git a/include/linux/signal.h b/include/linux/signal.h
>> index 92557bb..fae0618 100644
>> --- a/include/linux/signal.h
>> +++ b/include/linux/signal.h
>> @@ -280,6 +280,8 @@ extern int get_signal(struct ksignal *ksig);
>>  extern void signal_setup_done(int failed, struct ksignal *ksig, int stepping);
>>  extern void exit_signals(struct task_struct *tsk);
>>  extern void kernel_sigaction(int, __sighandler_t);
>> +extern int set_sigcookie(unsigned long __user *location);
>> +extern int verify_clear_sigcookie(unsigned long __user *sig_cookie_ptr);
>>
>>  static inline void allow_signal(int sig)
>>  {
>> diff --git a/kernel/signal.c b/kernel/signal.c
>> index f3f1f7a..11fc2b2 100644
>> --- a/kernel/signal.c
>> +++ b/kernel/signal.c
>> @@ -34,6 +34,7 @@
>>  #include <linux/compat.h>
>>  #include <linux/cn_proc.h>
>>  #include <linux/compiler.h>
>> +#include <linux/hash.h>
>>
>>  #define CREATE_TRACE_POINTS
>>  #include <trace/events/signal.h>
>> @@ -2430,6 +2431,58 @@ out:
>>         }
>>  }
>>
>> +static unsigned long gen_sigcookie(unsigned long __user *location)
>> +{
>> +
>> +       unsigned long sig_cookie;
>> +
>> +       sig_cookie = (unsigned long) location ^ current->sig_cookie;
>> +
> 
> If the cookie is secret, is there a need for the hashing?
> 

That's what I was wondering, and the answer is probably no. I did the hash because Andy L.
suggested doing a hash and I was somewhat concerned about someone being able to leak the
user cookie and leak a stack address in which case it would be easy to find the secret sig_cookie. Of course
the hash probably has the same issue. More over, if someone can leak both a stack address and
the cookie they've probably already owned the application and all bets are off at this point.

I can remove it, as it seems like its unnecessary. I'll probably rename the current->sig_cookie to be current->sig_secret.