[v10,06/11] seccomp: add SECCOMP_RET_ERRNO

This change adds the SECCOMP_RET_ERRNO as a valid return value from a
seccomp filter.  Additionally, it makes the first use of the lower
16-bits for storing a filter-supplied errno.  16-bits is more than
enough for the errno-base.h calls.

Returning errors instead of immediately terminating processes that
violate seccomp policy allow for broader use of this functionality
for kernel attack surface reduction.  For example, a linux container
could maintain a whitelist of pre-existing system calls but drop
all new ones with errnos.  This would keep a logically static attack
surface while providing errnos that may allow for graceful failure
without the downside of do_exit() on a bad call.

v10: - change loaders to fn
 v9: - n/a
 v8: - update Kconfig to note new need for syscall_set_return_value.
     - reordered such that TRAP behavior follows on later.
     - made the for loop a little less indent-y
 v7: - introduced

Signed-off-by: Will Drewry <wad@chromium.org>
---
 arch/Kconfig            |    6 ++++--
 include/linux/seccomp.h |   15 +++++++++++----
 kernel/seccomp.c        |   39 ++++++++++++++++++++++++++++-----------
 3 files changed, 43 insertions(+), 17 deletions(-)

On Tue, Feb 21, 2012 at 11:30:30AM -0600, Will Drewry wrote:
> diff --git a/kernel/seccomp.c b/kernel/seccomp.c
> index 0043b7e..23f1844 100644
> --- a/kernel/seccomp.c
> +++ b/kernel/seccomp.c
> @@ -136,22 +136,18 @@ static void *bpf_load(const void *nr, int off, unsigned int size, void *buf)
>  static u32 seccomp_run_filters(int syscall)
>  {
>  	struct seccomp_filter *f;
> -	u32 ret = SECCOMP_RET_KILL;
>  	static const struct bpf_load_fn fns = {
>  		bpf_load,
>  		sizeof(struct seccomp_data),
>  	};
> +	u32 ret = SECCOMP_RET_ALLOW;
>  	const void *sc_ptr = (const void *)(uintptr_t)syscall;
> -
>  	/*
>  	 * All filters are evaluated in order of youngest to oldest. The lowest
>  	 * BPF return value always takes priority.
>  	 */
> -	for (f = current->seccomp.filter; f; f = f->prev) {
> -		ret = bpf_run_filter(sc_ptr, f->insns, &fns);
> -		if (ret != SECCOMP_RET_ALLOW)
> -			break;
> -	}
> +	for (f = current->seccomp.filter; f; f = f->prev)
> +		ret = min_t(u32, ret, bpf_run_filter(sc_ptr, f->insns, &fns));
>  	return ret;
>  }

I'd like to see this fail closed in the (theoretically impossible, but
why risk it) case of there being no filters at all. Could do something
like this:

	u32 ret = current->seccomp.filter ? SECCOMP_RET_ALLOW : SECCOMP_RET_KILL;

Or, just this, to catch the misbehavior:

	if (unlikely(current->seccomp.filter == NULL))
		return SECCOMP_RET_KILL;

-Kees

On Tue, Feb 21, 2012 at 4:41 PM, Kees Cook <keescook@chromium.org> wrote:
> On Tue, Feb 21, 2012 at 11:30:30AM -0600, Will Drewry wrote:
>> diff --git a/kernel/seccomp.c b/kernel/seccomp.c
>> index 0043b7e..23f1844 100644
>> --- a/kernel/seccomp.c
>> +++ b/kernel/seccomp.c
>> @@ -136,22 +136,18 @@ static void *bpf_load(const void *nr, int off, unsigned int size, void *buf)
>>  static u32 seccomp_run_filters(int syscall)
>>  {
>>       struct seccomp_filter *f;
>> -     u32 ret = SECCOMP_RET_KILL;
>>       static const struct bpf_load_fn fns = {
>>               bpf_load,
>>               sizeof(struct seccomp_data),
>>       };
>> +     u32 ret = SECCOMP_RET_ALLOW;
>>       const void *sc_ptr = (const void *)(uintptr_t)syscall;
>> -
>>       /*
>>        * All filters are evaluated in order of youngest to oldest. The lowest
>>        * BPF return value always takes priority.
>>        */
>> -     for (f = current->seccomp.filter; f; f = f->prev) {
>> -             ret = bpf_run_filter(sc_ptr, f->insns, &fns);
>> -             if (ret != SECCOMP_RET_ALLOW)
>> -                     break;
>> -     }
>> +     for (f = current->seccomp.filter; f; f = f->prev)
>> +             ret = min_t(u32, ret, bpf_run_filter(sc_ptr, f->insns, &fns));
>>       return ret;
>>  }
>
> I'd like to see this fail closed in the (theoretically impossible, but
> why risk it) case of there being no filters at all. Could do something
> like this:
>
>        u32 ret = current->seccomp.filter ? SECCOMP_RET_ALLOW : SECCOMP_RET_KILL;
>
> Or, just this, to catch the misbehavior:
>
>        if (unlikely(current->seccomp.filter == NULL))
>                return SECCOMP_RET_KILL;

I think the last one makes the most sense to me.  I'll add it and rev the patch.

thanks!
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