Re: [RFC,PATCH 1/2] seccomp_filters: system call filtering using BPF

[Will Drewry <wad@chromium.org>]

On Tue, Jan 17, 2012 at 2:34 PM, Kees Cook <keescook@chromium.org> wrote:
> On Mon, Jan 16, 2012 at 10:46 PM, Indan Zupancic <indan@nul.nu> wrote:
>> So call it once and store the value in a long. Then copy the low half
>> to the right place and then the upper half when on 64 bits. It may not
>> look too pretty, but the compiler should be able to optimise almost all
>> overhead away and end up with 6 (or 12) int copies. Something like this:
>>
>> struct bpf_data {
>>        uint32 syscall_nr;
>>        uint32 arg_low[MAX_SC_ARGS];
>>        uint32 arg_high[MAX_SC_ARGS];
>> };
>>
>> void fill_bpf_data(struct task_struct *t, struct pt_regs *r, struct bpf_data *d)
>> {
>>        int i;
>>        unsigned long arg;
>>
>>        d->syscall_nr = syscall_get_nr(t, r);
>>        for (i = 0; i < MAX_SC_ARGS; ++i){
>>                syscall_get_arguments(t, r, i, 1, &arg);
>>                d->arg_low[i] = arg;
>>                d->arg_high[i] = arg >> 32;
>>        }
>> }
>
> If this turns out to be expensive, it might be possible to break it up
> and load the arguments on demand (and cache them); i.e. have
> load_pointer() or similar notice when it is about to access something
> other than bpf_data.syscall_nr.

Makes perfect sense!  In theory (as a few other people pointed this
out off list), it is entirely possible to never populate any data for
load_pointer except an optional cache.  Just provide a custom
load_pointer that knows to take the offset return the syscall nr or
the args or some slice of the returned data.

This is even easier if the struct looks like:
struct {
  int nr;
  union {
    uint32_t args32[6];
    uint64_t args64[6];
  }
};

since you can just use the offset without doing any endian-based
splitting.  Another suggestion (thanks roland!) was to add
  int syscall_arch;
to the struct populated with the AUDIT_ARCH_* defines.  This would
help the case Indan was worried about -- portable filter programs.

It looks like there'd be some cross-arch plumbing to make the
AUDIT_ARCH_ data available, but not too bad.

Seem sane? I'm headed down this path now and I think it'll work out
assuming there aren't major objections to the syscall_arch piece.

thanks!
will