Re: [PATCH 2/3] [RFC] seccomp: give BPF x32 bit when restoring x32 filter

[Kees Cook]

On Fri, Jul 11, 2014 at 3:48 PM, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> On Fri, Jul 11, 2014 at 12:36 PM, Paul Moore <pmoore@xxxxxxxxxx> wrote:
>> Anyway, getting back to the idea I mentioned earlier ... as many of you may
>> know, Kees (added to the CC line) is working on some seccomp filter
>> improvements which will result in a new seccomp syscall.  Perhaps one way
>> forward is to preserve everything as it is currently with the prctl()
>> interface, but with the new seccomp() based interface we fixup x32 and use the
>> new AUDIT_ARCH_X32 token?  It might result in a bit of ugliness in some of the
>> kernel, but I don't think it would be too bad, and I think it would address
>> both our concerns.
>
> Adding AUDIT_ARCH_X32: yes please. (On that note, the comment "/* Both
> x32 and x86_64 are considered "64-bit". */" should be changed...)
>
> Just so I understand: currently x86_64 and x32 both present as
> AUDIT_ARCH_X86_64. The x32 syscalls are seen as in a different range
> (due to the set high bit).
>
> The seccomp used in Chrome, Chrome OS, and vsftpd should all only do
> whitelisting by both arch and syscall, so adding AUDIT_ARCH_X32
> without setting __X32_SYSCALL_BIT would be totally fine (it would
> catch the arch instead of the syscall). This sounds similar to how
> libseccomp is doing things, so these should be fine.

I should clarify: seccomp expects to find whatever is sent as the
syscall nr... as in the __NR_read used like this:

                BPF_STMT(BPF_LD+BPF_W+BPF_ABS,
                        offsetof(struct seccomp_data, nr)),
                BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_read, 0, 1),
                BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_KILL),
                BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW),

Are there native x32 users yet? What does __NR_read resolve to via the
uapi on a native x32 userspace?

-Kees

> The only project I know of doing blacklisting is lxc, and Eric's
> example looks a lot like a discussion I saw with lxc and init_module.
> :) So it sounds like we can get this right there.
>
> I'd like to avoid carrying a delta on filter logic based on the prctl
> vs syscall entry. Can we find any userspace filters being used that a
> "correct" fix would break? (If so, then yes, we'll need to do this
> proposed "via prctl or via syscall?" change.)
>
> -Kees
>
> --
> Kees Cook
> Chrome OS Security



-- 
Kees Cook
Chrome OS Security
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