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

[Kees Cook]

On Fri, Jul 11, 2014 at 3:55 PM, H. Peter Anvin <hpa@xxxxxxxxx> wrote:
> It includes the X32 bit.

If the uapi for __NR_* includes the x32 bit, then that's what seccomp
filters must be seeing. Building seccomp filters is documented to use
the __NR_* values.

-Kees

>
> On July 11, 2014 3:52:42 PM PDT, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>>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
>
> --
> Sent from my mobile phone.  Please pardon brevity and lack of formatting.



-- 
Kees Cook
Chrome OS Security
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