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

[H. Peter Anvin]

It includes the X32 bit.

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

-- 
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