Re: [PATCH 2/3] [RFC] seccomp: give BPF x32 bit when restoring x32 filter
On Fri, Jul 11, 2014 at 3:48 PM, Kees Cook <keescook(a)chromium.org> wrote:
On Fri, Jul 11, 2014 at 12:36 PM, Paul Moore
<pmoore(a)redhat.com> 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