From: Kees Cook <keescook@chromium.org>
To: Jann Horn <jannh@google.com>
Cc: YiFei Zhu <yifeifz2@illinois.edu>,
Christian Brauner <christian.brauner@ubuntu.com>,
Tycho Andersen <tycho@tycho.pizza>,
Andy Lutomirski <luto@amacapital.net>,
Will Drewry <wad@chromium.org>,
Andrea Arcangeli <aarcange@redhat.com>,
Giuseppe Scrivano <gscrivan@redhat.com>,
Tobin Feldman-Fitzthum <tobin@ibm.com>,
Dimitrios Skarlatos <dskarlat@cs.cmu.edu>,
Valentin Rothberg <vrothber@redhat.com>,
Hubertus Franke <frankeh@us.ibm.com>,
Jack Chen <jianyan2@illinois.edu>,
Josep Torrellas <torrella@illinois.edu>,
Tianyin Xu <tyxu@illinois.edu>, bpf <bpf@vger.kernel.org>,
Linux Containers <containers@lists.linux-foundation.org>,
Linux API <linux-api@vger.kernel.org>,
kernel list <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 1/6] seccomp: Introduce SECCOMP_PIN_ARCHITECTURE
Date: Thu, 24 Sep 2020 00:11:01 -0700 [thread overview]
Message-ID: <202009232353.FD011DAA0@keescook> (raw)
In-Reply-To: <CAG48ez17XK2Co+1LbUWTc4x_W7nza=TObNh2Kpz6P+ba3OKPsw@mail.gmail.com>
On Thu, Sep 24, 2020 at 02:41:36AM +0200, Jann Horn wrote:
> On Thu, Sep 24, 2020 at 1:29 AM Kees Cook <keescook@chromium.org> wrote:
> > For systems that provide multiple syscall maps based on audit
> > architectures (e.g. AUDIT_ARCH_X86_64 and AUDIT_ARCH_I386 via
> > CONFIG_COMPAT) or via syscall masks (e.g. x86_x32), allow a fast way
> > to pin the process to a specific syscall table, instead of needing
> > to generate all filters with an architecture check as the first filter
> > action.
> >
> > This creates the internal representation that seccomp itself can use
> > (which is separate from the filters, which need to stay runtime
> > agnostic). Additionally paves the way for constant-action bitmaps.
>
> I don't really see the point in providing this UAPI - the syscall
> number checking will probably have much more performance cost than the
> architecture number check, and it's not like this lets us avoid the
> check, we're just moving it over into C code.
It's desirable for libseccomp and is a request from systemd (which is,
at this point, the largest seccomp user I know of), as they have no way
to force an arch without doing it in filters, which doesn't help much
with reducing filter runtime.
>
> > Signed-off-by: Kees Cook <keescook@chromium.org>
> > ---
> > include/linux/seccomp.h | 9 +++
> > include/uapi/linux/seccomp.h | 1 +
> > kernel/seccomp.c | 79 ++++++++++++++++++-
> > tools/testing/selftests/seccomp/seccomp_bpf.c | 33 ++++++++
> > 4 files changed, 120 insertions(+), 2 deletions(-)
> >
> > diff --git a/include/linux/seccomp.h b/include/linux/seccomp.h
> > index 02aef2844c38..0be20bc81ea9 100644
> > --- a/include/linux/seccomp.h
> > +++ b/include/linux/seccomp.h
> > @@ -20,12 +20,18 @@
> > #include <linux/atomic.h>
> > #include <asm/seccomp.h>
> >
> > +#define SECCOMP_ARCH_IS_NATIVE 1
> > +#define SECCOMP_ARCH_IS_COMPAT 2
>
> FYI, mips has three different possible "arch" values (per kernel build
> config; the __AUDIT_ARCH_LE flag can also be set, but that's fixed
> based on the config):
>
> - AUDIT_ARCH_MIPS
> - AUDIT_ARCH_MIPS | __AUDIT_ARCH_64BIT
> - AUDIT_ARCH_MIPS | __AUDIT_ARCH_64BIT | __AUDIT_ARCH_CONVENTION_MIPS64_N32
>
> But I guess we can deal with that once someone wants to actually add
> support for this on mips.
Yup!
>
> > +#define SECCOMP_ARCH_IS_MULTIPLEX 3
>
> Why should X32 be handled specially? If the seccomp filter allows
Because it's a masked lookup into a separate table; the syscalls don't
map to x86_64's table; so for seccomp to correctly figure out which
bitmap to use, it has to do this decoding.
> specific syscalls (as it should), we don't have to care about X32.
> Only in weird cases where the seccomp filter wants to deny specific
> syscalls (a horrible idea), X32 is a concern, and in such cases, the
> userspace code can generate a single conditional jump to deal with it.
I feel like I must not understand what you mean. The x32-aware seccomp
filters are using syscall tests with 0x40000000 included in the values.
So seccomp's bitmap cannot handle this because it must know how many
syscalls to include in a linearly-allocated bitmap.
> And when seccomp is used properly to allow specific syscalls, the
> kernel will just waste time uselessly checking this X32 stuff.
It not measurable in my tests -- seccomp_data::nr is rather hot in the
cache. ;) That said, if it's unwanted, then CONFIG_X86_X32=n is the way
to go.
> [...]
> > diff --git a/kernel/seccomp.c b/kernel/seccomp.c
> [...]
> > +static long seccomp_pin_architecture(void)
> > +{
> > +#ifdef SECCOMP_ARCH
> > + struct task_struct *task = current;
> > +
> > + u8 arch = seccomp_get_arch(syscall_get_arch(task),
> > + syscall_get_nr(task, task_pt_regs(task)));
> > +
> > + /* How did you even get here? */
>
> Via a racing TSYNC, that's how.
Yes; thanks. This will need to take ¤t->sighand->siglock.
>
> > + if (task->seccomp.arch && task->seccomp.arch != arch)
> > + return -EBUSY;
> > +
> > + task->seccomp.arch = arch;
> > +#endif
> > + return 0;
> > +}
>
> Why does this return 0 if SECCOMP_ARCH is not defined? That suggests
> to userspace that we have successfully pinned the ABI, even though
> we're actually unable to do so.
Yup; thanks for the catch. This is a logical leftover from the RFC. This
should be, I think:
+ task->seccomp.arch = arch;
+ return 0;
+#else
+ return -EINVAL;
+#endif
--
Kees Cook
next prev parent reply other threads:[~2020-09-24 7:11 UTC|newest]
Thread overview: 40+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-09-23 23:29 [PATCH v1 0/6] seccomp: Implement constant action bitmaps Kees Cook
2020-09-23 23:29 ` [PATCH 1/6] seccomp: Introduce SECCOMP_PIN_ARCHITECTURE Kees Cook
2020-09-24 0:41 ` Jann Horn
2020-09-24 7:11 ` Kees Cook [this message]
2020-09-23 23:29 ` [PATCH 2/6] x86: Enable seccomp architecture tracking Kees Cook
2020-09-24 0:45 ` Jann Horn
2020-09-24 7:12 ` Kees Cook
2020-09-23 23:29 ` [PATCH 3/6] seccomp: Implement constant action bitmaps Kees Cook
2020-09-24 0:25 ` Jann Horn
2020-09-24 7:36 ` Kees Cook
2020-09-24 8:07 ` YiFei Zhu
2020-09-24 8:15 ` Kees Cook
2020-09-24 8:22 ` YiFei Zhu
2020-09-24 12:28 ` Jann Horn
2020-09-24 12:37 ` David Laight
2020-09-24 12:56 ` Jann Horn
[not found] ` <DM6PR11MB271492D0565E91475D949F5DEF390@DM6PR11MB2714.namprd11.prod.outlook.com>
2020-09-24 0:36 ` YiFei Zhu
2020-09-24 7:38 ` Kees Cook
2020-09-24 7:51 ` YiFei Zhu
2020-09-23 23:29 ` [PATCH 4/6] seccomp: Emulate basic filters for constant action results Kees Cook
2020-09-23 23:47 ` Jann Horn
2020-09-24 7:46 ` Kees Cook
2020-09-24 15:28 ` Paul Moore
2020-09-24 19:52 ` Kees Cook
2020-09-24 20:46 ` Paul Moore
2020-09-24 21:35 ` Kees Cook
2020-09-23 23:29 ` [PATCH 5/6] selftests/seccomp: Compare bitmap vs filter overhead Kees Cook
2020-09-23 23:29 ` [PATCH 6/6] [DEBUG] seccomp: Report bitmap coverage ranges Kees Cook
2020-09-24 13:40 ` [PATCH v1 0/6] seccomp: Implement constant action bitmaps Rasmus Villemoes
2020-09-24 13:58 ` YiFei Zhu
2020-09-25 5:56 ` Rasmus Villemoes
2020-09-25 7:07 ` YiFei Zhu
2020-09-26 18:11 ` YiFei Zhu
2020-09-28 20:04 ` Kees Cook
2020-09-28 20:16 ` YiFei Zhu
2020-09-24 14:05 ` Jann Horn
2020-09-24 18:57 ` Andrea Arcangeli
2020-09-24 19:18 ` Jann Horn
[not found] ` <9dbe8e3bbdad43a1872202ff38c34ca2@DM5PR11MB1692.namprd11.prod.outlook.com>
2020-09-24 19:48 ` Tianyin Xu
2020-09-24 20:00 ` Kees Cook
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