linux-kernel - Re: [PATCH v4 seccomp 5/5] seccomp/cache: Report cache data through /proc/pid/seccomp

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Date:   Fri, 9 Oct 2020 23:45:00 +0200
From:   Jann Horn <jannh@...gle.com>
To:     YiFei Zhu <zhuyifei1999@...il.com>
Cc:     Linux Containers <containers@...ts.linux-foundation.org>,
        YiFei Zhu <yifeifz2@...inois.edu>, bpf <bpf@...r.kernel.org>,
        kernel list <linux-kernel@...r.kernel.org>,
        Aleksa Sarai <cyphar@...har.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andy Lutomirski <luto@...capital.net>,
        David Laight <David.Laight@...lab.com>,
        Dimitrios Skarlatos <dskarlat@...cmu.edu>,
        Giuseppe Scrivano <gscrivan@...hat.com>,
        Hubertus Franke <frankeh@...ibm.com>,
        Jack Chen <jianyan2@...inois.edu>,
        Josep Torrellas <torrella@...inois.edu>,
        Kees Cook <keescook@...omium.org>,
        Tianyin Xu <tyxu@...inois.edu>,
        Tobin Feldman-Fitzthum <tobin@....com>,
        Tycho Andersen <tycho@...ho.pizza>,
        Valentin Rothberg <vrothber@...hat.com>,
        Will Drewry <wad@...omium.org>
Subject: Re: [PATCH v4 seccomp 5/5] seccomp/cache: Report cache data through /proc/pid/seccomp_cache

On Fri, Oct 9, 2020 at 7:15 PM YiFei Zhu <zhuyifei1999@...il.com> wrote:
> Currently the kernel does not provide an infrastructure to translate
> architecture numbers to a human-readable name. Translating syscall
> numbers to syscall names is possible through FTRACE_SYSCALL
> infrastructure but it does not provide support for compat syscalls.
>
> This will create a file for each PID as /proc/pid/seccomp_cache.
> The file will be empty when no seccomp filters are loaded, or be
> in the format of:
> <arch name> <decimal syscall number> <ALLOW | FILTER>
> where ALLOW means the cache is guaranteed to allow the syscall,
> and filter means the cache will pass the syscall to the BPF filter.
>
> For the docker default profile on x86_64 it looks like:
> x86_64 0 ALLOW
> x86_64 1 ALLOW
> x86_64 2 ALLOW
> x86_64 3 ALLOW
> [...]
> x86_64 132 ALLOW
> x86_64 133 ALLOW
> x86_64 134 FILTER
> x86_64 135 FILTER
> x86_64 136 FILTER
> x86_64 137 ALLOW
> x86_64 138 ALLOW
> x86_64 139 FILTER
> x86_64 140 ALLOW
> x86_64 141 ALLOW
> [...]
>
> This file is guarded by CONFIG_SECCOMP_CACHE_DEBUG with a default
> of N because I think certain users of seccomp might not want the
> application to know which syscalls are definitely usable. For
> the same reason, it is also guarded by CAP_SYS_ADMIN.
>
> Suggested-by: Jann Horn <jannh@...gle.com>
> Link: https://lore.kernel.org/lkml/CAG48ez3Ofqp4crXGksLmZY6=fGrF_tWyUCg7PBkAetvbbOPeOA@mail.gmail.com/
> Signed-off-by: YiFei Zhu <yifeifz2@...inois.edu>
[...]
> diff --git a/arch/Kconfig b/arch/Kconfig
[...]
> +config SECCOMP_CACHE_DEBUG
> +       bool "Show seccomp filter cache status in /proc/pid/seccomp_cache"
> +       depends on SECCOMP
> +       depends on SECCOMP_FILTER
> +       depends on PROC_FS
> +       help
> +         This is enables /proc/pid/seccomp_cache interface to monitor

nit: s/This is enables/This enables the/

> +         seccomp cache data. The file format is subject to change. Reading
> +         the file requires CAP_SYS_ADMIN.
> +
> +         This option is for debugging only. Enabling present the risk that

nit: *presents

> +         an adversary may be able to infer the seccomp filter logic.

[...]
> +int proc_pid_seccomp_cache(struct seq_file *m, struct pid_namespace *ns,
> +                          struct pid *pid, struct task_struct *task)
> +{
> +       struct seccomp_filter *f;
> +       unsigned long flags;
> +
> +       /*
> +        * We don't want some sandboxed process know what their seccomp

s/know/to know/

> +        * filters consist of.
> +        */
> +       if (!file_ns_capable(m->file, &init_user_ns, CAP_SYS_ADMIN))
> +               return -EACCES;
> +
> +       if (!lock_task_sighand(task, &flags))
> +               return 0;

maybe return -ESRCH here so that userspace can distinguish between an
exiting process and a process with no filters?

> +       f = READ_ONCE(task->seccomp.filter);
> +       if (!f) {
> +               unlock_task_sighand(task, &flags);
> +               return 0;
> +       }
[...]