Re: [PATCH v4 seccomp 5/5] seccomp/cache: Report cache data through /proc/pid/seccomp_cache

Jann Horn via Containers
2020-10-09T21:45:33Z

On Fri, Oct 9, 2020 at 7:15 PM YiFei Zhu <zhuyifei1999@gmail.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@google.com>
> Link: https://lore.kernel.org/lkml/CAG48ez3Ofqp4crXGksLmZY6=fGrF_tWyUCg7PBkAetvbbOPeOA@mail.gmail.com/
> Signed-off-by: YiFei Zhu <yifeifz2@illinois.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;
> + } …