Re: [PATCH 5/6] seccomp: add a way to attach a filter via eBPF fd

Michael Kerrisk (man-pages)
2015-09-05T07:13:16Z

On 09/04/2015 10:41 PM, Kees Cook wrote:
> On Fri, Sep 4, 2015 at 9:04 AM, Tycho Andersen
> <tycho.andersen@canonical.com> wrote:
>> This is the final bit needed to support seccomp filters created via the bpf
>> syscall.

Hmm. Thanks Kees, for CCinf linux-api@. That really should have been done at
the outset.

Tycho, where's the man-pages patch describing this new kernel-userspace
API feature? :-)

>> One concern with this patch is exactly what the interface should look like
>> for users, since seccomp()'s second argument is a pointer, we could ask
>> people to pass a pointer to the fd, but implies we might write to it which
>> seems impolite. Right now we cast the pointer (and force the user to cast
>> it), which generates ugly warnings. I'm not sure what the right answer is
>> here.
>>
>> Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
>> CC: Kees Cook <keescook@chromium.org>
>> CC: Will Drewry <wad@chromium.org>
>> CC: Oleg Nesterov <oleg@redhat.com>
>> CC: Andy Lutomirski <luto@amacapital.net>
>> CC: Pavel Emelyanov <xemul@parallels.com>
>> CC: Serge E. Hallyn <serge.hallyn@ubuntu.com>
>> CC: Alexei Starovoitov <ast@kernel.org>
>> CC: Daniel Borkmann <daniel@iogearbox.net>
>> ---
>> include/linux/seccomp.h | 3 +-
>> include/uapi/linux/seccomp.h | 1 +
>> kernel/seccomp.c | 70 ++++++++++++++++++++++++++++++++++++--------
>> 3 files changed, 61 insertions(+), 13 deletions(-)
>>
>> diff --git a/include/linux/seccomp.h b/include/linux/seccomp.h
>> index d1a86ed..a725dd5 100644
>> --- a/include/linux/seccomp.h
>> +++ b/include/linux/seccomp.h
>> @@ -3,7 +3,8 @@
>>
>> #include <uapi/linux/seccomp.h>
>>
>> -#define SECCOMP_FILTER_FLAG_MASK (SECCOMP_FILTER_FLAG_TSYNC)
>> +#define SECCOMP_FILTER_FLAG_MASK (\
>> + SECCOMP_FILTER_FLAG_TSYNC | SECCOMP_FILTER_FLAG_EBPF)
>>
>> #ifdef CONFIG_SECCOMP
>>
>> diff --git a/include/uapi/linux/seccomp.h b/include/uapi/linux/seccomp.h
>> index 0f238a4..c29a423 100644
>> --- a/include/uapi/linux/seccomp.h
>> +++ b/include/uapi/linux/seccomp.h
>> @@ -16,6 +16,7 @@
>>
>> /* Valid flags for SECCOMP_SET_MODE_FILTER */
>> #define SECCOMP_FILTER_FLAG_TSYNC 1
>> +#define SECCOMP_FILTER_FLAG_EBPF (1 << 1)
>>
>> /*
>> * All BPF programs must return a 32-bit value.
>> diff --git a/kernel/seccomp.c b/kernel/seccomp.c
>> index a2c5b32..9c6bea6 100644
>> --- a/kernel/seccomp.c
>> +++ b/kernel/seccomp.c
>> @@ -355,17 +355,6 @@ static struct seccomp_filter *seccomp_prepare_filter(struct sock_fprog *fprog)
>>
>> BUG_ON(INT_MAX / fprog->len < sizeof(struct sock_filter));
>>
>> - /*
>> - * Installing a seccomp filter requires that the task has
>> - * CAP_SYS_ADMIN in its namespace or be running with no_new_privs.
>> - * This avoids scenarios where unprivileged tasks can affect the
>> - * behavior of privileged children.
>> - */
>> - if (!task_no_new_privs(current) &&
>> - security_capable_noaudit(current_cred(), current_user_ns(),
>> - CAP_SYS_ADMIN) != 0)
>> - return ERR_PTR(-EACCES);
>> -
>> /* Allocate a new seccomp_filter */
>> sfilter = kzalloc(sizeof(*sfilter), GFP_KERNEL | __GFP_NOWARN);
>> if (!sfilter)
>> @@ -509,6 +498,48 @@ static void seccomp_send_sigsys(int syscall, int reason)
>> info.si_syscall = syscall;
>> force_sig_info(SIGSYS, &info, current);
>> }
>> +
>> +#ifdef CONFIG_BPF_SYSCALL
>> +static struct seccomp_filter *seccomp_prepare_ebpf(const char __user *filter)
>> +{
>> + /* XXX: this cast generates a warning. should we make people pass in
>> + * &fd, or is there some nicer way of doing this?
>> + */
>> + u32 fd = (u32) filter;
>
> I think this is probably the right way to do it, modulo getting the
> warning fixed. Let me invoke the great linux-api subscribers to get
> some more opinions.

Sigh. It's sad, but the using a cast does seem the simplest option.
But, how about another idea...

> tl;dr: adding SECCOMP_FILTER_FLAG_EBPF to the flags changes the
> pointer argument into an fd argument. Is this sane, should it be a
> pointer to an fd, or should it not be a flag at all, creating a new
> seccomp command instead (SECCOMP_MODE_FILTER_EBPF)?

What about

seccomp(SECCOMP_MODE_FILTER_EBPF, flags, structp)

Where structp is a pointer to something like

struct seccomp_ebpf {
int size; /* Size of this whole struct */
int fd;
}

'size' allows for future expansion of the struct (in case we want to
expand it later), and placing 'fd' inside a struct avoids unpleasant
implication that would be made by passing a pointer to an fd as the
third argument.

Cheers,

Michael

> -Kees
>
>> + struct seccomp_filter *ret;
>> + struct bpf_prog *prog;
>> +
>> + prog = bpf_prog_get(fd);
>> + if (IS_ERR(prog))
>> + return (struct seccomp_filter *) prog;
>> +
>> + if (prog->type != BPF_PROG_TYPE_SECCOMP) {
>> + bpf_prog_put(prog);
>> + return ERR_PTR(-EINVAL);
>> + }
>> +
>> + ret = kzalloc(sizeof(*ret), GFP_KERNEL | __GFP_NOWARN);
>> + if (!ret) {
>> + bpf_prog_put(prog);
>> + return ERR_PTR(-ENOMEM);
>> + }
>> +
>> + ret->prog = prog;
>> + atomic_set(&ret->usage, 1);
>> +
>> + /* Intentionally don't bpf_prog_put() here, because the underlying prog
>> + * is refcounted too and we're holding a reference from the struct
>> + * seccomp_filter object.
>> + */
>> +
>> + return ret;
>> +}
>> +#else
>> +static struct seccomp_filter *seccomp_prepare_ebpf(const char __user *filter)
>> +{
>> + return ERR_PTR(-EINVAL);
>> +}
>> +#endif
>> #endif /* CONFIG_SECCOMP_FILTER */
>>
>> /*
>> @@ -775,8 +806,23 @@ static long seccomp_set_mode_filter(unsigned int flags,
>> if (flags & ~SECCOMP_FILTER_FLAG_MASK)
>> return -EINVAL;
>>
>> + /*
>> + * Installing a seccomp filter requires that the task has
>> + * CAP_SYS_ADMIN in its namespace or be running with no_new_privs.
>> + * This avoids scenarios where unprivileged tasks can affect the
>> + * behavior of privileged children.
>> + */
>> + if (!task_no_new_privs(current) &&
>> + security_capable_noaudit(current_cred(), current_user_ns(),
>> + CAP_SYS_ADMIN) != 0)
>> + return -EACCES;
>> +
>> /* Prepare the new filter before holding any locks. */
>> - prepared = seccomp_prepare_user_filter(filter);
>> + if (flags & SECCOMP_FILTER_FLAG_EBPF)
>> + prepared = seccomp_prepare_ebpf(filter);
>> + else
>> + prepared = seccomp_prepare_user_filter(filter);
>> +
>> if (IS_ERR(prepared))
>> return PTR_ERR(prepared);
>>
>> --
>> 2.1.4
>>
>
>
>

--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/