Subject: Re: [PATCH v2 5/5] seccomp: add a way to attach a filter via eBPF fd

From: Daniel Borkmann <[email-masked]>
Date: Fri, 11 Sep 2015 14:37:59 +0200

On 09/11/2015 02:21 AM, Tycho Andersen wrote:
> This is the final bit needed to support seccomp filters created via the bpf
> syscall. The patch adds a new seccomp operation SECCOMP_MODE_FILTER_EBPF,
> which takes exactly one command (presumably to be expanded upon later when
> seccomp EBPFs support more interesting things) and an argument struct
> similar to that of bpf(), although the size is explicit in the struct to
> avoid changing the signature of seccomp().
>
> v2: Don't abuse seccomp's third argument; use a separate command and a
>      pointer to a structure instead.

Comments below ...

> Signed-off-by: Tycho Andersen <[email-masked]>
> CC: Kees Cook <[email-masked]>
> CC: Will Drewry <[email-masked]>
> CC: Oleg Nesterov <[email-masked]>
> CC: Andy Lutomirski <[email-masked]>
> CC: Pavel Emelyanov <[email-masked]>
> CC: Serge E. Hallyn <[email-masked]>
> CC: Alexei Starovoitov <[email-masked]>
> CC: Daniel Borkmann <[email-masked]>
> ---
>   include/uapi/linux/seccomp.h |  16 +++++
>   kernel/seccomp.c             | 135 ++++++++++++++++++++++++++++++++++++++-----
>   2 files changed, 138 insertions(+), 13 deletions(-)
>
> diff --git a/include/uapi/linux/seccomp.h b/include/uapi/linux/seccomp.h
> index 0f238a4..a8694e2 100644
> --- a/include/uapi/linux/seccomp.h
> +++ b/include/uapi/linux/seccomp.h
> @@ -13,10 +13,14 @@
>   /* Valid operations for seccomp syscall. */
>   #define SECCOMP_SET_MODE_STRICT	0
>   #define SECCOMP_SET_MODE_FILTER	1
> +#define SECCOMP_MODE_FILTER_EBPF	2

Should this be SECCOMP_SET_MODE_FILTER_EBPF or just SECCOMP_SET_MODE_EBPF?

>   /* Valid flags for SECCOMP_SET_MODE_FILTER */
>   #define SECCOMP_FILTER_FLAG_TSYNC	1
>
> +/* Valid cmds for SECCOMP_MODE_FILTER_EBPF */
> +#define SECCOMP_EBPF_ADD_FD	0
> +
>   /*
>    * All BPF programs must return a 32-bit value.
>    * The bottom 16-bits are for optional return data.
> @@ -51,4 +55,16 @@ struct seccomp_data {
>   	__u64 args[6];
>   };
>
> +struct seccomp_ebpf {
> +	unsigned int size;
> +
> +	union {
> +		/* SECCOMP_EBPF_ADD_FD */
> +		struct {
> +			unsigned int	add_flags;
> +			__u32		add_fd;
> +		};
> +	};
> +};
> +
>   #endif /* _UAPI_LINUX_SECCOMP_H */
> diff --git a/kernel/seccomp.c b/kernel/seccomp.c
> index 1856f69..e78175a 100644
> --- a/kernel/seccomp.c
> +++ b/kernel/seccomp.c
> @@ -65,6 +65,9 @@ struct seccomp_filter {
>   /* Limit any path through the tree to 256KB worth of instructions. */
>   #define MAX_INSNS_PER_PATH ((1 << 18) / sizeof(struct sock_filter))
>
> +static long seccomp_install_filter(unsigned int flags,
> +				   struct seccomp_filter *prepared);
> +
>   /*
>    * Endianness is explicitly ignored and left for BPF program authors to manage
>    * as per the specific architecture.
> @@ -356,17 +359,6 @@ 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)
> @@ -510,8 +502,105 @@ static void seccomp_send_sigsys(int syscall, int reason)
>   	info.si_syscall = syscall;
>   	force_sig_info(SIGSYS, &info, current);
>   }
> +
>   #endif	/* CONFIG_SECCOMP_FILTER */
>
> +#if defined(CONFIG_BPF_SYSCALL) && defined(CONFIG_SECCOMP_FILTER)
> +static struct seccomp_filter *seccomp_prepare_ebpf(int fd)
> +{
> +	struct seccomp_filter *ret;
> +	struct bpf_prog *prog;
> +
> +	prog = bpf_prog_get(fd);
> +	if (IS_ERR(prog))
> +		return (struct seccomp_filter *) prog;

ERR_CAST()

> +
> +	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;
> +}
> +
> +static long seccomp_ebpf_add_fd(struct seccomp_ebpf *ebpf)
> +{
> +	struct seccomp_filter *prepared;
> +
> +	prepared = seccomp_prepare_ebpf(ebpf->add_fd);
> +	if (IS_ERR(prepared))
> +		return PTR_ERR(prepared);
> +
> +	return seccomp_install_filter(ebpf->add_flags, prepared);
> +}
> +
> +static long seccomp_mode_filter_ebpf(unsigned int cmd, const char __user *uargs)
> +{
> +	const struct seccomp_ebpf __user *uebpf;
> +	struct seccomp_ebpf ebpf;
> +	unsigned int size;
> +	long ret = -EFAULT;
> +
> +	uebpf = (const struct seccomp_ebpf __user *) uargs;
> +
> +	if (get_user(size, &uebpf->size) != 0)
> +		return -EFAULT;
> +
> +	/* If we're handed a bigger struct than we know of,
> +	 * ensure all the unknown bits are 0 - i.e. new
> +	 * user-space does not rely on any kernel feature
> +	 * extensions we dont know about yet.
> +	 */
> +	if (size > sizeof(ebpf)) {
> +		unsigned char __user *addr;
> +		unsigned char __user *end;
> +		unsigned char val;
> +
> +		addr = (void __user *)uebpf + sizeof(ebpf);
> +		end  = (void __user *)uebpf + size;
> +
> +		for (; addr < end; addr++) {
> +			int err = get_user(val, addr);
> +
> +			if (err)
> +				return err;
> +			if (val)
> +				return -E2BIG;
> +		}
> +		size = sizeof(ebpf);
> +	}
> +
> +	if (copy_from_user(&ebpf, uebpf, size) != 0)
> +		return -EFAULT;

Not sure it's worth adding all this bpf(2)-alike interface complexity into
this, but fair enough, I guess there are some very good reasons and bigger
additions coming then ...

> +	switch (cmd) {
> +	case SECCOMP_EBPF_ADD_FD:
> +		ret = seccomp_ebpf_add_fd(&ebpf);
> +		break;
> +	}
> +
> +	return ret;
> +}
> +#else
> +static long seccomp_mode_filter_ebpf(unsigned int cmd, const char __user *uargs)
> +{
> +	return -EINVAL;
> +}
> +#endif
> +
>   /*
>    * Secure computing mode 1 allows only read/write/exit/sigreturn.
>    * To be fully secure this must be combined with rlimit
> @@ -760,9 +849,7 @@ out:
>   static long seccomp_set_mode_filter(unsigned int flags,
>   				    const char __user *filter)
>   {
> -	const unsigned long seccomp_mode = SECCOMP_MODE_FILTER;
>   	struct seccomp_filter *prepared = NULL;
> -	long ret = -EINVAL;
>
>   	/* Validate flags. */
>   	if (flags & ~SECCOMP_FILTER_FLAG_MASK)
> @@ -773,6 +860,26 @@ static long seccomp_set_mode_filter(unsigned int flags,
>   	if (IS_ERR(prepared))
>   		return PTR_ERR(prepared);
>
> +	return seccomp_install_filter(flags, prepared);

I (truly) hope, I'm overseeing something ;) ...

... but why doing all the (classic) seccomp-BPF preparation work (which is rather
a lot) up to this point, where you have it ready, only to *then* find out we don't
have the actual permissions ?!

Plus, when seccomp_install_filter() fails with -EACCES, who is releasing all the
allocated foo resp. dropping taken program refs !?

I see the same in seccomp_ebpf_add_fd().

So, an unprivileged child could increase the parent's bpf_prog's reference count
w/o having the actual permissions to do so, and thus controlling it to the point
where the next bpf_prog_put() would unintentionally release it?

(So yeah, I'm hoping I misread something ... ;))

> +}
> +
> +static long seccomp_install_filter(unsigned int flags,
> +				   struct seccomp_filter *prepared)
> +{
> +	const unsigned long seccomp_mode = SECCOMP_MODE_FILTER;
> +	long ret = -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;
> +
>   	/*
>   	 * Make sure we cannot change seccomp or nnp state via TSYNC
>   	 * while another thread is in the middle of calling exec.
> @@ -875,6 +982,8 @@ static long do_seccomp(unsigned int op, unsigned int flags,
>   		return seccomp_set_mode_strict();
>   	case SECCOMP_SET_MODE_FILTER:
>   		return seccomp_set_mode_filter(flags, uargs);
> +	case SECCOMP_MODE_FILTER_EBPF:
> +		return seccomp_mode_filter_ebpf(flags, uargs);
>   	default:
>   		return -EINVAL;
>   	}
>