[RFC PATCH 2/2] seccomp: Introduce SECCOMP_ATTACH_FILTER operation

[Hengqi Chen <hengqi.chen@gmail.com>]

The SECCOMP_ATTACH_FILTER operation is used to attach
a loaded filter to the current process. The loaded filter
is represented by a fd which is either returned by the
SECCOMP_LOAD_FILTER operation or obtained from bpffs using
bpf syscall.

Signed-off-by: Hengqi Chen <hengqi.chen@gmail.com>
---
 include/uapi/linux/seccomp.h |  1 +
 kernel/seccomp.c             | 74 ++++++++++++++++++++++++++++++++++--
 2 files changed, 71 insertions(+), 4 deletions(-)

diff --git a/include/uapi/linux/seccomp.h b/include/uapi/linux/seccomp.h
index ee2c83697810..fbe30262fdfc 100644
--- a/include/uapi/linux/seccomp.h
+++ b/include/uapi/linux/seccomp.h
@@ -17,6 +17,7 @@
 #define SECCOMP_GET_ACTION_AVAIL	2
 #define SECCOMP_GET_NOTIF_SIZES		3
 #define SECCOMP_LOAD_FILTER		4
+#define SECCOMP_ATTACH_FILTER		5
 
 /* Valid flags for SECCOMP_SET_MODE_FILTER */
 #define SECCOMP_FILTER_FLAG_TSYNC		(1UL << 0)
diff --git a/kernel/seccomp.c b/kernel/seccomp.c
index 7aff22f56a91..adfafee4c3da 100644
--- a/kernel/seccomp.c
+++ b/kernel/seccomp.c
@@ -862,7 +862,7 @@ static void seccomp_cache_prepare(struct seccomp_filter *sfilter)
 #endif /* SECCOMP_ARCH_NATIVE */
 
 /**
- * seccomp_attach_filter: validate and attach filter
+ * seccomp_do_attach_filter: validate and attach filter
  * @flags:  flags to change filter behavior
  * @filter: seccomp filter to add to the current process
  *
@@ -873,8 +873,8 @@ static void seccomp_cache_prepare(struct seccomp_filter *sfilter)
  *     seccomp mode or did not have an ancestral seccomp filter
  *   - in NEW_LISTENER mode: the fd of the new listener
  */
-static long seccomp_attach_filter(unsigned int flags,
-				  struct seccomp_filter *filter)
+static long seccomp_do_attach_filter(unsigned int flags,
+				     struct seccomp_filter *filter)
 {
 	unsigned long total_insns;
 	struct seccomp_filter *walker;
@@ -1969,7 +1969,7 @@ static long seccomp_set_mode_filter(unsigned int flags,
 		goto out;
 	}
 
-	ret = seccomp_attach_filter(flags, prepared);
+	ret = seccomp_do_attach_filter(flags, prepared);
 	if (ret)
 		goto out;
 	/* Do not free the successfully attached filter. */
@@ -2050,6 +2050,62 @@ static long seccomp_load_filter(const char __user *filter)
 out:
 	return ret;
 }
+
+static long seccomp_attach_filter(const char __user *ufd)
+{
+	const unsigned long seccomp_mode = SECCOMP_MODE_FILTER;
+	struct seccomp_filter *sfilter;
+	struct bpf_prog *prog;
+	struct file *filp;
+	int flags = 0;
+	int fd, ret;
+
+	if (copy_from_user(&fd, ufd, sizeof(fd)))
+		return -EFAULT;
+
+	filp = fget(fd);
+	if (!filp)
+		return -EBADF;
+
+	if (filp->f_op != &bpf_prog_fops) {
+		ret = -EINVAL;
+		goto out;
+	}
+
+	prog = filp->private_data;
+
+	sfilter = kzalloc(sizeof(*sfilter), GFP_KERNEL | __GFP_NOWARN);
+	if (!sfilter) {
+		ret = -ENOMEM;
+		goto out;
+	}
+
+	sfilter->prog = prog;
+	refcount_set(&sfilter->refs, 1);
+	refcount_set(&sfilter->users, 1);
+	mutex_init(&sfilter->notify_lock);
+	init_waitqueue_head(&sfilter->wqh);
+
+	spin_lock_irq(&current->sighand->siglock);
+
+	ret = -EINVAL;
+	if (!seccomp_may_assign_mode(seccomp_mode))
+		goto out_unlock;
+
+	ret = seccomp_do_attach_filter(flags, sfilter);
+	if (ret)
+		goto out_unlock;
+
+	sfilter = NULL;
+	seccomp_assign_mode(current, seccomp_mode, flags);
+
+out_unlock:
+	spin_unlock_irq(&current->sighand->siglock);
+	seccomp_filter_free(sfilter);
+out:
+	fput(filp);
+	return ret;
+}
 #else
 static inline long seccomp_set_mode_filter(unsigned int flags,
 					   const char __user *filter)
@@ -2061,6 +2117,11 @@ static inline long seccomp_load_filter(const char __user *filter)
 {
 	return -EINVAL;
 }
+
+static inline long seccomp_attach_filter(const char __user *ufd)
+{
+	return -EINVAL;
+}
 #endif
 
 static long seccomp_get_action_avail(const char __user *uaction)
@@ -2127,6 +2188,11 @@ static long do_seccomp(unsigned int op, unsigned int flags,
 			return -EINVAL;
 
 		return seccomp_load_filter(uargs);
+	case SECCOMP_ATTACH_FILTER:
+		if (flags != 0)
+			return -EINVAL;
+
+		return seccomp_attach_filter(uargs);
 	default:
 		return -EINVAL;
 	}
-- 
2.34.1