[RFC PATCH bpf-next seccomp 03/12] seccomp, ptrace: Add a mechanism to retrieve attached eBPF seccomp filters

[YiFei Zhu <zhuyifei1999@gmail.com>]

From: Sargun Dhillon <sargun@sargun.me>

This extends the ptrace API to allow fetching eBPF seccomp filters
attached to programs. This is to enable checkpoint / restore cases.
The user will have to use the traditional PTRACE_SECCOMP_GET_FILTER
API call, and if they get an invalid medium type error they can switch
over to the eBPF variant of the API -- PTRACE_SECCOMP_GET_FILTER_EXTENDED.

Signed-off-by: Sargun Dhillon <sargun@sargun.me>
Link: https://lists.linux-foundation.org/pipermail/containers/2018-February/038478.html
[YiFei: increase ptrace number to 0x4210]
Signed-off-by: YiFei Zhu <yifeifz2@illinois.edu>
---
 include/linux/seccomp.h     | 12 ++++++++++++
 include/uapi/linux/ptrace.h |  2 ++
 kernel/ptrace.c             |  4 ++++
 kernel/seccomp.c            | 37 +++++++++++++++++++++++++++++++++++++
 4 files changed, 55 insertions(+)

diff --git a/include/linux/seccomp.h b/include/linux/seccomp.h
index c0750dc05de5..7ce9e3b3fa80 100644
--- a/include/linux/seccomp.h
+++ b/include/linux/seccomp.h
@@ -122,6 +122,18 @@ static inline long seccomp_get_metadata(struct task_struct *task,
 	return -EINVAL;
 }
 #endif /* CONFIG_SECCOMP_FILTER && CONFIG_CHECKPOINT_RESTORE */
+#if defined(CONFIG_SECCOMP_FILTER_EXTENDED) && defined(CONFIG_CHECKPOINT_RESTORE)
+extern long seccomp_get_filter_extended(struct task_struct *task,
+					unsigned long n,
+					void __user *data);
+#else
+static inline long seccomp_get_filter_extended(struct task_struct *task,
+					       unsigned long n,
+					       void __user *data)
+{
+	return -EINVAL;
+}
+#endif /* CONFIG_SECCOMP_FILTER_EXTENDED && CONFIG_CHECKPOINT_RESTORE */
 
 #ifdef CONFIG_SECCOMP_CACHE_DEBUG
 struct seq_file;
diff --git a/include/uapi/linux/ptrace.h b/include/uapi/linux/ptrace.h
index 3747bf816f9a..725a03614c28 100644
--- a/include/uapi/linux/ptrace.h
+++ b/include/uapi/linux/ptrace.h
@@ -112,6 +112,8 @@ struct ptrace_rseq_configuration {
 	__u32 pad;
 };
 
+#define PTRACE_SECCOMP_GET_FILTER_EXTENDED	0x4210
+
 /*
  * These values are stored in task->ptrace_message
  * by tracehook_report_syscall_* to describe the current syscall-stop.
diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index 76f09456ec4b..1e8d2155231f 100644
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -1247,6 +1247,10 @@ int ptrace_request(struct task_struct *child, long request,
 		break;
 #endif
 
+	case PTRACE_SECCOMP_GET_FILTER_EXTENDED:
+		ret = seccomp_get_filter_extended(child, addr, datavp);
+		break;
+
 	default:
 		break;
 	}
diff --git a/kernel/seccomp.c b/kernel/seccomp.c
index 1ef26a5bf93f..8550ae885245 100644
--- a/kernel/seccomp.c
+++ b/kernel/seccomp.c
@@ -2168,6 +2168,43 @@ long seccomp_get_metadata(struct task_struct *task,
 }
 #endif
 
+#if defined(CONFIG_SECCOMP_FILTER_EXTENDED) && defined(CONFIG_CHECKPOINT_RESTORE)
+long seccomp_get_filter_extended(struct task_struct *task,
+				 unsigned long filter_off,
+				 void __user *data)
+{
+	struct seccomp_filter *filter;
+	struct bpf_prog *prog;
+	long ret;
+
+	if (!capable(CAP_SYS_ADMIN) ||
+	    current->seccomp.mode != SECCOMP_MODE_DISABLED) {
+		return -EACCES;
+	}
+
+	filter = get_nth_filter(task, filter_off);
+	if (IS_ERR(filter))
+		return PTR_ERR(filter);
+
+	if (bpf_prog_was_classic(filter->prog)) {
+		ret = -EMEDIUMTYPE;
+		goto out;
+	}
+	prog = bpf_prog_inc_not_zero(filter->prog);
+	if (IS_ERR(prog)) {
+		ret = PTR_ERR(prog);
+		goto out;
+	}
+
+	ret = bpf_prog_new_fd(filter->prog);
+	if (ret < 0)
+		bpf_prog_put(prog);
+out:
+	__put_seccomp_filter(filter);
+	return ret;
+}
+#endif
+
 #ifdef CONFIG_SYSCTL
 
 /* Human readable action names for friendly sysctl interaction */
-- 
2.31.1