[RFC PATCH bpf-next seccomp 10/12] seccomp-ebpf: Add ability to read user memory

[YiFei Zhu <zhuyifei1999@gmail.com>]

From: YiFei Zhu <yifeifz2@illinois.edu>

This uses helpers bpf_probe_read_user{,str}. To repect unprivileged
users may also load filters, when the loader of the filter does not
have CAP_SYS_PTRACE, attempting to read user memory when current mm
is non-dumpable results in -EPERM.

Right now this is not sleepable, -EFAULT may happen for valid memory
addresses. Future work might be adding support to bpf_copy_from_user
via sleepable filters.

Use of memory data to implement policy is discouraged until there is
a solution for time-of-check to time-of-use.

Signed-off-by: YiFei Zhu <yifeifz2@illinois.edu>
---
 include/linux/bpf.h      |  4 ++++
 kernel/seccomp.c         |  8 ++++++++
 kernel/trace/bpf_trace.c | 42 ++++++++++++++++++++++++++++++++++++++++
 3 files changed, 54 insertions(+)

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index 86f3e8784e43..2019c0893250 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -1965,6 +1965,10 @@ extern const struct bpf_func_proto bpf_get_socket_ptr_cookie_proto;
 extern const struct bpf_func_proto bpf_task_storage_get_proto;
 extern const struct bpf_func_proto bpf_task_storage_delete_proto;
 extern const struct bpf_func_proto bpf_for_each_map_elem_proto;
+extern const struct bpf_func_proto bpf_probe_read_user_proto;
+extern const struct bpf_func_proto bpf_probe_read_user_dumpable_proto;
+extern const struct bpf_func_proto bpf_probe_read_user_str_proto;
+extern const struct bpf_func_proto bpf_probe_read_user_dumpable_str_proto;
 
 const struct bpf_func_proto *bpf_tracing_func_proto(
 	enum bpf_func_id func_id, const struct bpf_prog *prog);
diff --git a/kernel/seccomp.c b/kernel/seccomp.c
index b9ed9951a05b..330e9c365cdc 100644
--- a/kernel/seccomp.c
+++ b/kernel/seccomp.c
@@ -2449,6 +2449,14 @@ seccomp_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
 		return &bpf_get_current_uid_gid_proto;
 	case BPF_FUNC_get_current_pid_tgid:
 		return &bpf_get_current_pid_tgid_proto;
+	case BPF_FUNC_probe_read_user:
+		return ns_capable(current_user_ns(), CAP_SYS_PTRACE) ?
+			&bpf_probe_read_user_proto :
+			&bpf_probe_read_user_dumpable_proto;
+	case BPF_FUNC_probe_read_user_str:
+		return ns_capable(current_user_ns(), CAP_SYS_PTRACE) ?
+			&bpf_probe_read_user_str_proto :
+			&bpf_probe_read_user_dumpable_str_proto;
 	default:
 		break;
 	}
diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
index d2d7cf6cfe83..a1d6d64bde08 100644
--- a/kernel/trace/bpf_trace.c
+++ b/kernel/trace/bpf_trace.c
@@ -175,6 +175,27 @@ const struct bpf_func_proto bpf_probe_read_user_proto = {
 	.arg3_type	= ARG_ANYTHING,
 };
 
+BPF_CALL_3(bpf_probe_read_user_dumpable, void *, dst, u32, size,
+	   const void __user *, unsafe_ptr)
+{
+	int ret = -EPERM;
+
+	if (get_dumpable(current->mm))
+		ret = copy_from_user_nofault(dst, unsafe_ptr, size);
+	if (unlikely(ret < 0))
+		memset(dst, 0, size);
+	return ret;
+}
+
+const struct bpf_func_proto bpf_probe_read_user_dumpable_proto = {
+	.func		= bpf_probe_read_user_dumpable,
+	.gpl_only	= true,
+	.ret_type	= RET_INTEGER,
+	.arg1_type	= ARG_PTR_TO_UNINIT_MEM,
+	.arg2_type	= ARG_CONST_SIZE_OR_ZERO,
+	.arg3_type	= ARG_ANYTHING,
+};
+
 static __always_inline int
 bpf_probe_read_user_str_common(void *dst, u32 size,
 			       const void __user *unsafe_ptr)
@@ -212,6 +233,27 @@ const struct bpf_func_proto bpf_probe_read_user_str_proto = {
 	.arg3_type	= ARG_ANYTHING,
 };
 
+BPF_CALL_3(bpf_probe_read_user_dumpable_str, void *, dst, u32, size,
+	   const void __user *, unsafe_ptr)
+{
+	int ret = -EPERM;
+
+	if (get_dumpable(current->mm))
+		ret = strncpy_from_user_nofault(dst, unsafe_ptr, size);
+	if (unlikely(ret < 0))
+		memset(dst, 0, size);
+	return ret;
+}
+
+const struct bpf_func_proto bpf_probe_read_user_dumpable_str_proto = {
+	.func		= bpf_probe_read_user_dumpable_str,
+	.gpl_only	= true,
+	.ret_type	= RET_INTEGER,
+	.arg1_type	= ARG_PTR_TO_UNINIT_MEM,
+	.arg2_type	= ARG_CONST_SIZE_OR_ZERO,
+	.arg3_type	= ARG_ANYTHING,
+};
+
 static __always_inline int
 bpf_probe_read_kernel_common(void *dst, u32 size, const void *unsafe_ptr)
 {
-- 
2.31.1