bpf: Add bpf_bprm_opts_set helper

The helper allows modification of certain bits on the linux_binprm
struct starting with the secureexec bit which can be updated using the
BPF_F_BPRM_SECUREEXEC flag.

secureexec can be set by the LSM for privilege gaining executions to set
the AT_SECURE auxv for glibc.  When set, the dynamic linker disables the
use of certain environment variables (like LD_PRELOAD).

Signed-off-by: KP Singh <kpsingh@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Link: https://lore.kernel.org/bpf/20201117232929.2156341-1-kpsingh@chromium.org

include/uapi/linux/bpf.h

@@ -3787,6 +3787,16 @@ union bpf_attr {
  *		*ARG_PTR_TO_BTF_ID* of type *task_struct*.
  *	Return
  *		Pointer to the current task.
+ *
+ * long bpf_bprm_opts_set(struct linux_binprm *bprm, u64 flags)
+ *	Description
+ *		Set or clear certain options on *bprm*:
+ *
+ *		**BPF_F_BPRM_SECUREEXEC** Set the secureexec bit
+ *		which sets the **AT_SECURE** auxv for glibc. The bit
+ *		is cleared if the flag is not specified.
+ *	Return
+ *		**-EINVAL** if invalid *flags* are passed, zero otherwise.
  */
 #define __BPF_FUNC_MAPPER(FN)		\
 	FN(unspec),			\
@@ -3948,6 +3958,7 @@ union bpf_attr {
 	FN(task_storage_get),		\
 	FN(task_storage_delete),	\
 	FN(get_current_task_btf),	\
+	FN(bprm_opts_set),		\
 	/* */
 
 /* integer value in 'imm' field of BPF_CALL instruction selects which helper
@@ -4119,6 +4130,11 @@ enum bpf_lwt_encap_mode {
 	BPF_LWT_ENCAP_IP,
 };
 
+/* Flags for bpf_bprm_opts_set helper */
+enum {
+	BPF_F_BPRM_SECUREEXEC	= (1ULL << 0),
+};
+
 #define __bpf_md_ptr(type, name)	\
 union {					\
 	type name;			\

kernel/bpf/bpf_lsm.c

@@ -7,6 +7,7 @@
 #include <linux/filter.h>
 #include <linux/bpf.h>
 #include <linux/btf.h>
+#include <linux/binfmts.h>
 #include <linux/lsm_hooks.h>
 #include <linux/bpf_lsm.h>
 #include <linux/kallsyms.h>
@@ -51,6 +52,29 @@ int bpf_lsm_verify_prog(struct bpf_verifier_log *vlog,
 	return 0;
 }
 
+/* Mask for all the currently supported BPRM option flags */
+#define BPF_F_BRPM_OPTS_MASK	BPF_F_BPRM_SECUREEXEC
+
+BPF_CALL_2(bpf_bprm_opts_set, struct linux_binprm *, bprm, u64, flags)
+{
+	if (flags & ~BPF_F_BRPM_OPTS_MASK)
+		return -EINVAL;
+
+	bprm->secureexec = (flags & BPF_F_BPRM_SECUREEXEC);
+	return 0;
+}
+
+BTF_ID_LIST_SINGLE(bpf_bprm_opts_set_btf_ids, struct, linux_binprm)
+
+const static struct bpf_func_proto bpf_bprm_opts_set_proto = {
+	.func		= bpf_bprm_opts_set,
+	.gpl_only	= false,
+	.ret_type	= RET_INTEGER,
+	.arg1_type	= ARG_PTR_TO_BTF_ID,
+	.arg1_btf_id	= &bpf_bprm_opts_set_btf_ids[0],
+	.arg2_type	= ARG_ANYTHING,
+};
+
 static const struct bpf_func_proto *
 bpf_lsm_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
 {
@@ -71,6 +95,8 @@ bpf_lsm_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
 		return &bpf_task_storage_get_proto;
 	case BPF_FUNC_task_storage_delete:
 		return &bpf_task_storage_delete_proto;
+	case BPF_FUNC_bprm_opts_set:
+		return &bpf_bprm_opts_set_proto;
 	default:
 		return tracing_prog_func_proto(func_id, prog);
 	}

scripts/bpf_helpers_doc.py

@@ -418,6 +418,7 @@ class PrinterHelpers(Printer):
             'struct bpf_tcp_sock',
             'struct bpf_tunnel_key',
             'struct bpf_xfrm_state',
+            'struct linux_binprm',
             'struct pt_regs',
             'struct sk_reuseport_md',
             'struct sockaddr',
@@ -465,6 +466,7 @@ class PrinterHelpers(Printer):
             'struct bpf_tcp_sock',
             'struct bpf_tunnel_key',
             'struct bpf_xfrm_state',
+            'struct linux_binprm',
             'struct pt_regs',
             'struct sk_reuseport_md',
             'struct sockaddr',

tools/include/uapi/linux/bpf.h

@@ -3787,6 +3787,16 @@ union bpf_attr {
  *		*ARG_PTR_TO_BTF_ID* of type *task_struct*.
  *	Return
  *		Pointer to the current task.
+ *
+ * long bpf_bprm_opts_set(struct linux_binprm *bprm, u64 flags)
+ *	Description
+ *		Set or clear certain options on *bprm*:
+ *
+ *		**BPF_F_BPRM_SECUREEXEC** Set the secureexec bit
+ *		which sets the **AT_SECURE** auxv for glibc. The bit
+ *		is cleared if the flag is not specified.
+ *	Return
+ *		**-EINVAL** if invalid *flags* are passed, zero otherwise.
  */
 #define __BPF_FUNC_MAPPER(FN)		\
 	FN(unspec),			\
@@ -3948,6 +3958,7 @@ union bpf_attr {
 	FN(task_storage_get),		\
 	FN(task_storage_delete),	\
 	FN(get_current_task_btf),	\
+	FN(bprm_opts_set),		\
 	/* */
 
 /* integer value in 'imm' field of BPF_CALL instruction selects which helper
@@ -4119,6 +4130,11 @@ enum bpf_lwt_encap_mode {
 	BPF_LWT_ENCAP_IP,
 };
 
+/* Flags for bpf_bprm_opts_set helper */
+enum {
+	BPF_F_BPRM_SECUREEXEC	= (1ULL << 0),
+};
+
 #define __bpf_md_ptr(type, name)	\
 union {					\
 	type name;			\