To fulfill the BPF signing contract, represented as Sig(I_loader ||
H_meta), the generated trusted loader program must verify the integrity
of the metadata. This signature cryptographically binds the loader's
instructions (I_loader) to a hash of the metadata (H_meta).

The verification process is embedded directly into the loader program.
Upon execution, the loader loads the runtime hash from struct bpf_map
i.e. BPF_PSEUDO_MAP_IDX and compares this runtime hash against an
expected hash value that has been hardcoded directly by
bpf_obj__gen_loader.

The load from bpf_map can be improved by calling
BPF_OBJ_GET_INFO_BY_FD from the kernel context after BPF_OBJ_GET_INFO_BY_FD
has been updated for being called from the kernel context.

The following instructions are generated:

    ld_imm64 r1, const_ptr_to_map // insn[0].src_reg == BPF_PSEUDO_MAP_IDX
    r2 = *(u64 *)(r1 + 0);
    ld_imm64 r3, sha256_of_map_part1 // constant precomputed by
bpftool (part of H_meta)
    if r2 != r3 goto out;

    r2 = *(u64 *)(r1 + 8);
    ld_imm64 r3, sha256_of_map_part2 // (part of H_meta)
    if r2 != r3 goto out;

    r2 = *(u64 *)(r1 + 16);
    ld_imm64 r3, sha256_of_map_part3 // (part of H_meta)
    if r2 != r3 goto out;

    r2 = *(u64 *)(r1 + 24);
    ld_imm64 r3, sha256_of_map_part4 // (part of H_meta)
    if r2 != r3 goto out;
    ...

Signed-off-by: KP Singh <kpsingh@kernel.org>
---
 tools/lib/bpf/bpf_gen_internal.h |  2 ++
 tools/lib/bpf/gen_loader.c       | 52 ++++++++++++++++++++++++++++++++
 tools/lib/bpf/libbpf.h           |  3 +-
 3 files changed, 56 insertions(+), 1 deletion(-)

diff --git a/tools/lib/bpf/bpf_gen_internal.h b/tools/lib/bpf/bpf_gen_internal.h
index 6ff963a491d9..49af4260b8e6 100644
--- a/tools/lib/bpf/bpf_gen_internal.h
+++ b/tools/lib/bpf/bpf_gen_internal.h

@@ -4,6 +4,7 @@
 #define __BPF_GEN_INTERNAL_H

 #include "bpf.h"
+#include "libbpf_internal.h"

 struct ksym_relo_desc {
        const char *name;

@@ -50,6 +51,7 @@ struct bpf_gen {
        __u32 nr_ksyms;
        int fd_array;
        int nr_fd_array;
+       int hash_insn_offset[SHA256_DWORD_SIZE];
 };

 void bpf_gen__init(struct bpf_gen *gen, int log_level, int nr_progs, int nr_maps);

diff --git a/tools/lib/bpf/gen_loader.c b/tools/lib/bpf/gen_loader.c
index 113ae4abd345..3d672c09e948 100644
--- a/tools/lib/bpf/gen_loader.c
+++ b/tools/lib/bpf/gen_loader.c

@@ -110,6 +110,7 @@ static void emit2(struct bpf_gen *gen, struct bpf_insn insn1, struct bpf_insn in

 static int add_data(struct bpf_gen *gen, const void *data, __u32 size);
 static void emit_sys_close_blob(struct bpf_gen *gen, int blob_off);
+static void bpf_gen__signature_match(struct bpf_gen *gen);

 void bpf_gen__init(struct bpf_gen *gen, int log_level, int nr_progs, int nr_maps)
 {

@@ -152,6 +153,8 @@ void bpf_gen__init(struct bpf_gen *gen, int log_level, int nr_progs, int nr_maps
        /* R7 contains the error code from sys_bpf. Copy it into R0 and exit. */
        emit(gen, BPF_MOV64_REG(BPF_REG_0, BPF_REG_7));
        emit(gen, BPF_EXIT_INSN());
+       if (gen->opts->gen_hash)
+               bpf_gen__signature_match(gen);
 }

 static int add_data(struct bpf_gen *gen, const void *data, __u32 size)

@@ -368,6 +371,25 @@ static void emit_sys_close_blob(struct bpf_gen *gen, int blob_off)
        __emit_sys_close(gen);
 }

+static int compute_sha_udpate_offsets(struct bpf_gen *gen)
+{
+       __u64 sha[SHA256_DWORD_SIZE];
+       int i, err;
+
+       err = libbpf_sha256(gen->data_start, gen->data_cur - gen->data_start, sha);
+       if (err < 0) {
+               pr_warn("sha256 computation of the metadata failed");
+               return err;
+       }
+       for (i = 0; i < SHA256_DWORD_SIZE; i++) {
+               struct bpf_insn *insn =
+                       (struct bpf_insn *)(gen->insn_start + gen->hash_insn_offset[i]);

+       }
+       return 0;
+}
+
 int bpf_gen__finish(struct bpf_gen *gen, int nr_progs, int nr_maps)
 {
        int i;

@@ -394,6 +416,12 @@ int bpf_gen__finish(struct bpf_gen *gen, int nr_progs, int nr_maps)
                              blob_fd_array_off(gen, i));
        emit(gen, BPF_MOV64_IMM(BPF_REG_0, 0));
        emit(gen, BPF_EXIT_INSN());
+       if (gen->opts->gen_hash) {
+               gen->error = compute_sha_udpate_offsets(gen);
+               if (gen->error)
+                       return gen->error;
+       }
+
        pr_debug("gen: finish %s\n", errstr(gen->error));
        if (!gen->error) {
                struct gen_loader_opts *opts = gen->opts;

@@ -557,6 +585,30 @@ void bpf_gen__map_create(struct bpf_gen *gen,
                emit_sys_close_stack(gen, stack_off(inner_map_fd));
 }

+static void bpf_gen__signature_match(struct bpf_gen *gen)
+{
+       __s64 off = -(gen->insn_cur - gen->insn_start - gen->cleanup_label) / 8 - 1;
+       int i;
+
+       for (i = 0; i < SHA256_DWORD_SIZE; i++) {
+               emit2(gen, BPF_LD_IMM64_RAW_FULL(BPF_REG_1, BPF_PSEUDO_MAP_IDX,
+                                                0, 0, 0, 0));
+               emit(gen, BPF_LDX_MEM(BPF_DW, BPF_REG_2, BPF_REG_1, i * sizeof(__u64)));
+               gen->hash_insn_offset[i] = gen->insn_cur - gen->insn_start;