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// SPDX-License-Identifier: GPL-2.0
/* Copyright (c) 2023 Meta Platforms, Inc. and affiliates. */
#include "vmlinux.h"
#include <errno.h>
#include <bpf/bpf_helpers.h>
#include <bpf/bpf_tracing.h>
#include "bpf_kfuncs.h"
char _license[] SEC("license") = "GPL";
#ifndef SHA256_DIGEST_SIZE
#define SHA256_DIGEST_SIZE 32
#endif
#define MAX_SIG_SIZE 1024
/* By default, "fsverity sign" signs a file with fsverity_formatted_digest
* of the file. fsverity_formatted_digest on the kernel side is only used
* with CONFIG_FS_VERITY_BUILTIN_SIGNATURES. However, BPF LSM doesn't not
* require CONFIG_FS_VERITY_BUILTIN_SIGNATURES, so vmlinux.h may not have
* fsverity_formatted_digest. In this test, we intentionally avoid using
* fsverity_formatted_digest.
*
* Luckily, fsverity_formatted_digest is simply 8-byte magic followed by
* fsverity_digest. We use a char array of size fsverity_formatted_digest
* plus SHA256_DIGEST_SIZE. The magic part of it is filled by user space,
* and the rest of it is filled by bpf_get_fsverity_digest.
*
* Note that, generating signatures based on fsverity_formatted_digest is
* the design choice of this selftest (and "fsverity sign"). With BPF
* LSM, we have the flexibility to generate signature based on other data
* sets, for example, fsverity_digest or only the digest[] part of it.
*/
#define MAGIC_SIZE 8
#define SIZEOF_STRUCT_FSVERITY_DIGEST 4 /* sizeof(struct fsverity_digest) */
char digest[MAGIC_SIZE + SIZEOF_STRUCT_FSVERITY_DIGEST + SHA256_DIGEST_SIZE];
__u32 monitored_pid;
char sig[MAX_SIG_SIZE];
__u32 sig_size;
__u32 user_keyring_serial;
SEC("lsm.s/file_open")
int BPF_PROG(test_file_open, struct file *f)
{
struct bpf_dynptr digest_ptr, sig_ptr;
struct bpf_key *trusted_keyring;
__u32 pid;
int ret;
pid = bpf_get_current_pid_tgid() >> 32;
if (pid != monitored_pid)
return 0;
/* digest_ptr points to fsverity_digest */
bpf_dynptr_from_mem(digest + MAGIC_SIZE, sizeof(digest) - MAGIC_SIZE, 0, &digest_ptr);
ret = bpf_get_fsverity_digest(f, &digest_ptr);
/* No verity, allow access */
if (ret < 0)
return 0;
/* Move digest_ptr to fsverity_formatted_digest */
bpf_dynptr_from_mem(digest, sizeof(digest), 0, &digest_ptr);
/* Read signature from xattr */
bpf_dynptr_from_mem(sig, sizeof(sig), 0, &sig_ptr);
ret = bpf_get_file_xattr(f, "user.sig", &sig_ptr);
/* No signature, reject access */
if (ret < 0)
return -EPERM;
trusted_keyring = bpf_lookup_user_key(user_keyring_serial, 0);
if (!trusted_keyring)
return -ENOENT;
/* Verify signature */
ret = bpf_verify_pkcs7_signature(&digest_ptr, &sig_ptr, trusted_keyring);
bpf_key_put(trusted_keyring);
return ret;
}
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