1 // SPDX-License-Identifier: GPL-2.0
2 /* Copyright (c) 2023 Meta Platforms, Inc. and affiliates. */
3
4 #include "vmlinux.h"
5 #include <errno.h>
6 #include <bpf/bpf_helpers.h>
7 #include <bpf/bpf_tracing.h>
8 #include "bpf_kfuncs.h"
9 #include "err.h"
10
11 char _license[] SEC("license") = "GPL";
12
13 #ifndef SHA256_DIGEST_SIZE
14 #define SHA256_DIGEST_SIZE 32
15 #endif
16
17 #define MAX_SIG_SIZE 1024
18
19 /* By default, "fsverity sign" signs a file with fsverity_formatted_digest
20 * of the file. fsverity_formatted_digest on the kernel side is only used
21 * with CONFIG_FS_VERITY_BUILTIN_SIGNATURES. However, BPF LSM doesn't not
22 * require CONFIG_FS_VERITY_BUILTIN_SIGNATURES, so vmlinux.h may not have
23 * fsverity_formatted_digest. In this test, we intentionally avoid using
24 * fsverity_formatted_digest.
25 *
26 * Luckily, fsverity_formatted_digest is simply 8-byte magic followed by
27 * fsverity_digest. We use a char array of size fsverity_formatted_digest
28 * plus SHA256_DIGEST_SIZE. The magic part of it is filled by user space,
29 * and the rest of it is filled by bpf_get_fsverity_digest.
30 *
31 * Note that, generating signatures based on fsverity_formatted_digest is
32 * the design choice of this selftest (and "fsverity sign"). With BPF
33 * LSM, we have the flexibility to generate signature based on other data
34 * sets, for example, fsverity_digest or only the digest[] part of it.
35 */
36 #define MAGIC_SIZE 8
37 #define SIZEOF_STRUCT_FSVERITY_DIGEST 4 /* sizeof(struct fsverity_digest) */
38 char digest[MAGIC_SIZE + SIZEOF_STRUCT_FSVERITY_DIGEST + SHA256_DIGEST_SIZE];
39
40 __u32 monitored_pid;
41 char sig[MAX_SIG_SIZE];
42 __u32 sig_size;
43 __u32 user_keyring_serial;
44
45 SEC("lsm.s/file_open")
46 int BPF_PROG(test_file_open, struct file *f)
47 {
48 struct bpf_dynptr digest_ptr, sig_ptr;
49 struct bpf_key *trusted_keyring;
50 __u32 pid;
51 int ret;
52
53 pid = bpf_get_current_pid_tgid() >> 32;
54 if (pid != monitored_pid)
55 return 0;
56
57 /* digest_ptr points to fsverity_digest */
58 bpf_dynptr_from_mem(digest + MAGIC_SIZE, sizeof(digest) - MAGIC_SIZE, 0, &digest_ptr);
59
60 ret = bpf_get_fsverity_digest(f, &digest_ptr);
61 /* No verity, allow access */
62 if (ret < 0)
63 return 0;
64
65 /* Move digest_ptr to fsverity_formatted_digest */
66 bpf_dynptr_from_mem(digest, sizeof(digest), 0, &digest_ptr);
67
68 /* Read signature from xattr */
69 bpf_dynptr_from_mem(sig, sizeof(sig), 0, &sig_ptr);
70 ret = bpf_get_file_xattr(f, "user.sig", &sig_ptr);
71 /* No signature, reject access */
72 if (ret < 0)
73 return -EPERM;
74
75 trusted_keyring = bpf_lookup_user_key(user_keyring_serial, 0);
76 if (!trusted_keyring)
77 return -ENOENT;
78
79 /* Verify signature */
80 ret = bpf_verify_pkcs7_signature(&digest_ptr, &sig_ptr, trusted_keyring);
81
82 bpf_key_put(trusted_keyring);
83
84 set_if_not_errno_or_zero(ret, -EFAULT);
85
86 return ret;
87 }
88
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