measure.c

  1// SPDX-License-Identifier: GPL-2.0
  2/*
  3 * Ioctl to get a verity file's digest
  4 *
  5 * Copyright 2019 Google LLC
  6 */
  7
  8#include "fsverity_private.h"
  9
 10#include <linux/bpf.h>
 11#include <linux/btf.h>
 12#include <linux/export.h>
 13#include <linux/uaccess.h>
 14
 15/**
 16 * fsverity_ioctl_measure() - get a verity file's digest
 17 * @filp: file to get digest of
 18 * @_uarg: user pointer to fsverity_digest
 19 *
 20 * Retrieve the file digest that the kernel is enforcing for reads from a verity
 21 * file.  See the "FS_IOC_MEASURE_VERITY" section of
 22 * Documentation/filesystems/fsverity.rst for the documentation.
 23 *
 24 * Return: 0 on success, -errno on failure
 25 */
 26int fsverity_ioctl_measure(struct file *filp, void __user *_uarg)
 27{
 28	const struct inode *inode = file_inode(filp);
 29	struct fsverity_digest __user *uarg = _uarg;
 30	const struct fsverity_info *vi;
 31	const struct fsverity_hash_alg *hash_alg;
 32	struct fsverity_digest arg;
 33
 34	vi = fsverity_get_info(inode);
 35	if (!vi)
 36		return -ENODATA; /* not a verity file */
 37	hash_alg = vi->tree_params.hash_alg;
 38
 39	/*
 40	 * The user specifies the digest_size their buffer has space for; we can
 41	 * return the digest if it fits in the available space.  We write back
 42	 * the actual size, which may be shorter than the user-specified size.
 43	 */
 44
 45	if (get_user(arg.digest_size, &uarg->digest_size))
 46		return -EFAULT;
 47	if (arg.digest_size < hash_alg->digest_size)
 48		return -EOVERFLOW;
 49
 50	memset(&arg, 0, sizeof(arg));
 51	arg.digest_algorithm = hash_alg - fsverity_hash_algs;
 52	arg.digest_size = hash_alg->digest_size;
 53
 54	if (copy_to_user(uarg, &arg, sizeof(arg)))
 55		return -EFAULT;
 56
 57	if (copy_to_user(uarg->digest, vi->file_digest, hash_alg->digest_size))
 58		return -EFAULT;
 59
 60	return 0;
 61}
 62EXPORT_SYMBOL_GPL(fsverity_ioctl_measure);
 63
 64/**
 65 * fsverity_get_digest() - get a verity file's digest
 66 * @inode: inode to get digest of
 67 * @raw_digest: (out) the raw file digest
 68 * @alg: (out) the digest's algorithm, as a FS_VERITY_HASH_ALG_* value
 69 * @halg: (out) the digest's algorithm, as a HASH_ALGO_* value
 70 *
 71 * Retrieves the fsverity digest of the given file.  The file must have been
 72 * opened at least once since the inode was last loaded into the inode cache;
 73 * otherwise this function will not recognize when fsverity is enabled.
 74 *
 75 * The file's fsverity digest consists of @raw_digest in combination with either
 76 * @alg or @halg.  (The caller can choose which one of @alg or @halg to use.)
 77 *
 78 * IMPORTANT: Callers *must* make use of one of the two algorithm IDs, since
 79 * @raw_digest is meaningless without knowing which algorithm it uses!  fsverity
 80 * provides no security guarantee for users who ignore the algorithm ID, even if
 81 * they use the digest size (since algorithms can share the same digest size).
 82 *
 83 * Return: The size of the raw digest in bytes, or 0 if the file doesn't have
 84 *	   fsverity enabled.
 85 */
 86int fsverity_get_digest(struct inode *inode,
 87			u8 raw_digest[FS_VERITY_MAX_DIGEST_SIZE],
 88			u8 *alg, enum hash_algo *halg)
 89{
 90	const struct fsverity_info *vi;
 91	const struct fsverity_hash_alg *hash_alg;
 92
 93	vi = fsverity_get_info(inode);
 94	if (!vi)
 95		return 0; /* not a verity file */
 96
 97	hash_alg = vi->tree_params.hash_alg;
 98	memcpy(raw_digest, vi->file_digest, hash_alg->digest_size);
 99	if (alg)
100		*alg = hash_alg - fsverity_hash_algs;
101	if (halg)
102		*halg = hash_alg->algo_id;
103	return hash_alg->digest_size;
104}
105EXPORT_SYMBOL_GPL(fsverity_get_digest);
106
107#ifdef CONFIG_BPF_SYSCALL
108
109/* bpf kfuncs */
110__bpf_kfunc_start_defs();
111
112/**
113 * bpf_get_fsverity_digest: read fsverity digest of file
114 * @file: file to get digest from
115 * @digest_p: (out) dynptr for struct fsverity_digest
116 *
117 * Read fsverity_digest of *file* into *digest_ptr*.
118 *
119 * Return: 0 on success, a negative value on error.
120 */
121__bpf_kfunc int bpf_get_fsverity_digest(struct file *file, struct bpf_dynptr *digest_p)
122{
123	struct bpf_dynptr_kern *digest_ptr = (struct bpf_dynptr_kern *)digest_p;
124	const struct inode *inode = file_inode(file);
125	u32 dynptr_sz = __bpf_dynptr_size(digest_ptr);
126	struct fsverity_digest *arg;
127	const struct fsverity_info *vi;
128	const struct fsverity_hash_alg *hash_alg;
129	int out_digest_sz;
130
131	if (dynptr_sz < sizeof(struct fsverity_digest))
132		return -EINVAL;
133
134	arg = __bpf_dynptr_data_rw(digest_ptr, dynptr_sz);
135	if (!arg)
136		return -EINVAL;
137
138	if (!IS_ALIGNED((uintptr_t)arg, __alignof__(*arg)))
139		return -EINVAL;
140
141	vi = fsverity_get_info(inode);
142	if (!vi)
143		return -ENODATA; /* not a verity file */
144
145	hash_alg = vi->tree_params.hash_alg;
146
147	arg->digest_algorithm = hash_alg - fsverity_hash_algs;
148	arg->digest_size = hash_alg->digest_size;
149
150	out_digest_sz = dynptr_sz - sizeof(struct fsverity_digest);
151
152	/* copy digest */
153	memcpy(arg->digest, vi->file_digest,  min_t(int, hash_alg->digest_size, out_digest_sz));
154
155	/* fill the extra buffer with zeros */
156	if (out_digest_sz > hash_alg->digest_size)
157		memset(arg->digest + arg->digest_size, 0, out_digest_sz - hash_alg->digest_size);
158
159	return 0;
160}
161
162__bpf_kfunc_end_defs();
163
164BTF_KFUNCS_START(fsverity_set_ids)
165BTF_ID_FLAGS(func, bpf_get_fsverity_digest, KF_TRUSTED_ARGS)
166BTF_KFUNCS_END(fsverity_set_ids)
167
168static int bpf_get_fsverity_digest_filter(const struct bpf_prog *prog, u32 kfunc_id)
169{
170	if (!btf_id_set8_contains(&fsverity_set_ids, kfunc_id))
171		return 0;
172
173	/* Only allow to attach from LSM hooks, to avoid recursion */
174	return prog->type != BPF_PROG_TYPE_LSM ? -EACCES : 0;
175}
176
177static const struct btf_kfunc_id_set bpf_fsverity_set = {
178	.owner = THIS_MODULE,
179	.set = &fsverity_set_ids,
180	.filter = bpf_get_fsverity_digest_filter,
181};
182
183void __init fsverity_init_bpf(void)
184{
185	register_btf_kfunc_id_set(BPF_PROG_TYPE_LSM, &bpf_fsverity_set);
186}
187
188#endif /* CONFIG_BPF_SYSCALL */