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// SPDX-License-Identifier: GPL-2.0
/*
* fs/verity/signature.c: verification of builtin signatures
*
* Copyright 2019 Google LLC
*/
#include "fsverity_private.h"
#include <linux/cred.h>
#include <linux/key.h>
#include <linux/slab.h>
#include <linux/verification.h>
/*
* /proc/sys/fs/verity/require_signatures
* If 1, all verity files must have a valid builtin signature.
*/
static int fsverity_require_signatures;
/*
* Keyring that contains the trusted X.509 certificates.
*
* Only root (kuid=0) can modify this. Also, root may use
* keyctl_restrict_keyring() to prevent any more additions.
*/
static struct key *fsverity_keyring;
/**
* fsverity_verify_signature() - check a verity file's signature
* @vi: the file's fsverity_info
* @desc: the file's fsverity_descriptor
* @desc_size: size of @desc
*
* If the file's fs-verity descriptor includes a signature of the file
* measurement, verify it against the certificates in the fs-verity keyring.
*
* Return: 0 on success (signature valid or not required); -errno on failure
*/
int fsverity_verify_signature(const struct fsverity_info *vi,
const struct fsverity_descriptor *desc,
size_t desc_size)
{
const struct inode *inode = vi->inode;
const struct fsverity_hash_alg *hash_alg = vi->tree_params.hash_alg;
const u32 sig_size = le32_to_cpu(desc->sig_size);
struct fsverity_signed_digest *d;
int err;
if (sig_size == 0) {
if (fsverity_require_signatures) {
fsverity_err(inode,
"require_signatures=1, rejecting unsigned file!");
return -EPERM;
}
return 0;
}
if (sig_size > desc_size - sizeof(*desc)) {
fsverity_err(inode, "Signature overflows verity descriptor");
return -EBADMSG;
}
if (fsverity_keyring->keys.nr_leaves_on_tree == 0) {
/*
* The ".fs-verity" keyring is empty, due to builtin signatures
* being supported by the kernel but not actually being used.
* In this case, verify_pkcs7_signature() would always return an
* error, usually ENOKEY. It could also be EBADMSG if the
* PKCS#7 is malformed, but that isn't very important to
* distinguish. So, just skip to ENOKEY to avoid the attack
* surface of the PKCS#7 parser, which would otherwise be
* reachable by any task able to execute FS_IOC_ENABLE_VERITY.
*/
fsverity_err(inode,
"fs-verity keyring is empty, rejecting signed file!");
return -ENOKEY;
}
d = kzalloc(sizeof(*d) + hash_alg->digest_size, GFP_KERNEL);
if (!d)
return -ENOMEM;
memcpy(d->magic, "FSVerity", 8);
d->digest_algorithm = cpu_to_le16(hash_alg - fsverity_hash_algs);
d->digest_size = cpu_to_le16(hash_alg->digest_size);
memcpy(d->digest, vi->measurement, hash_alg->digest_size);
err = verify_pkcs7_signature(d, sizeof(*d) + hash_alg->digest_size,
desc->signature, sig_size,
fsverity_keyring,
VERIFYING_UNSPECIFIED_SIGNATURE,
NULL, NULL);
kfree(d);
if (err) {
if (err == -ENOKEY)
fsverity_err(inode,
"File's signing cert isn't in the fs-verity keyring");
else if (err == -EKEYREJECTED)
fsverity_err(inode, "Incorrect file signature");
else if (err == -EBADMSG)
fsverity_err(inode, "Malformed file signature");
else
fsverity_err(inode, "Error %d verifying file signature",
err);
return err;
}
pr_debug("Valid signature for file measurement %s:%*phN\n",
hash_alg->name, hash_alg->digest_size, vi->measurement);
return 0;
}
#ifdef CONFIG_SYSCTL
static struct ctl_table_header *fsverity_sysctl_header;
static const struct ctl_path fsverity_sysctl_path[] = {
{ .procname = "fs", },
{ .procname = "verity", },
{ }
};
static struct ctl_table fsverity_sysctl_table[] = {
{
.procname = "require_signatures",
.data = &fsverity_require_signatures,
.maxlen = sizeof(int),
.mode = 0644,
.proc_handler = proc_dointvec_minmax,
.extra1 = SYSCTL_ZERO,
.extra2 = SYSCTL_ONE,
},
{ }
};
static int __init fsverity_sysctl_init(void)
{
fsverity_sysctl_header = register_sysctl_paths(fsverity_sysctl_path,
fsverity_sysctl_table);
if (!fsverity_sysctl_header) {
pr_err("sysctl registration failed!\n");
return -ENOMEM;
}
return 0;
}
#else /* !CONFIG_SYSCTL */
static inline int __init fsverity_sysctl_init(void)
{
return 0;
}
#endif /* !CONFIG_SYSCTL */
int __init fsverity_init_signature(void)
{
struct key *ring;
int err;
ring = keyring_alloc(".fs-verity", KUIDT_INIT(0), KGIDT_INIT(0),
current_cred(), KEY_POS_SEARCH |
KEY_USR_VIEW | KEY_USR_READ | KEY_USR_WRITE |
KEY_USR_SEARCH | KEY_USR_SETATTR,
KEY_ALLOC_NOT_IN_QUOTA, NULL, NULL);
if (IS_ERR(ring))
return PTR_ERR(ring);
err = fsverity_sysctl_init();
if (err)
goto err_put_ring;
fsverity_keyring = ring;
return 0;
err_put_ring:
key_put(ring);
return err;
}
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