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// SPDX-License-Identifier: GPL-2.0
/*
* Verification of builtin signatures
*
* Copyright 2019 Google LLC
*/
/*
* This file implements verification of fs-verity builtin signatures. Please
* take great care before using this feature. It is not the only way to do
* signatures with fs-verity, and the alternatives (such as userspace signature
* verification, and IMA appraisal) can be much better. For details about the
* limitations of this feature, see Documentation/filesystems/fsverity.rst.
*/
#include "fsverity_private.h"
#include <linux/cred.h>
#include <linux/key.h>
#include <linux/security.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.
*/
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
* @signature: the file's built-in signature
* @sig_size: size of signature in bytes, or 0 if no signature
*
* If the file includes a signature of its fs-verity file digest, verify it
* against the certificates in the fs-verity keyring. Note that signatures
* are verified regardless of the state of the 'fsverity_require_signatures'
* variable and the LSM subsystem relies on this behavior to help enforce
* file integrity policies. Please discuss changes with the LSM list
* (thank you!).
*
* Return: 0 on success (signature valid or not required); -errno on failure
*/
int fsverity_verify_signature(const struct fsverity_info *vi,
const u8 *signature, size_t sig_size)
{
const struct inode *inode = vi->inode;
const struct fsverity_hash_alg *hash_alg = vi->tree_params.hash_alg;
struct fsverity_formatted_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 (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->file_digest, hash_alg->digest_size);
err = verify_pkcs7_signature(d, sizeof(*d) + hash_alg->digest_size,
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;
}
err = security_inode_setintegrity(inode,
LSM_INT_FSVERITY_BUILTINSIG_VALID,
signature,
sig_size);
if (err) {
fsverity_err(inode, "Error %d exposing file signature to LSMs",
err);
return err;
}
return 0;
}
void __init fsverity_init_signature(void)
{
fsverity_keyring =
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(fsverity_keyring))
panic("failed to allocate \".fs-verity\" keyring");
}
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