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919dc32095
If an fsverity builtin signature is given for a file but the
".fs-verity" keyring is empty, there's no real reason to run the PKCS#7
parser. Skip this to avoid the PKCS#7 attack surface when builtin
signature support is configured into the kernel but is not being used.
This is a hardening improvement, not a fix per se, but I've added
Fixes and Cc stable to get it out to more users.
Fixes: 432434c9f8 ("fs-verity: support builtin file signatures")
Cc: stable@vger.kernel.org
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Link: https://lore.kernel.org/r/20230820173237.2579-1-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>
123 lines
3.8 KiB
C
123 lines
3.8 KiB
C
// SPDX-License-Identifier: GPL-2.0
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/*
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* Verification of builtin signatures
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*
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* Copyright 2019 Google LLC
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*/
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/*
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* This file implements verification of fs-verity builtin signatures. Please
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* take great care before using this feature. It is not the only way to do
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* signatures with fs-verity, and the alternatives (such as userspace signature
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* verification, and IMA appraisal) can be much better. For details about the
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* limitations of this feature, see Documentation/filesystems/fsverity.rst.
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*/
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#include "fsverity_private.h"
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#include <linux/cred.h>
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#include <linux/key.h>
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#include <linux/slab.h>
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#include <linux/verification.h>
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/*
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* /proc/sys/fs/verity/require_signatures
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* If 1, all verity files must have a valid builtin signature.
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*/
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int fsverity_require_signatures;
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/*
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* Keyring that contains the trusted X.509 certificates.
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*
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* Only root (kuid=0) can modify this. Also, root may use
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* keyctl_restrict_keyring() to prevent any more additions.
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*/
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static struct key *fsverity_keyring;
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/**
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* fsverity_verify_signature() - check a verity file's signature
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* @vi: the file's fsverity_info
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* @signature: the file's built-in signature
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* @sig_size: size of signature in bytes, or 0 if no signature
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*
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* If the file includes a signature of its fs-verity file digest, verify it
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* against the certificates in the fs-verity keyring.
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*
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* Return: 0 on success (signature valid or not required); -errno on failure
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*/
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int fsverity_verify_signature(const struct fsverity_info *vi,
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const u8 *signature, size_t sig_size)
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{
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const struct inode *inode = vi->inode;
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const struct fsverity_hash_alg *hash_alg = vi->tree_params.hash_alg;
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struct fsverity_formatted_digest *d;
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int err;
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if (sig_size == 0) {
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if (fsverity_require_signatures) {
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fsverity_err(inode,
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"require_signatures=1, rejecting unsigned file!");
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return -EPERM;
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}
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return 0;
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}
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if (fsverity_keyring->keys.nr_leaves_on_tree == 0) {
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/*
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* The ".fs-verity" keyring is empty, due to builtin signatures
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* being supported by the kernel but not actually being used.
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* In this case, verify_pkcs7_signature() would always return an
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* error, usually ENOKEY. It could also be EBADMSG if the
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* PKCS#7 is malformed, but that isn't very important to
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* distinguish. So, just skip to ENOKEY to avoid the attack
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* surface of the PKCS#7 parser, which would otherwise be
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* reachable by any task able to execute FS_IOC_ENABLE_VERITY.
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*/
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fsverity_err(inode,
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"fs-verity keyring is empty, rejecting signed file!");
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return -ENOKEY;
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}
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d = kzalloc(sizeof(*d) + hash_alg->digest_size, GFP_KERNEL);
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if (!d)
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return -ENOMEM;
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memcpy(d->magic, "FSVerity", 8);
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d->digest_algorithm = cpu_to_le16(hash_alg - fsverity_hash_algs);
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d->digest_size = cpu_to_le16(hash_alg->digest_size);
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memcpy(d->digest, vi->file_digest, hash_alg->digest_size);
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err = verify_pkcs7_signature(d, sizeof(*d) + hash_alg->digest_size,
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signature, sig_size, fsverity_keyring,
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VERIFYING_UNSPECIFIED_SIGNATURE,
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NULL, NULL);
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kfree(d);
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if (err) {
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if (err == -ENOKEY)
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fsverity_err(inode,
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"File's signing cert isn't in the fs-verity keyring");
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else if (err == -EKEYREJECTED)
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fsverity_err(inode, "Incorrect file signature");
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else if (err == -EBADMSG)
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fsverity_err(inode, "Malformed file signature");
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else
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fsverity_err(inode, "Error %d verifying file signature",
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err);
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return err;
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}
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return 0;
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}
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void __init fsverity_init_signature(void)
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{
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fsverity_keyring =
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keyring_alloc(".fs-verity", KUIDT_INIT(0), KGIDT_INIT(0),
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current_cred(), KEY_POS_SEARCH |
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KEY_USR_VIEW | KEY_USR_READ | KEY_USR_WRITE |
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KEY_USR_SEARCH | KEY_USR_SETATTR,
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KEY_ALLOC_NOT_IN_QUOTA, NULL, NULL);
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if (IS_ERR(fsverity_keyring))
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panic("failed to allocate \".fs-verity\" keyring");
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}
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