verify.c
// SPDX-License-Identifier: GPL-2.0
/*
* Data verification functions, i.e. hooks for ->readahead()
*
* Copyright 2019 Google LLC
*/
#include "fsverity_private.h"
#include <crypto/hash.h>
#include <linux/bio.h>
static struct workqueue_struct *fsverity_read_workqueue;
/*
* Returns true if the hash block with index @hblock_idx in the tree, located in
* @hpage, has already been verified.
*/
static bool is_hash_block_verified(struct fsverity_info *vi, struct page *hpage,
unsigned long hblock_idx)
{
bool verified;
unsigned int blocks_per_page;
unsigned int i;
/*
* When the Merkle tree block size and page size are the same, then the
* ->hash_block_verified bitmap isn't allocated, and we use PG_checked
* to directly indicate whether the page's block has been verified.
*
* Using PG_checked also guarantees that we re-verify hash pages that
* get evicted and re-instantiated from the backing storage, as new
* pages always start out with PG_checked cleared.
*/
if (!vi->hash_block_verified)
return PageChecked(hpage);
/*
* When the Merkle tree block size and page size differ, we use a bitmap
* to indicate whether each hash block has been verified.
*
* However, we still need to ensure that hash pages that get evicted and
* re-instantiated from the backing storage are re-verified. To do
* this, we use PG_checked again, but now it doesn't really mean
* "checked". Instead, now it just serves as an indicator for whether
* the hash page is newly instantiated or not.
*
* The first thread that sees PG_checked=0 must clear the corresponding
* bitmap bits, then set PG_checked=1. This requires a spinlock. To
* avoid having to take this spinlock in the common case of
* PG_checked=1, we start with an opportunistic lockless read.
*/
if (PageChecked(hpage)) {
/*
* A read memory barrier is needed here to give ACQUIRE
* semantics to the above PageChecked() test.
*/
smp_rmb();
return test_bit(hblock_idx, vi->hash_block_verified);
}
spin_lock(&vi->hash_page_init_lock);
if (PageChecked(hpage)) {
verified = test_bit(hblock_idx, vi->hash_block_verified);
} else {
blocks_per_page = vi->tree_params.blocks_per_page;
hblock_idx = round_down(hblock_idx, blocks_per_page);
for (i = 0; i < blocks_per_page; i++)
clear_bit(hblock_idx + i, vi->hash_block_verified);
/*
* A write memory barrier is needed here to give RELEASE
* semantics to the below SetPageChecked() operation.
*/
smp_wmb();
SetPageChecked(hpage);
verified = false;
}
spin_unlock(&vi->hash_page_init_lock);
return verified;
}
/*
* Verify a single data block against the file's Merkle tree.
*
* In principle, we need to verify the entire path to the root node. However,
* for efficiency the filesystem may cache the hash blocks. Therefore we need
* only ascend the tree until an already-verified hash block is seen, and then
* verify the path to that block.
*
* Return: %true if the data block is valid, else %false.
*/
static bool
verify_data_block(struct inode *inode, struct fsverity_info *vi,
const void *data, u64 data_pos, unsigned long max_ra_pages)
{
const struct merkle_tree_params *params = &vi->tree_params;
const unsigned int hsize = params->digest_size;
int level;
u8 _want_hash[FS_VERITY_MAX_DIGEST_SIZE];
const u8 *want_hash;
u8 real_hash[FS_VERITY_MAX_DIGEST_SIZE];
/* The hash blocks that are traversed, indexed by level */
struct {
/* Page containing the hash block */
struct page *page;
/* Mapped address of the hash block (will be within @page) */
const void *addr;
/* Index of the hash block in the tree overall */
unsigned long index;
/* Byte offset of the wanted hash relative to @addr */
unsigned int hoffset;
} hblocks[FS_VERITY_MAX_LEVELS];
/*
* The index of the previous level's block within that level; also the
* index of that block's hash within the current level.
*/
u64 hidx = data_pos >> params->log_blocksize;
/* Up to 1 + FS_VERITY_MAX_LEVELS pages may be mapped at once */
BUILD_BUG_ON(1 + FS_VERITY_MAX_LEVELS > KM_MAX_IDX);
if (unlikely(data_pos >= inode->i_size)) {
/*
* This can happen in the data page spanning EOF when the Merkle
* tree block size is less than the page size. The Merkle tree
* doesn't cover data blocks fully past EOF. But the entire
* page spanning EOF can be visible to userspace via a mmap, and
* any part past EOF should be all zeroes. Therefore, we need
* to verify that any data blocks fully past EOF are all zeroes.
*/
if (memchr_inv(data, 0, params->block_size)) {
fsverity_err(inode,
"FILE CORRUPTED! Data past EOF is not zeroed");
return false;
}
return true;
}
/*
* Starting at the leaf level, ascend the tree saving hash blocks along
* the way until we find a hash block that has already been verified, or
* until we reach the root.
*/
for (level = 0; level < params->num_levels; level++) {
unsigned long next_hidx;
unsigned long hblock_idx;
pgoff_t hpage_idx;
unsigned int hblock_offset_in_page;
unsigned int hoffset;
struct page *hpage;
const void *haddr;
/*
* The index of the block in the current level; also the index
* of that block's hash within the next level.
*/
next_hidx = hidx >> params->log_arity;
/* Index of the hash block in the tree overall */
hblock_idx = params->level_start[level] + next_hidx;
/* Index of the hash page in the tree overall */
hpage_idx = hblock_idx >> params->log_blocks_per_page;
/* Byte offset of the hash block within the page */
hblock_offset_in_page =
(hblock_idx << params->log_blocksize) & ~PAGE_MASK;
/* Byte offset of the hash within the block */
hoffset = (hidx << params->log_digestsize) &
(params->block_size - 1);
hpage = inode->i_sb->s_vop->read_merkle_tree_page(inode,
hpage_idx, level == 0 ? min(max_ra_pages,
params->tree_pages - hpage_idx) : 0);
if (IS_ERR(hpage)) {
fsverity_err(inode,
"Error %ld reading Merkle tree page %lu",
PTR_ERR(hpage), hpage_idx);
goto error;
}
haddr = kmap_local_page(hpage) + hblock_offset_in_page;
if (is_hash_block_verified(vi, hpage, hblock_idx)) {
memcpy(_want_hash, haddr + hoffset, hsize);
want_hash = _want_hash;
kunmap_local(haddr);
put_page(hpage);
goto descend;
}
hblocks[level].page = hpage;
hblocks[level].addr = haddr;
hblocks[level].index = hblock_idx;
hblocks[level].hoffset = hoffset;
hidx = next_hidx;
}
want_hash = vi->root_hash;
descend:
/* Descend the tree verifying hash blocks. */
for (; level > 0; level--) {
struct page *hpage = hblocks[level - 1].page;
const void *haddr = hblocks[level - 1].addr;
unsigned long hblock_idx = hblocks[level - 1].index;
unsigned int hoffset = hblocks[level - 1].hoffset;
if (fsverity_hash_block(params, inode, haddr, real_hash) != 0)
goto error;
if (memcmp(want_hash, real_hash, hsize) != 0)
goto corrupted;
/*
* Mark the hash block as verified. This must be atomic and
* idempotent, as the same hash block might be verified by
* multiple threads concurrently.
*/
if (vi->hash_block_verified)
set_bit(hblock_idx, vi->hash_block_verified);
else
SetPageChecked(hpage);
memcpy(_want_hash, haddr + hoffset, hsize);
want_hash = _want_hash;
kunmap_local(haddr);
put_page(hpage);
}
/* Finally, verify the data block. */
if (fsverity_hash_block(params, inode, data, real_hash) != 0)
goto error;
if (memcmp(want_hash, real_hash, hsize) != 0)
goto corrupted;
return true;
corrupted:
fsverity_err(inode,
"FILE CORRUPTED! pos=%llu, level=%d, want_hash=%s:%*phN, real_hash=%s:%*phN",
data_pos, level - 1,
params->hash_alg->name, hsize, want_hash,
params->hash_alg->name, hsize, real_hash);
error:
for (; level > 0; level--) {
kunmap_local(hblocks[level - 1].addr);
put_page(hblocks[level - 1].page);
}
return false;
}
static bool
verify_data_blocks(struct folio *data_folio, size_t len, size_t offset,
unsigned long max_ra_pages)
{
struct inode *inode = data_folio->mapping->host;
struct fsverity_info *vi = inode->i_verity_info;
const unsigned int block_size = vi->tree_params.block_size;
u64 pos = (u64)data_folio->index << PAGE_SHIFT;
if (WARN_ON_ONCE(len <= 0 || !IS_ALIGNED(len | offset, block_size)))
return false;
if (WARN_ON_ONCE(!folio_test_locked(data_folio) ||
folio_test_uptodate(data_folio)))
return false;
do {
void *data;
bool valid;
data = kmap_local_folio(data_folio, offset);
valid = verify_data_block(inode, vi, data, pos + offset,
max_ra_pages);
kunmap_local(data);
if (!valid)
return false;
offset += block_size;
len -= block_size;
} while (len);
return true;
}
/**
* fsverity_verify_blocks() - verify data in a folio
* @folio: the folio containing the data to verify
* @len: the length of the data to verify in the folio
* @offset: the offset of the data to verify in the folio
*
* Verify data that has just been read from a verity file. The data must be
* located in a pagecache folio that is still locked and not yet uptodate. The
* length and offset of the data must be Merkle tree block size aligned.
*
* Return: %true if the data is valid, else %false.
*/
bool fsverity_verify_blocks(struct folio *folio, size_t len, size_t offset)
{
return verify_data_blocks(folio, len, offset, 0);
}
EXPORT_SYMBOL_GPL(fsverity_verify_blocks);
#ifdef CONFIG_BLOCK
/**
* fsverity_verify_bio() - verify a 'read' bio that has just completed
* @bio: the bio to verify
*
* Verify the bio's data against the file's Merkle tree. All bio data segments
* must be aligned to the file's Merkle tree block size. If any data fails
* verification, then bio->bi_status is set to an error status.
*
* This is a helper function for use by the ->readahead() method of filesystems
* that issue bios to read data directly into the page cache. Filesystems that
* populate the page cache without issuing bios (e.g. non block-based
* filesystems) must instead call fsverity_verify_page() directly on each page.
* All filesystems must also call fsverity_verify_page() on holes.
*/
void fsverity_verify_bio(struct bio *bio)
{
struct folio_iter fi;
unsigned long max_ra_pages = 0;
if (bio->bi_opf & REQ_RAHEAD) {
/*
* If this bio is for data readahead, then we also do readahead
* of the first (largest) level of the Merkle tree. Namely,
* when a Merkle tree page is read, we also try to piggy-back on
* some additional pages -- up to 1/4 the number of data pages.
*
* This improves sequential read performance, as it greatly
* reduces the number of I/O requests made to the Merkle tree.
*/
max_ra_pages = bio->bi_iter.bi_size >> (PAGE_SHIFT + 2);
}
bio_for_each_folio_all(fi, bio) {
if (!verify_data_blocks(fi.folio, fi.length, fi.offset,
max_ra_pages)) {
bio->bi_status = BLK_STS_IOERR;
break;
}
}
}
EXPORT_SYMBOL_GPL(fsverity_verify_bio);
#endif /* CONFIG_BLOCK */
/**
* fsverity_enqueue_verify_work() - enqueue work on the fs-verity workqueue
* @work: the work to enqueue
*
* Enqueue verification work for asynchronous processing.
*/
void fsverity_enqueue_verify_work(struct work_struct *work)
{
queue_work(fsverity_read_workqueue, work);
}
EXPORT_SYMBOL_GPL(fsverity_enqueue_verify_work);
void __init fsverity_init_workqueue(void)
{
/*
* Use a high-priority workqueue to prioritize verification work, which
* blocks reads from completing, over regular application tasks.
*
* For performance reasons, don't use an unbound workqueue. Using an
* unbound workqueue for crypto operations causes excessive scheduler
* latency on ARM64.
*/
fsverity_read_workqueue = alloc_workqueue("fsverity_read_queue",
WQ_HIGHPRI,
num_online_cpus());
if (!fsverity_read_workqueue)
panic("failed to allocate fsverity_read_queue");
}