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linux/fs/verity/verify.c
Andrey Albershteyn 8e43fb06e1 fsverity: remove hash page spin lock
The spin lock is not necessary here as it can be replaced with
memory barrier which should be better performance-wise.

When Merkle tree block size differs from page size, in
is_hash_block_verified() two things are modified during check - a
bitmap and PG_checked flag of the page.

Each bit in the bitmap represent verification status of the Merkle
tree blocks. PG_checked flag tells if page was just re-instantiated
or was in pagecache. Both of this states are shared between
verification threads. Page which was re-instantiated can not have
already verified blocks (bit set in bitmap).

The spin lock was used to allow only one thread to modify both of
these states and keep order of operations. The only requirement here
is that PG_Checked is set strictly after bitmap is updated.
This way other threads which see that PG_Checked=1 (page cached)
knows that bitmap is up-to-date. Otherwise, if PG_Checked is set
before bitmap is cleared, other threads can see bit=1 and therefore
will not perform verification of that Merkle tree block.

However, there's still the case when one thread is setting a bit in
verify_data_block() and other thread is clearing it in
is_hash_block_verified(). This can happen if two threads get to
!PageChecked branch and one of the threads is rescheduled before
resetting the bitmap. This is fine as at worst blocks are
re-verified in each thread.

Signed-off-by: Andrey Albershteyn <aalbersh@redhat.com>
[ebiggers: improved the comment and removed the 'verified' variable]
Link: https://lore.kernel.org/r/20240201052813.68380-1-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>
2024-02-01 15:19:23 -08:00

365 lines
12 KiB
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)
{
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. If the page is new, as
* indicated by PG_checked=0, we clear the bitmap bits for the page's
* blocks since they are untrustworthy, then set PG_checked=1.
* Otherwise we return the bitmap bit for the requested block.
*
* Multiple threads may execute this code concurrently on the same page.
* This is safe because we use memory barriers to ensure that if a
* thread sees PG_checked=1, then it also sees the associated bitmap
* clearing to have occurred. Also, all writes and their corresponding
* reads are atomic, and all writes are safe to repeat in the event that
* multiple threads get into the PG_checked=0 section. (Clearing a
* bitmap bit again at worst causes a hash block to be verified
* redundantly. That event should be very rare, so it's not worth using
* a lock to avoid. Setting PG_checked again has no effect.)
*/
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);
}
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);
return false;
}
/*
* 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");
}