Linux 6.13-rc4

[linux.git] / fs / verity / 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)
{
        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");
}