// SPDX-License-Identifier: GPL-2.0
/*
 * Data verification functions, i.e. hooks for ->readpages()
 *
 * Copyright 2019 Google LLC
 */

#include "fsverity_private.h"

#include <crypto/hash.h>
#include <linux/bio.h>
#include <linux/ratelimit.h>

static struct workqueue_struct *fsverity_read_workqueue;

/**
 * hash_at_level() - compute the location of the block's hash at the given level
 *
 * @params:     (in) the Merkle tree parameters
 * @dindex:     (in) the index of the data block being verified
 * @level:      (in) the level of hash we want (0 is leaf level)
 * @hindex:     (out) the index of the hash block containing the wanted hash
 * @hoffset:    (out) the byte offset to the wanted hash within the hash block
 */
static void hash_at_level(const struct merkle_tree_params *params,
                          pgoff_t dindex, unsigned int level, pgoff_t *hindex,
                          unsigned int *hoffset)
{
        pgoff_t position;

        /* Offset of the hash within the level's region, in hashes */
        position = dindex >> (level * params->log_arity);

        /* Index of the hash block in the tree overall */
        *hindex = params->level_start[level] + (position >> params->log_arity);

        /* Offset of the wanted hash (in bytes) within the hash block */
        *hoffset = (position & ((1 << params->log_arity) - 1)) <<
                   (params->log_blocksize - params->log_arity);
}

/* Extract a hash from a hash page */
static void extract_hash(struct page *hpage, unsigned int hoffset,
                         unsigned int hsize, u8 *out)
{
        void *virt = kmap_atomic(hpage);

        memcpy(out, virt + hoffset, hsize);
        kunmap_atomic(virt);
}

static inline int cmp_hashes(const struct fsverity_info *vi,
                             const u8 *want_hash, const u8 *real_hash,
                             pgoff_t index, int level)
{
        const unsigned int hsize = vi->tree_params.digest_size;

        if (memcmp(want_hash, real_hash, hsize) == 0)
                return 0;

        fsverity_err(vi->inode,
                     "FILE CORRUPTED! index=%lu, level=%d, want_hash=%s:%*phN, real_hash=%s:%*phN",
                     index, level,
                     vi->tree_params.hash_alg->name, hsize, want_hash,
                     vi->tree_params.hash_alg->name, hsize, real_hash);
        return -EBADMSG;
}

/*
 * Verify a single data page 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 pages.  Therefore we need
 * only ascend the tree until an already-verified page is seen, as indicated by
 * the PageChecked bit being set; then verify the path to that page.
 *
 * This code currently only supports the case where the verity block size is
 * equal to PAGE_SIZE.  Doing otherwise would be possible but tricky, since we
 * wouldn't be able to use the PageChecked bit.
 *
 * Note that multiple processes may race to verify a hash page and mark it
 * Checked, but it doesn't matter; the result will be the same either way.
 *
 * Return: true if the page is valid, else false.
 */
static bool verify_page(struct inode *inode, const struct fsverity_info *vi,
                        struct ahash_request *req, struct page *data_page,
                        unsigned long level0_ra_pages)
{
        const struct merkle_tree_params *params = &vi->tree_params;
        const unsigned int hsize = params->digest_size;
        const pgoff_t index = data_page->index;
        int level;
        u8 _want_hash[FS_VERITY_MAX_DIGEST_SIZE];
        const u8 *want_hash;
        u8 real_hash[FS_VERITY_MAX_DIGEST_SIZE];
        struct page *hpages[FS_VERITY_MAX_LEVELS];
        unsigned int hoffsets[FS_VERITY_MAX_LEVELS];
        int err;

        if (WARN_ON_ONCE(!PageLocked(data_page) || PageUptodate(data_page)))
                return false;

        pr_debug_ratelimited("Verifying data page %lu...\n", index);

        /*
         * Starting at the leaf level, ascend the tree saving hash pages along
         * the way until we find a verified hash page, indicated by PageChecked;
         * or until we reach the root.
         */
        for (level = 0; level < params->num_levels; level++) {
                pgoff_t hindex;
                unsigned int hoffset;
                struct page *hpage;

                hash_at_level(params, index, level, &hindex, &hoffset);

                pr_debug_ratelimited("Level %d: hindex=%lu, hoffset=%u\n",
                                     level, hindex, hoffset);

                hpage = inode->i_sb->s_vop->read_merkle_tree_page(inode, hindex,
                                level == 0 ? level0_ra_pages : 0);
                if (IS_ERR(hpage)) {
                        err = PTR_ERR(hpage);
                        fsverity_err(inode,
                                     "Error %d reading Merkle tree page %lu",
                                     err, hindex);
                        goto out;
                }

                if (PageChecked(hpage)) {
                        extract_hash(hpage, hoffset, hsize, _want_hash);
                        want_hash = _want_hash;
                        put_page(hpage);
                        pr_debug_ratelimited("Hash page already checked, want %s:%*phN\n",
                                             params->hash_alg->name,
                                             hsize, want_hash);
                        goto descend;
                }
                pr_debug_ratelimited("Hash page not yet checked\n");
                hpages[level] = hpage;
                hoffsets[level] = hoffset;
        }

        want_hash = vi->root_hash;
        pr_debug("Want root hash: %s:%*phN\n",
                 params->hash_alg->name, hsize, want_hash);
descend:
        /* Descend the tree verifying hash pages */
        for (; level > 0; level--) {
                struct page *hpage = hpages[level - 1];
                unsigned int hoffset = hoffsets[level - 1];

                err = fsverity_hash_page(params, inode, req, hpage, real_hash);
                if (err)
                        goto out;
                err = cmp_hashes(vi, want_hash, real_hash, index, level - 1);
                if (err)
                        goto out;
                SetPageChecked(hpage);
                extract_hash(hpage, hoffset, hsize, _want_hash);
                want_hash = _want_hash;
                put_page(hpage);
                pr_debug("Verified hash page at level %d, now want %s:%*phN\n",
                         level - 1, params->hash_alg->name, hsize, want_hash);
        }

        /* Finally, verify the data page */
        err = fsverity_hash_page(params, inode, req, data_page, real_hash);
        if (err)
                goto out;
        err = cmp_hashes(vi, want_hash, real_hash, index, -1);
out:
        for (; level > 0; level--)
                put_page(hpages[level - 1]);

        return err == 0;
}

/**
 * fsverity_verify_page() - verify a data page
 * @page: the page to verity
 *
 * Verify a page that has just been read from a verity file.  The page must be a
 * pagecache page that is still locked and not yet uptodate.
 *
 * Return: true if the page is valid, else false.
 */
bool fsverity_verify_page(struct page *page)
{
        struct inode *inode = page->mapping->host;
        const struct fsverity_info *vi = inode->i_verity_info;
        struct ahash_request *req;
        bool valid;

        /* This allocation never fails, since it's mempool-backed. */
        req = fsverity_alloc_hash_request(vi->tree_params.hash_alg, GFP_NOFS);

        valid = verify_page(inode, vi, req, page, 0);

        fsverity_free_hash_request(vi->tree_params.hash_alg, req);

        return valid;
}
EXPORT_SYMBOL_GPL(fsverity_verify_page);

#ifdef CONFIG_BLOCK
/**
 * fsverity_verify_bio() - verify a 'read' bio that has just completed
 * @bio: the bio to verify
 *
 * Verify a set of pages that have just been read from a verity file.  The pages
 * must be pagecache pages that are still locked and not yet uptodate.  Pages
 * that fail verification are set to the Error state.  Verification is skipped
 * for pages already in the Error state, e.g. due to fscrypt decryption failure.
 *
 * This is a helper function for use by the ->readpages() 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 inode *inode = bio_first_page_all(bio)->mapping->host;
        const struct fsverity_info *vi = inode->i_verity_info;
        const struct merkle_tree_params *params = &vi->tree_params;
        struct ahash_request *req;
        struct bio_vec *bv;
        struct bvec_iter_all iter_all;
        unsigned long max_ra_pages = 0;

        /* This allocation never fails, since it's mempool-backed. */
        req = fsverity_alloc_hash_request(params->hash_alg, GFP_NOFS);

        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.
                 */
                bio_for_each_segment_all(bv, bio, iter_all)
                        max_ra_pages++;
                max_ra_pages /= 4;
        }

        bio_for_each_segment_all(bv, bio, iter_all) {
                struct page *page = bv->bv_page;
                unsigned long level0_index = page->index >> params->log_arity;
                unsigned long level0_ra_pages =
                        min(max_ra_pages, params->level0_blocks - level0_index);

                if (!PageError(page) &&
                    !verify_page(inode, vi, req, page, level0_ra_pages))
                        SetPageError(page);
        }

        fsverity_free_hash_request(params->hash_alg, req);
}
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);

int __init fsverity_init_workqueue(void)
{
        /*
         * Use an unbound workqueue to allow bios to be verified in parallel
         * even when they happen to complete on the same CPU.  This sacrifices
         * locality, but it's worthwhile since hashing is CPU-intensive.
         *
         * Also use a high-priority workqueue to prioritize verification work,
         * which blocks reads from completing, over regular application tasks.
         */
        fsverity_read_workqueue = alloc_workqueue("fsverity_read_queue",
                                                  WQ_UNBOUND | WQ_HIGHPRI,
                                                  num_online_cpus());
        if (!fsverity_read_workqueue)
                return -ENOMEM;
        return 0;
}

void __init fsverity_exit_workqueue(void)
{
        destroy_workqueue(fsverity_read_workqueue);
        fsverity_read_workqueue = NULL;
}