[v5,09/16] fs-verity: add data verification hooks for ->readpages()

Commit Message

Eric Biggers June 20, 2019, 8:50 p.m. UTC

From: Eric Biggers <[e-mail]>

Add functions that verify data pages that have been read from a
fs-verity file, against that file's Merkle tree.  These will be called
from filesystems' ->readpage() and ->readpages() methods.

Since data verification can block, a workqueue is provided for these
methods to enqueue verification work from their bio completion callback.

See the "Verifying data" section of
Documentation/filesystems/fsverity.rst for more information.

Reviewed-by: Theodore Ts'o <[e-mail]>
Signed-off-by: Eric Biggers <[e-mail]>
---
 fs/verity/Makefile           |   3 +-
 fs/verity/fsverity_private.h |   5 +
 fs/verity/init.c             |   8 +
 fs/verity/open.c             |   6 +
 fs/verity/verify.c           | 275 +++++++++++++++++++++++++++++++++++
 include/linux/fsverity.h     |  56 +++++++
 6 files changed, 352 insertions(+), 1 deletion(-)
 create mode 100644 fs/verity/verify.c

Comments

Jaegeuk Kim June 22, 2019, 10:32 p.m. UTC | #1

On 06/20, Eric Biggers wrote:
> From: Eric Biggers <[e-mail]>
> 
> Add functions that verify data pages that have been read from a
> fs-verity file, against that file's Merkle tree.  These will be called
> from filesystems' ->readpage() and ->readpages() methods.
> 
> Since data verification can block, a workqueue is provided for these
> methods to enqueue verification work from their bio completion callback.
> 
> See the "Verifying data" section of
> Documentation/filesystems/fsverity.rst for more information.
> 
> Reviewed-by: Theodore Ts'o <[e-mail]>

Reviewed-by: Jaegeuk Kim <[e-mail]>

> Signed-off-by: Eric Biggers <[e-mail]>
> ---
>  fs/verity/Makefile           |   3 +-
>  fs/verity/fsverity_private.h |   5 +
>  fs/verity/init.c             |   8 +
>  fs/verity/open.c             |   6 +
>  fs/verity/verify.c           | 275 +++++++++++++++++++++++++++++++++++
>  include/linux/fsverity.h     |  56 +++++++
>  6 files changed, 352 insertions(+), 1 deletion(-)
>  create mode 100644 fs/verity/verify.c
> 
> diff --git a/fs/verity/Makefile b/fs/verity/Makefile
> index e6a8951c493a5e..7fa628cd5eba24 100644
> --- a/fs/verity/Makefile
> +++ b/fs/verity/Makefile
> @@ -2,4 +2,5 @@
>  
>  obj-$(CONFIG_FS_VERITY) += hash_algs.o \
>  			   init.o \
> -			   open.o
> +			   open.o \
> +			   verify.o
> diff --git a/fs/verity/fsverity_private.h b/fs/verity/fsverity_private.h
> index c79746ff335e14..eaa2b3b93bbf6b 100644
> --- a/fs/verity/fsverity_private.h
> +++ b/fs/verity/fsverity_private.h
> @@ -134,5 +134,10 @@ void fsverity_set_info(struct inode *inode, struct fsverity_info *vi);
>  void fsverity_free_info(struct fsverity_info *vi);
>  
>  int __init fsverity_init_info_cache(void);
> +void __init fsverity_exit_info_cache(void);
> +
> +/* verify.c */
> +
> +int __init fsverity_init_workqueue(void);
>  
>  #endif /* _FSVERITY_PRIVATE_H */
> diff --git a/fs/verity/init.c b/fs/verity/init.c
> index fff1fd6343357d..b593805aafcc89 100644
> --- a/fs/verity/init.c
> +++ b/fs/verity/init.c
> @@ -41,7 +41,15 @@ static int __init fsverity_init(void)
>  	if (err)
>  		return err;
>  
> +	err = fsverity_init_workqueue();
> +	if (err)
> +		goto err_exit_info_cache;
> +
>  	pr_debug("Initialized fs-verity\n");
>  	return 0;
> +
> +err_exit_info_cache:
> +	fsverity_exit_info_cache();
> +	return err;
>  }
>  late_initcall(fsverity_init)
> diff --git a/fs/verity/open.c b/fs/verity/open.c
> index 21ae0ef254a695..7a2cd000dc4f06 100644
> --- a/fs/verity/open.c
> +++ b/fs/verity/open.c
> @@ -338,3 +338,9 @@ int __init fsverity_init_info_cache(void)
>  		return -ENOMEM;
>  	return 0;
>  }
> +
> +void __init fsverity_exit_info_cache(void)
> +{
> +	kmem_cache_destroy(fsverity_info_cachep);
> +	fsverity_info_cachep = NULL;
> +}
> diff --git a/fs/verity/verify.c b/fs/verity/verify.c
> new file mode 100644
> index 00000000000000..2a0f9e2ebc9f16
> --- /dev/null
> +++ b/fs/verity/verify.c
> @@ -0,0 +1,275 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * fs/verity/verify.c: 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)
> +{
> +	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);
> +		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
> + *
> + * 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;
> +
> +	req = ahash_request_alloc(vi->tree_params.hash_alg->tfm, GFP_NOFS);
> +	if (unlikely(!req))
> +		return false;
> +
> +	valid = verify_page(inode, vi, req, page);
> +
> +	ahash_request_free(req);
> +
> +	return valid;
> +}
> +EXPORT_SYMBOL_GPL(fsverity_verify_page);
> +
> +#ifdef CONFIG_BLOCK
> +/**
> + * fsverity_verify_bio - verify a 'read' bio that has just completed
> + *
> + * 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;
> +	struct ahash_request *req;
> +	struct bio_vec *bv;
> +	struct bvec_iter_all iter_all;
> +
> +	req = ahash_request_alloc(vi->tree_params.hash_alg->tfm, GFP_NOFS);
> +	if (unlikely(!req)) {
> +		bio_for_each_segment_all(bv, bio, iter_all)
> +			SetPageError(bv->bv_page);
> +		return;
> +	}
> +
> +	bio_for_each_segment_all(bv, bio, iter_all) {
> +		struct page *page = bv->bv_page;
> +
> +		if (!PageError(page) && !verify_page(inode, vi, req, page))
> +			SetPageError(page);
> +	}
> +
> +	ahash_request_free(req);
> +}
> +EXPORT_SYMBOL_GPL(fsverity_verify_bio);
> +#endif /* CONFIG_BLOCK */
> +
> +/**
> + * fsverity_enqueue_verify_work - enqueue work on the fs-verity workqueue
> + *
> + * 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;
> +}
> diff --git a/include/linux/fsverity.h b/include/linux/fsverity.h
> index cbcc358d073652..ecd47e748c7f64 100644
> --- a/include/linux/fsverity.h
> +++ b/include/linux/fsverity.h
> @@ -33,6 +33,23 @@ struct fsverity_operations {
>  	 */
>  	int (*get_verity_descriptor)(struct inode *inode, void *buf,
>  				     size_t bufsize);
> +
> +	/**
> +	 * Read a Merkle tree page of the given inode.
> +	 *
> +	 * @inode: the inode
> +	 * @index: 0-based index of the page within the Merkle tree
> +	 *
> +	 * This can be called at any time on an open verity file, as well as
> +	 * between ->begin_enable_verity() and ->end_enable_verity().  It may be
> +	 * called by multiple processes concurrently, even with the same page.
> +	 *
> +	 * Note that this must retrieve a *page*, not necessarily a *block*.
> +	 *
> +	 * Return: the page on success, ERR_PTR() on failure
> +	 */
> +	struct page *(*read_merkle_tree_page)(struct inode *inode,
> +					      pgoff_t index);
>  };
>  
>  #ifdef CONFIG_FS_VERITY
> @@ -49,6 +66,12 @@ extern int fsverity_file_open(struct inode *inode, struct file *filp);
>  extern int fsverity_prepare_setattr(struct dentry *dentry, struct iattr *attr);
>  extern void fsverity_cleanup_inode(struct inode *inode);
>  
> +/* verify.c */
> +
> +extern bool fsverity_verify_page(struct page *page);
> +extern void fsverity_verify_bio(struct bio *bio);
> +extern void fsverity_enqueue_verify_work(struct work_struct *work);
> +
>  #else /* !CONFIG_FS_VERITY */
>  
>  static inline struct fsverity_info *fsverity_get_info(const struct inode *inode)
> @@ -73,6 +96,39 @@ static inline void fsverity_cleanup_inode(struct inode *inode)
>  {
>  }
>  
> +/* verify.c */
> +
> +static inline bool fsverity_verify_page(struct page *page)
> +{
> +	WARN_ON(1);
> +	return false;
> +}
> +
> +static inline void fsverity_verify_bio(struct bio *bio)
> +{
> +	WARN_ON(1);
> +}
> +
> +static inline void fsverity_enqueue_verify_work(struct work_struct *work)
> +{
> +	WARN_ON(1);
> +}
> +
>  #endif	/* !CONFIG_FS_VERITY */
>  
> +/**
> + * fsverity_active() - do reads from the inode need to go through fs-verity?
> + *
> + * This checks whether ->i_verity_info has been set.
> + *
> + * Filesystems call this from ->readpages() to check whether the pages need to
> + * be verified or not.  Don't use IS_VERITY() for this purpose; it's subject to
> + * a race condition where the file is being read concurrently with
> + * FS_IOC_ENABLE_VERITY completing.  (S_VERITY is set before ->i_verity_info.)
> + */
> +static inline bool fsverity_active(const struct inode *inode)
> +{
> +	return fsverity_get_info(inode) != NULL;
> +}
> +
>  #endif	/* _LINUX_FSVERITY_H */
> -- 
> 2.22.0.410.gd8fdbe21b5-goog

Patch

diff --git a/fs/verity/Makefile b/fs/verity/Makefile
index e6a8951c493a5e..7fa628cd5eba24 100644
--- a/fs/verity/Makefile
+++ b/fs/verity/Makefile
@@ -2,4 +2,5 @@ 
 
 obj-$(CONFIG_FS_VERITY) += hash_algs.o \
 			   init.o \
-			   open.o
+			   open.o \
+			   verify.o
diff --git a/fs/verity/fsverity_private.h b/fs/verity/fsverity_private.h
index c79746ff335e14..eaa2b3b93bbf6b 100644
--- a/fs/verity/fsverity_private.h
+++ b/fs/verity/fsverity_private.h
@@ -134,5 +134,10 @@  void fsverity_set_info(struct inode *inode, struct fsverity_info *vi);
 void fsverity_free_info(struct fsverity_info *vi);
 
 int __init fsverity_init_info_cache(void);
+void __init fsverity_exit_info_cache(void);
+
+/* verify.c */
+
+int __init fsverity_init_workqueue(void);
 
 #endif /* _FSVERITY_PRIVATE_H */
diff --git a/fs/verity/init.c b/fs/verity/init.c
index fff1fd6343357d..b593805aafcc89 100644
--- a/fs/verity/init.c
+++ b/fs/verity/init.c
@@ -41,7 +41,15 @@  static int __init fsverity_init(void)
 	if (err)
 		return err;
 
+	err = fsverity_init_workqueue();
+	if (err)
+		goto err_exit_info_cache;
+
 	pr_debug("Initialized fs-verity\n");
 	return 0;
+
+err_exit_info_cache:
+	fsverity_exit_info_cache();
+	return err;
 }
 late_initcall(fsverity_init)
diff --git a/fs/verity/open.c b/fs/verity/open.c
index 21ae0ef254a695..7a2cd000dc4f06 100644
--- a/fs/verity/open.c
+++ b/fs/verity/open.c
@@ -338,3 +338,9 @@  int __init fsverity_init_info_cache(void)
 		return -ENOMEM;
 	return 0;
 }
+
+void __init fsverity_exit_info_cache(void)
+{
+	kmem_cache_destroy(fsverity_info_cachep);
+	fsverity_info_cachep = NULL;
+}
diff --git a/fs/verity/verify.c b/fs/verity/verify.c
new file mode 100644
index 00000000000000..2a0f9e2ebc9f16
--- /dev/null
+++ b/fs/verity/verify.c
@@ -0,0 +1,275 @@ 
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * fs/verity/verify.c: 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)
+{
+	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);
+		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
+ *
+ * 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;
+
+	req = ahash_request_alloc(vi->tree_params.hash_alg->tfm, GFP_NOFS);
+	if (unlikely(!req))
+		return false;
+
+	valid = verify_page(inode, vi, req, page);
+
+	ahash_request_free(req);
+
+	return valid;
+}
+EXPORT_SYMBOL_GPL(fsverity_verify_page);
+
+#ifdef CONFIG_BLOCK
+/**
+ * fsverity_verify_bio - verify a 'read' bio that has just completed
+ *
+ * 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;
+	struct ahash_request *req;
+	struct bio_vec *bv;
+	struct bvec_iter_all iter_all;
+
+	req = ahash_request_alloc(vi->tree_params.hash_alg->tfm, GFP_NOFS);
+	if (unlikely(!req)) {
+		bio_for_each_segment_all(bv, bio, iter_all)
+			SetPageError(bv->bv_page);
+		return;
+	}
+
+	bio_for_each_segment_all(bv, bio, iter_all) {
+		struct page *page = bv->bv_page;
+
+		if (!PageError(page) && !verify_page(inode, vi, req, page))
+			SetPageError(page);
+	}
+
+	ahash_request_free(req);
+}
+EXPORT_SYMBOL_GPL(fsverity_verify_bio);
+#endif /* CONFIG_BLOCK */
+
+/**
+ * fsverity_enqueue_verify_work - enqueue work on the fs-verity workqueue
+ *
+ * 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;
+}
diff --git a/include/linux/fsverity.h b/include/linux/fsverity.h
index cbcc358d073652..ecd47e748c7f64 100644
--- a/include/linux/fsverity.h
+++ b/include/linux/fsverity.h
@@ -33,6 +33,23 @@  struct fsverity_operations {
 	 */
 	int (*get_verity_descriptor)(struct inode *inode, void *buf,
 				     size_t bufsize);
+
+	/**
+	 * Read a Merkle tree page of the given inode.
+	 *
+	 * @inode: the inode
+	 * @index: 0-based index of the page within the Merkle tree
+	 *
+	 * This can be called at any time on an open verity file, as well as
+	 * between ->begin_enable_verity() and ->end_enable_verity().  It may be
+	 * called by multiple processes concurrently, even with the same page.
+	 *
+	 * Note that this must retrieve a *page*, not necessarily a *block*.
+	 *
+	 * Return: the page on success, ERR_PTR() on failure
+	 */
+	struct page *(*read_merkle_tree_page)(struct inode *inode,
+					      pgoff_t index);
 };
 
 #ifdef CONFIG_FS_VERITY
@@ -49,6 +66,12 @@  extern int fsverity_file_open(struct inode *inode, struct file *filp);
 extern int fsverity_prepare_setattr(struct dentry *dentry, struct iattr *attr);
 extern void fsverity_cleanup_inode(struct inode *inode);
 
+/* verify.c */
+
+extern bool fsverity_verify_page(struct page *page);
+extern void fsverity_verify_bio(struct bio *bio);
+extern void fsverity_enqueue_verify_work(struct work_struct *work);
+
 #else /* !CONFIG_FS_VERITY */
 
 static inline struct fsverity_info *fsverity_get_info(const struct inode *inode)
@@ -73,6 +96,39 @@  static inline void fsverity_cleanup_inode(struct inode *inode)
 {
 }
 
+/* verify.c */
+
+static inline bool fsverity_verify_page(struct page *page)
+{
+	WARN_ON(1);
+	return false;
+}
+
+static inline void fsverity_verify_bio(struct bio *bio)
+{
+	WARN_ON(1);
+}
+
+static inline void fsverity_enqueue_verify_work(struct work_struct *work)
+{
+	WARN_ON(1);
+}
+
 #endif	/* !CONFIG_FS_VERITY */
 
+/**
+ * fsverity_active() - do reads from the inode need to go through fs-verity?
+ *
+ * This checks whether ->i_verity_info has been set.
+ *
+ * Filesystems call this from ->readpages() to check whether the pages need to
+ * be verified or not.  Don't use IS_VERITY() for this purpose; it's subject to
+ * a race condition where the file is being read concurrently with
+ * FS_IOC_ENABLE_VERITY completing.  (S_VERITY is set before ->i_verity_info.)
+ */
+static inline bool fsverity_active(const struct inode *inode)
+{
+	return fsverity_get_info(inode) != NULL;
+}
+
 #endif	/* _LINUX_FSVERITY_H */