Re: [PATCH v5 07/24] fsverity: support block-based Merkle tree caching

Eric Biggers
2024-03-06T03:56:24Z

On Mon, Mar 04, 2024 at 08:10:30PM +0100, Andrey Albershteyn wrote:
> diff --git a/fs/verity/fsverity_private.h b/fs/verity/fsverity_private.h
> index b3506f56e180..dad33e6ff0d6 100644
> --- a/fs/verity/fsverity_private.h
> +++ b/fs/verity/fsverity_private.h
> @@ -154,4 +154,12 @@ static inline void fsverity_init_signature(void)
>
> void __init fsverity_init_workqueue(void);
>
> +/*
> + * Drop 'block' obtained with ->read_merkle_tree_block(). Calls out back to
> + * filesystem if ->drop_block() is set, otherwise, drop the reference in the
> + * block->context.
> + */
> +void fsverity_drop_block(struct inode *inode,
> + struct fsverity_blockbuf *block);
> +
> #endif /* _FSVERITY_PRIVATE_H */

This should be paired with a helper function that reads a Merkle tree block by
calling ->read_merkle_tree_block or ->read_merkle_tree_page as needed. Besides
being consistent with having a helper function for drop, this would prevent code
duplication between verify_data_block() and fsverity_read_merkle_tree().

I recommend that it look like this:

int fsverity_read_merkle_tree_block(struct inode *inode, u64 pos,
unsigned long ra_bytes,
struct fsverity_blockbuf *block);

'pos' would be the byte position of the block in the Merkle tree, and 'ra_bytes'
would be the number of bytes for the filesystem to (optionally) readahead if the
block is not yet cached. I think that things work out simpler if these values
are measured in bytes, not blocks. 'block' would be at the end because it's an
output, and it can be confusing to interleave inputs and outputs in parameters.

Also, the drop function should be named consistently:

void fsverity_drop_merkle_tree_block(struct inode *inode,
struct fsverity_blockbuf *block);

It would be a bit long, but I think it's helpful for consistency and also
because there are other types of "blocks" that it could be confused with.
(Data blocks, filesystem blocks, internal blocks of the hash function.)

> diff --git a/fs/verity/open.c b/fs/verity/open.c
> index fdeb95eca3af..6e6922b4b014 100644
> --- a/fs/verity/open.c
> +++ b/fs/verity/open.c
> @@ -213,7 +213,13 @@ struct fsverity_info *fsverity_create_info(const struct inode *inode,
> if (err)
> goto fail;
>
> - if (vi->tree_params.block_size != PAGE_SIZE) {
> + /*
> + * If fs passes Merkle tree blocks to fs-verity (e.g. XFS), then
> + * fs-verity should use hash_block_verified bitmap as there's no page
> + * to mark it with PG_checked.
> + */
> + if (vi->tree_params.block_size != PAGE_SIZE ||
> + inode->i_sb->s_vop->read_merkle_tree_block) {

Technically, all filesystems "pass Merkle tree blocks to fs-verity". Maybe
stick to calling it "block-based Merkle tree caching", as you have in the commit
title, as that seems clearer.

> diff --git a/fs/verity/read_metadata.c b/fs/verity/read_metadata.c
> index f58432772d9e..5da40b5a81af 100644
> --- a/fs/verity/read_metadata.c
> +++ b/fs/verity/read_metadata.c
> @@ -18,50 +18,68 @@ static int fsverity_read_merkle_tree(struct inode *inode,
> {
> const struct fsverity_operations *vops = inode->i_sb->s_vop;
> u64 end_offset;
> - unsigned int offs_in_page;
> + unsigned int offs_in_block;
> pgoff_t index, last_index;
> int retval = 0;
> int err = 0;
> + const unsigned int block_size = vi->tree_params.block_size;
> + const u8 log_blocksize = vi->tree_params.log_blocksize;
>
> end_offset = min(offset + length, vi->tree_params.tree_size);
> if (offset >= end_offset)
> return 0;
> - offs_in_page = offset_in_page(offset);
> - last_index = (end_offset - 1) >> PAGE_SHIFT;
> + offs_in_block = offset & (block_size - 1);
> + last_index = (end_offset - 1) >> log_blocksize;
>
> /*
> - * Iterate through each Merkle tree page in the requested range and copy
> - * the requested portion to userspace. Note that the Merkle tree block
> - * size isn't important here, as we are returning a byte stream; i.e.,
> - * we can just work with pages even if the tree block size != PAGE_SIZE.
> + * Iterate through each Merkle tree block in the requested range and
> + * copy the requested portion to userspace. Note that we are returning
> + * a byte stream.
> */
> - for (index = offset >> PAGE_SHIFT; index <= last_index; index++) {
> + for (index = offset >> log_blocksize; index <= last_index; index++) {
> unsigned long num_ra_pages =
> min_t(unsigned long, last_index - index + 1,
> inode->i_sb->s_bdi->io_pages);
> unsigned int bytes_to_copy = min_t(u64, end_offset - offset,
> - PAGE_SIZE - offs_in_page);
> - struct page *page;
> - const void *virt;
> + block_size - offs_in_block);
> + struct fsverity_blockbuf block = {
> + .size = block_size,
> + };
>
> - page = vops->read_merkle_tree_page(inode, index, num_ra_pages);
> - if (IS_ERR(page)) {
> - err = PTR_ERR(page);
> + if (!vops->read_merkle_tree_block) {
> + unsigned int blocks_per_page =
> + vi->tree_params.blocks_per_page;
> + unsigned long page_idx =
> + round_down(index, blocks_per_page);
> + struct page *page = vops->read_merkle_tree_page(inode,
> + page_idx, num_ra_pages);
> +
> + if (IS_ERR(page)) {
> + err = PTR_ERR(page);
> + } else {
> + block.kaddr = kmap_local_page(page) +
> + ((index - page_idx) << log_blocksize);
> + block.context = page;
> + }
> + } else {
> + err = vops->read_merkle_tree_block(inode,
> + index << log_blocksize,
> + &block, log_blocksize, num_ra_pages);
> + }
> +
> + if (err) {
> fsverity_err(inode,
> - "Error %d reading Merkle tree page %lu",
> - err, index);
> + "Error %d reading Merkle tree block %lu",
> + err, index << log_blocksize);
> break;
> }
>
> - virt = kmap_local_page(page);
> - if (copy_to_user(buf, virt + offs_in_page, bytes_to_copy)) {
> - kunmap_local(virt);
> - put_page(page);
> + if (copy_to_user(buf, block.kaddr + offs_in_block, bytes_to_copy)) {
> + fsverity_drop_block(inode, &block);
> err = -EFAULT;
> break;
> }
> - kunmap_local(virt);
> - put_page(page);
> + fsverity_drop_block(inode, &block);
>
> retval += bytes_to_copy;
> buf += bytes_to_copy;
> @@ -72,7 +90,7 @@ static int fsverity_read_merkle_tree(struct inode *inode,
> break;
> }
> cond_resched();
> - offs_in_page = 0;
> + offs_in_block = 0;
> }
> return retval ? retval : err;
> }

This is one of the two places that would be simplified quite a bit by adding an
fsverity_read_merkle_tree_block() helper function, especially if the loop
variable was changed to measure bytes instead of blocks so that the Merkle tree
block position and readahead amount can more easily be computed in bytes. E.g.:

static int fsverity_read_merkle_tree(struct inode *inode,
const struct fsverity_info *vi,
void __user *buf, u64 pos, int length)
{
const u64 end_pos = min(pos + length, vi->tree_params.tree_size);
[...]

while (pos < end_pos) {
// bytes_to_copy = remaining length in current block
[...]
err = fsverity_read_merkle_tree_block(inode,
pos - offs_in_block,
ra_bytes,
&block);
[...]
pos += bytes_to_copy;
[...]
}

> diff --git a/fs/verity/verify.c b/fs/verity/verify.c
> index 4fcad0825a12..de71911d400c 100644
> --- a/fs/verity/verify.c
> +++ b/fs/verity/verify.c
> @@ -13,14 +13,17 @@
> 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.
> + * Returns true if the hash block with index @hblock_idx in the tree has
> + * already been verified.
> */
> -static bool is_hash_block_verified(struct fsverity_info *vi, struct page *hpage,
> +static bool is_hash_block_verified(struct inode *inode,
> + struct fsverity_blockbuf *block,
> unsigned long hblock_idx)
> {
> unsigned int blocks_per_page;
> unsigned int i;
> + struct fsverity_info *vi = inode->i_verity_info;
> + struct page *hpage = (struct page *)block->context;
>
> /*
> * When the Merkle tree block size and page size are the same, then the
> @@ -34,6 +37,12 @@ static bool is_hash_block_verified(struct fsverity_info *vi, struct page *hpage,
> if (!vi->hash_block_verified)
> return PageChecked(hpage);
>
> + /*
> + * Filesystems which use block based caching (e.g. XFS) always use
> + * bitmap.
> + */
> + if (inode->i_sb->s_vop->read_merkle_tree_block)
> + return test_bit(hblock_idx, vi->hash_block_verified);
> /*
> * When the Merkle tree block size and page size differ, we use a bitmap
> * to indicate whether each hash block has been verified.

This is hard to follow because the code goes from handling page-based caching to
handling block-based caching to handling page-based caching again. Also, the
comment says that block-based caching uses the bitmap, but it doesn't give any
hint at why the bitmap is being handled differently in that case from the
page-based caching case that also uses the bitmap.

How about doing something like the following where the block-based caching case
is handled first, before either block-based caching case:

/*
* If the filesystem uses block-based caching, then
* ->hash_block_verified is always used and the filesystem pushes
* invalidations to it as needed.
*/
if (inode->i_sb->s_vop->read_merkle_tree_block)
return test_bit(hblock_idx, vi->hash_block_verified);

/* Otherwise, the filesystem uses page-based caching. */
hpage = (struct page *)block->context;

Note, to avoid confusion, the 'hpage' variable shouldn't be set until it's been
verified that the filesystem is using page-based caching.

> @@ -95,15 +104,15 @@ verify_data_block(struct inode *inode, struct fsverity_info *vi,
> verify_data_block(struct inode *inode, struct fsverity_info *vi,
> const void *data, u64 data_pos, unsigned long max_ra_pages)

max_ra_pages should become max_ra_bytes.

> + int num_ra_pages;

unsigned long ra_bytes. Also, this should be declared in the scope later on
where it's actually needed.

> @@ -144,10 +153,11 @@ verify_data_block(struct inode *inode, struct fsverity_info *vi,
> unsigned long next_hidx;
> unsigned long hblock_idx;
> pgoff_t hpage_idx;
> + u64 hblock_pos;
> unsigned int hblock_offset_in_page;

hpage_idx and hblock_offset_in_page should both go away.
fsverity_read_merkle_tree_block() should do the conversion to/from pages.

> unsigned int hoffset;
> struct page *hpage;
> - const void *haddr;
> + struct fsverity_blockbuf *block = &hblocks[level].block;
>
> /*
> * The index of the block in the current level; also the index
> @@ -165,29 +175,49 @@ verify_data_block(struct inode *inode, struct fsverity_info *vi,
> /* Index of the hash block in the tree overall */
> hblock_idx = params->level_start[level] + next_hidx;
>
> /* Byte offset of the hash block within the page */
> hblock_offset_in_page =
> (hblock_idx << params->log_blocksize) & ~PAGE_MASK;
>
> + /* Offset of the Merkle tree block into the tree */
> + hblock_pos = hblock_idx << params->log_blocksize;

The comment for hblock_pos should say:

/* Byte offset of the hash block in the tree overall */

... so that it's consistent with the one for hblock_idx which says:

/* Index of the hash block in the tree overall */

> + num_ra_pages = level == 0 ?
> + min(max_ra_pages, params->tree_pages - hpage_idx) : 0;
> +
> + if (inode->i_sb->s_vop->read_merkle_tree_block) {
> + err = inode->i_sb->s_vop->read_merkle_tree_block(
> + inode, hblock_pos, block, params->log_blocksize,
> + num_ra_pages);
> + } else {
> + unsigned int blocks_per_page =
> + vi->tree_params.blocks_per_page;
> + hblock_idx = round_down(hblock_idx, blocks_per_page);
> + hpage = inode->i_sb->s_vop->read_merkle_tree_page(
> + inode, hpage_idx, (num_ra_pages << PAGE_SHIFT));
> +
> + if (IS_ERR(hpage)) {
> + err = PTR_ERR(hpage);
> + } else {
> + block->kaddr = kmap_local_page(hpage) +
> + hblock_offset_in_page;
> + block->context = hpage;
> + }
> + }

This has the weirdness with the readahead amount again, where it's still being
measured in pages. I think it would make more sense to measure it in bytes. It
would stay bytes when passed to ->read_merkle_tree_block, or be converted to
pages at the last minute for ->read_merkle_tree_page which works in pages.

> + if (err) {
> fsverity_err(inode,
> - "Error %ld reading Merkle tree page %lu",
> - PTR_ERR(hpage), hpage_idx);
> + "Error %d reading Merkle tree block %lu",
> + err, hblock_idx);
> goto error;
> }

This error message should go in fsverity_read_merkle_tree_block().

Doing that would also eliminate the need to add an 'err' variable to
verify_data_block(), which could be confusing because it returns a bool.

> /* Descend the tree verifying hash blocks. */
> for (; level > 0; level--) {
> - struct page *hpage = hblocks[level - 1].page;
> - const void *haddr = hblocks[level - 1].addr;
> + struct fsverity_blockbuf *block = &hblocks[level - 1].block;
> + const void *haddr = block->kaddr;
> unsigned long hblock_idx = hblocks[level - 1].index;
> unsigned int hoffset = hblocks[level - 1].hoffset;
> + struct page *hpage = (struct page *)block->context;
>
> if (fsverity_hash_block(params, inode, haddr, real_hash) != 0)
> goto error;
> @@ -217,8 +248,7 @@ verify_data_block(struct inode *inode, struct fsverity_info *vi,
> if (vi->hash_block_verified)
> set_bit(hblock_idx, vi->hash_block_verified);
> else
> SetPageChecked(hpage);

Since the page is only valid when vi->hash_block_verified is NULL, remove the
hpage variable and do 'SetPageChecked((struct page *)block->context);'.
Yes, there's no issue until the pointer is actually dereferenced, but it's
confusing to have incorrectly-typed pointer variables sitting around.

> +/**
> + * fsverity_invalidate_block() - invalidate Merkle tree block
> + * @inode: inode to which this Merkle tree blocks belong
> + * @block: block to be invalidated
> + *
> + * This function invalidates/clears "verified" state of Merkle tree block
> + * in the fs-verity bitmap. The block needs to have ->offset set.
> + */
> +void fsverity_invalidate_block(struct inode *inode,
> + struct fsverity_blockbuf *block)
> +{
> + struct fsverity_info *vi = inode->i_verity_info;
> + const unsigned int log_blocksize = vi->tree_params.log_blocksize;
> +
> + if (block->offset > vi->tree_params.tree_size) {
> + fsverity_err(inode,
> +"Trying to invalidate beyond Merkle tree (tree %lld, offset %lld)",
> + vi->tree_params.tree_size, block->offset);
> + return;
> + }
> +
> + clear_bit(block->offset >> log_blocksize, vi->hash_block_verified);
> +}
> +EXPORT_SYMBOL_GPL(fsverity_invalidate_block);

I don't think this function should be using struct fsverity_blockbuf. It only
uses the 'offset' field, which is orthogonal to what ->read_merkle_tree_block
and ->drop_merkle_tree_block use.

The function name fsverity_invalidate_block() also has the issue where it's not
clear what type of block it's talking about.

How about changing the prototype to:

void fsverity_invalidate_merkle_tree_block(struct inode *inode, u64 pos);

Also, is it a kernel bug for the pos to be beyond the end of the Merkle tree, or
can it happen in cases like filesystem corruption? If it can only happen due to
kernel bugs, WARN_ON_ONCE() might be more appropriate than an error message.

Please note that there's also an off-by-one error. pos > tree_size should be
pos >= tree_size.

> +void fsverity_drop_block(struct inode *inode,
> + struct fsverity_blockbuf *block)

fsverity_drop_merkle_tree_block

> +{
> + if (inode->i_sb->s_vop->drop_block)
> + inode->i_sb->s_vop->drop_block(block);
> + else {

Add braces above.

(See line 213 of Documentation/process/coding-style.rst)

> + struct page *page = (struct page *)block->context;
> +
> + kunmap_local(block->kaddr);
> + put_page(page);

put_page((struct page *)block->context), and remove the page variable

> + }
> + block->kaddr = NULL;

Also set block->context = NULL

> +/**
> + * struct fsverity_blockbuf - Merkle Tree block buffer
> + * @kaddr: virtual address of the block's data
> + * @offset: block's offset into Merkle tree
> + * @size: the Merkle tree block size
> + * @context: filesystem private context
> + *
> + * Buffer containing single Merkle Tree block. These buffers are passed
> + * - to filesystem, when fs-verity is building merkel tree,
> + * - from filesystem, when fs-verity is reading merkle tree from a disk.
> + * Filesystems sets kaddr together with size to point to a memory which contains
> + * Merkle tree block. Same is done by fs-verity when Merkle tree is need to be
> + * written down to disk.
> + *
> + * While reading the tree, fs-verity calls ->read_merkle_tree_block followed by
> + * ->drop_block to let filesystem know that memory can be freed.
> + *
> + * The context is optional. This field can be used by filesystem to passthrough
> + * state from ->read_merkle_tree_block to ->drop_block.
> + */
> +struct fsverity_blockbuf {
> + void *kaddr;
> + u64 offset;
> + unsigned int size;
> + void *context;
> +};

If this is only used by ->read_merkle_tree_block and ->drop_merkle_tree_block,
then it can be simplified. The only fields needed are kaddr and context.

The comment also still incorrectly claims that the struct is used for writes.

> + /**
> + * Read a Merkle tree block of the given inode.
> + * @inode: the inode
> + * @pos: byte offset of the block within the Merkle tree
> + * @block: block buffer for filesystem to point it to the block
> + * @log_blocksize: log2 of the size of the expected block
> + * @ra_bytes: The number of bytes that should be
> + * prefetched starting at @pos if the page at @pos
> + * isn't already cached. Implementations may ignore this
> + * argument; it's only a performance optimization.
> + *
> + * This can be called at any time on an open verity file. It may be
> + * called by multiple processes concurrently.
> + *
> + * In case that block was evicted from the memory filesystem has to use
> + * fsverity_invalidate_block() to let fsverity know that block's
> + * verification state is not valid anymore.
> + *
> + * Return: 0 on success, -errno on failure
> + */
> + int (*read_merkle_tree_block)(struct inode *inode,
> + u64 pos,
> + struct fsverity_blockbuf *block,
> + unsigned int log_blocksize,
> + u64 ra_bytes);

How about:

int (*read_merkle_tree_block)(struct inode *inode,
u64 pos,
unsigned int size,
unsigned long ra_bytes,
struct fsverity_blockbuf *block);

- size instead of log_blocksize, for consistency with other functions and
because it's what XFS actually wants

- ra_bytes as unsigned long, since u64 readahead amounts don't really make
sense. (This isn't too important though; it could be u64 if you really want.)

- block at the end since it's an output parameter

It also needs to be clearly documented that filesystems must implement either
->read_merkle_tree_page, *or* both ->read_merkle_tree_block and
->drop_merkle_tree_block.

Also, the comment about invalidating the block is still unclear regarding the
timing of when the filesystem must do it.

> +
> + /**
> + * Release the reference to a Merkle tree block
> + *
> + * @block: the block to release
> + *
> + * This is called when fs-verity is done with a block obtained with
> + * ->read_merkle_tree_block().
> + */
> + void (*drop_block)(struct fsverity_blockbuf *block);

drop_merkle_tree_block, so that it's clearly paired with read_merkle_tree_block

- Eric