linux/fs/ext4/verity.c
Eric Biggers c93d8f8858 ext4: add basic fs-verity support
Add most of fs-verity support to ext4.  fs-verity is a filesystem
feature that enables transparent integrity protection and authentication
of read-only files.  It uses a dm-verity like mechanism at the file
level: a Merkle tree is used to verify any block in the file in
log(filesize) time.  It is implemented mainly by helper functions in
fs/verity/.  See Documentation/filesystems/fsverity.rst for the full
documentation.

This commit adds all of ext4 fs-verity support except for the actual
data verification, including:

- Adding a filesystem feature flag and an inode flag for fs-verity.

- Implementing the fsverity_operations to support enabling verity on an
  inode and reading/writing the verity metadata.

- Updating ->write_begin(), ->write_end(), and ->writepages() to support
  writing verity metadata pages.

- Calling the fs-verity hooks for ->open(), ->setattr(), and ->ioctl().

ext4 stores the verity metadata (Merkle tree and fsverity_descriptor)
past the end of the file, starting at the first 64K boundary beyond
i_size.  This approach works because (a) verity files are readonly, and
(b) pages fully beyond i_size aren't visible to userspace but can be
read/written internally by ext4 with only some relatively small changes
to ext4.  This approach avoids having to depend on the EA_INODE feature
and on rearchitecturing ext4's xattr support to support paging
multi-gigabyte xattrs into memory, and to support encrypting xattrs.
Note that the verity metadata *must* be encrypted when the file is,
since it contains hashes of the plaintext data.

This patch incorporates work by Theodore Ts'o and Chandan Rajendra.

Reviewed-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Eric Biggers <ebiggers@google.com>
2019-08-12 19:33:50 -07:00

368 lines
9.8 KiB
C

// SPDX-License-Identifier: GPL-2.0
/*
* fs/ext4/verity.c: fs-verity support for ext4
*
* Copyright 2019 Google LLC
*/
/*
* Implementation of fsverity_operations for ext4.
*
* ext4 stores the verity metadata (Merkle tree and fsverity_descriptor) past
* the end of the file, starting at the first 64K boundary beyond i_size. This
* approach works because (a) verity files are readonly, and (b) pages fully
* beyond i_size aren't visible to userspace but can be read/written internally
* by ext4 with only some relatively small changes to ext4. This approach
* avoids having to depend on the EA_INODE feature and on rearchitecturing
* ext4's xattr support to support paging multi-gigabyte xattrs into memory, and
* to support encrypting xattrs. Note that the verity metadata *must* be
* encrypted when the file is, since it contains hashes of the plaintext data.
*
* Using a 64K boundary rather than a 4K one keeps things ready for
* architectures with 64K pages, and it doesn't necessarily waste space on-disk
* since there can be a hole between i_size and the start of the Merkle tree.
*/
#include <linux/quotaops.h>
#include "ext4.h"
#include "ext4_extents.h"
#include "ext4_jbd2.h"
static inline loff_t ext4_verity_metadata_pos(const struct inode *inode)
{
return round_up(inode->i_size, 65536);
}
/*
* Read some verity metadata from the inode. __vfs_read() can't be used because
* we need to read beyond i_size.
*/
static int pagecache_read(struct inode *inode, void *buf, size_t count,
loff_t pos)
{
while (count) {
size_t n = min_t(size_t, count,
PAGE_SIZE - offset_in_page(pos));
struct page *page;
void *addr;
page = read_mapping_page(inode->i_mapping, pos >> PAGE_SHIFT,
NULL);
if (IS_ERR(page))
return PTR_ERR(page);
addr = kmap_atomic(page);
memcpy(buf, addr + offset_in_page(pos), n);
kunmap_atomic(addr);
put_page(page);
buf += n;
pos += n;
count -= n;
}
return 0;
}
/*
* Write some verity metadata to the inode for FS_IOC_ENABLE_VERITY.
* kernel_write() can't be used because the file descriptor is readonly.
*/
static int pagecache_write(struct inode *inode, const void *buf, size_t count,
loff_t pos)
{
if (pos + count > inode->i_sb->s_maxbytes)
return -EFBIG;
while (count) {
size_t n = min_t(size_t, count,
PAGE_SIZE - offset_in_page(pos));
struct page *page;
void *fsdata;
void *addr;
int res;
res = pagecache_write_begin(NULL, inode->i_mapping, pos, n, 0,
&page, &fsdata);
if (res)
return res;
addr = kmap_atomic(page);
memcpy(addr + offset_in_page(pos), buf, n);
kunmap_atomic(addr);
res = pagecache_write_end(NULL, inode->i_mapping, pos, n, n,
page, fsdata);
if (res < 0)
return res;
if (res != n)
return -EIO;
buf += n;
pos += n;
count -= n;
}
return 0;
}
static int ext4_begin_enable_verity(struct file *filp)
{
struct inode *inode = file_inode(filp);
const int credits = 2; /* superblock and inode for ext4_orphan_add() */
handle_t *handle;
int err;
if (ext4_verity_in_progress(inode))
return -EBUSY;
/*
* Since the file was opened readonly, we have to initialize the jbd
* inode and quotas here and not rely on ->open() doing it. This must
* be done before evicting the inline data.
*/
err = ext4_inode_attach_jinode(inode);
if (err)
return err;
err = dquot_initialize(inode);
if (err)
return err;
err = ext4_convert_inline_data(inode);
if (err)
return err;
if (!ext4_test_inode_flag(inode, EXT4_INODE_EXTENTS)) {
ext4_warning_inode(inode,
"verity is only allowed on extent-based files");
return -EOPNOTSUPP;
}
/*
* ext4 uses the last allocated block to find the verity descriptor, so
* we must remove any other blocks past EOF which might confuse things.
*/
err = ext4_truncate(inode);
if (err)
return err;
handle = ext4_journal_start(inode, EXT4_HT_INODE, credits);
if (IS_ERR(handle))
return PTR_ERR(handle);
err = ext4_orphan_add(handle, inode);
if (err == 0)
ext4_set_inode_state(inode, EXT4_STATE_VERITY_IN_PROGRESS);
ext4_journal_stop(handle);
return err;
}
/*
* ext4 stores the verity descriptor beginning on the next filesystem block
* boundary after the Merkle tree. Then, the descriptor size is stored in the
* last 4 bytes of the last allocated filesystem block --- which is either the
* block in which the descriptor ends, or the next block after that if there
* weren't at least 4 bytes remaining.
*
* We can't simply store the descriptor in an xattr because it *must* be
* encrypted when ext4 encryption is used, but ext4 encryption doesn't encrypt
* xattrs. Also, if the descriptor includes a large signature blob it may be
* too large to store in an xattr without the EA_INODE feature.
*/
static int ext4_write_verity_descriptor(struct inode *inode, const void *desc,
size_t desc_size, u64 merkle_tree_size)
{
const u64 desc_pos = round_up(ext4_verity_metadata_pos(inode) +
merkle_tree_size, i_blocksize(inode));
const u64 desc_end = desc_pos + desc_size;
const __le32 desc_size_disk = cpu_to_le32(desc_size);
const u64 desc_size_pos = round_up(desc_end + sizeof(desc_size_disk),
i_blocksize(inode)) -
sizeof(desc_size_disk);
int err;
err = pagecache_write(inode, desc, desc_size, desc_pos);
if (err)
return err;
return pagecache_write(inode, &desc_size_disk, sizeof(desc_size_disk),
desc_size_pos);
}
static int ext4_end_enable_verity(struct file *filp, const void *desc,
size_t desc_size, u64 merkle_tree_size)
{
struct inode *inode = file_inode(filp);
const int credits = 2; /* superblock and inode for ext4_orphan_del() */
handle_t *handle;
int err = 0;
int err2;
if (desc != NULL) {
/* Succeeded; write the verity descriptor. */
err = ext4_write_verity_descriptor(inode, desc, desc_size,
merkle_tree_size);
/* Write all pages before clearing VERITY_IN_PROGRESS. */
if (!err)
err = filemap_write_and_wait(inode->i_mapping);
}
/* If we failed, truncate anything we wrote past i_size. */
if (desc == NULL || err)
ext4_truncate(inode);
/*
* We must always clean up by clearing EXT4_STATE_VERITY_IN_PROGRESS and
* deleting the inode from the orphan list, even if something failed.
* If everything succeeded, we'll also set the verity bit in the same
* transaction.
*/
ext4_clear_inode_state(inode, EXT4_STATE_VERITY_IN_PROGRESS);
handle = ext4_journal_start(inode, EXT4_HT_INODE, credits);
if (IS_ERR(handle)) {
ext4_orphan_del(NULL, inode);
return PTR_ERR(handle);
}
err2 = ext4_orphan_del(handle, inode);
if (err2)
goto out_stop;
if (desc != NULL && !err) {
struct ext4_iloc iloc;
err = ext4_reserve_inode_write(handle, inode, &iloc);
if (err)
goto out_stop;
ext4_set_inode_flag(inode, EXT4_INODE_VERITY);
ext4_set_inode_flags(inode);
err = ext4_mark_iloc_dirty(handle, inode, &iloc);
}
out_stop:
ext4_journal_stop(handle);
return err ?: err2;
}
static int ext4_get_verity_descriptor_location(struct inode *inode,
size_t *desc_size_ret,
u64 *desc_pos_ret)
{
struct ext4_ext_path *path;
struct ext4_extent *last_extent;
u32 end_lblk;
u64 desc_size_pos;
__le32 desc_size_disk;
u32 desc_size;
u64 desc_pos;
int err;
/*
* Descriptor size is in last 4 bytes of last allocated block.
* See ext4_write_verity_descriptor().
*/
if (!ext4_test_inode_flag(inode, EXT4_INODE_EXTENTS)) {
EXT4_ERROR_INODE(inode, "verity file doesn't use extents");
return -EFSCORRUPTED;
}
path = ext4_find_extent(inode, EXT_MAX_BLOCKS - 1, NULL, 0);
if (IS_ERR(path))
return PTR_ERR(path);
last_extent = path[path->p_depth].p_ext;
if (!last_extent) {
EXT4_ERROR_INODE(inode, "verity file has no extents");
ext4_ext_drop_refs(path);
kfree(path);
return -EFSCORRUPTED;
}
end_lblk = le32_to_cpu(last_extent->ee_block) +
ext4_ext_get_actual_len(last_extent);
desc_size_pos = (u64)end_lblk << inode->i_blkbits;
ext4_ext_drop_refs(path);
kfree(path);
if (desc_size_pos < sizeof(desc_size_disk))
goto bad;
desc_size_pos -= sizeof(desc_size_disk);
err = pagecache_read(inode, &desc_size_disk, sizeof(desc_size_disk),
desc_size_pos);
if (err)
return err;
desc_size = le32_to_cpu(desc_size_disk);
/*
* The descriptor is stored just before the desc_size_disk, but starting
* on a filesystem block boundary.
*/
if (desc_size > INT_MAX || desc_size > desc_size_pos)
goto bad;
desc_pos = round_down(desc_size_pos - desc_size, i_blocksize(inode));
if (desc_pos < ext4_verity_metadata_pos(inode))
goto bad;
*desc_size_ret = desc_size;
*desc_pos_ret = desc_pos;
return 0;
bad:
EXT4_ERROR_INODE(inode, "verity file corrupted; can't find descriptor");
return -EFSCORRUPTED;
}
static int ext4_get_verity_descriptor(struct inode *inode, void *buf,
size_t buf_size)
{
size_t desc_size = 0;
u64 desc_pos = 0;
int err;
err = ext4_get_verity_descriptor_location(inode, &desc_size, &desc_pos);
if (err)
return err;
if (buf_size) {
if (desc_size > buf_size)
return -ERANGE;
err = pagecache_read(inode, buf, desc_size, desc_pos);
if (err)
return err;
}
return desc_size;
}
static struct page *ext4_read_merkle_tree_page(struct inode *inode,
pgoff_t index)
{
index += ext4_verity_metadata_pos(inode) >> PAGE_SHIFT;
return read_mapping_page(inode->i_mapping, index, NULL);
}
static int ext4_write_merkle_tree_block(struct inode *inode, const void *buf,
u64 index, int log_blocksize)
{
loff_t pos = ext4_verity_metadata_pos(inode) + (index << log_blocksize);
return pagecache_write(inode, buf, 1 << log_blocksize, pos);
}
const struct fsverity_operations ext4_verityops = {
.begin_enable_verity = ext4_begin_enable_verity,
.end_enable_verity = ext4_end_enable_verity,
.get_verity_descriptor = ext4_get_verity_descriptor,
.read_merkle_tree_page = ext4_read_merkle_tree_page,
.write_merkle_tree_block = ext4_write_merkle_tree_block,
};