[PATCH v10 1/5] fscrypt: add functions for direct I/O support

Eric Biggers
2022-01-20T07:16:24Z

From: Eric Biggers <ebiggers@google.com>

Encrypted files traditionally haven't supported DIO, due to the need to
encrypt/decrypt the data. However, when the encryption is implemented
using inline encryption (blk-crypto) instead of the traditional
filesystem-layer encryption, it is straightforward to support DIO.

In preparation for supporting this, add the following functions:

- fscrypt_dio_unsupported() checks whether a DIO request is unsupported
due to encryption constraints. Encrypted files will only support DIO
when inline encryption is used and the I/O request is properly
aligned; this function checks these preconditions.

- fscrypt_limit_io_blocks() limits the length of a bio to avoid crossing
a place in the file that a bio with an encryption context cannot
cross due to a DUN discontiguity. This function is needed by
filesystems that use the iomap DIO implementation (which operates
directly on logical ranges, so it won't use fscrypt_mergeable_bio())
and that support FSCRYPT_POLICY_FLAG_IV_INO_LBLK_32.

Co-developed-by: Satya Tangirala <satyat@google.com>
Signed-off-by: Satya Tangirala <satyat@google.com>
Signed-off-by: Eric Biggers <ebiggers@google.com>
---
fs/crypto/crypto.c | 8 ++++
fs/crypto/inline_crypt.c | 90 ++++++++++++++++++++++++++++++++++++++++
include/linux/fscrypt.h | 18 ++++++++
3 files changed, 116 insertions(+)

diff --git a/fs/crypto/crypto.c b/fs/crypto/crypto.c
index 4ef3f714046aa..4fcca79f39aeb 100644
--- a/fs/crypto/crypto.c
+++ b/fs/crypto/crypto.c
@@ -69,6 +69,14 @@ void fscrypt_free_bounce_page(struct page *bounce_page)
}
EXPORT_SYMBOL(fscrypt_free_bounce_page);

+/*
+ * Generate the IV for the given logical block number within the given file.
+ * For filenames encryption, lblk_num == 0.
+ *
+ * Keep this in sync with fscrypt_limit_io_blocks(). fscrypt_limit_io_blocks()
+ * needs to know about any IV generation methods where the low bits of IV don't
+ * simply contain the lblk_num (e.g., IV_INO_LBLK_32).
+ */
void fscrypt_generate_iv(union fscrypt_iv *iv, u64 lblk_num,
const struct fscrypt_info *ci)
{
diff --git a/fs/crypto/inline_crypt.c b/fs/crypto/inline_crypt.c
index c57bebfa48fea..304ae414cbbf2 100644
--- a/fs/crypto/inline_crypt.c
+++ b/fs/crypto/inline_crypt.c
@@ -17,6 +17,7 @@
#include <linux/buffer_head.h>
#include <linux/sched/mm.h>
#include <linux/slab.h>
+#include <linux/uio.h>

#include "fscrypt_private.h"

@@ -315,6 +316,10 @@ EXPORT_SYMBOL_GPL(fscrypt_set_bio_crypt_ctx_bh);
*
* fscrypt_set_bio_crypt_ctx() must have already been called on the bio.
*
+ * This function isn't required in cases where crypto-mergeability is ensured in
+ * another way, such as I/O targeting only a single file (and thus a single key)
+ * combined with fscrypt_limit_io_blocks() to ensure DUN contiguity.
+ *
* Return: true iff the I/O is mergeable
*/
bool fscrypt_mergeable_bio(struct bio *bio, const struct inode *inode,
@@ -363,3 +368,88 @@ bool fscrypt_mergeable_bio_bh(struct bio *bio,
return fscrypt_mergeable_bio(bio, inode, next_lblk);
}
EXPORT_SYMBOL_GPL(fscrypt_mergeable_bio_bh);
+
+/**
+ * fscrypt_dio_unsupported() - check whether a DIO (direct I/O) request is
+ * unsupported due to encryption constraints
+ * @iocb: the file and position the I/O is targeting
+ * @iter: the I/O data segment(s)
+ *
+ * Return: true if DIO is unsupported
+ */
+bool fscrypt_dio_unsupported(struct kiocb *iocb, struct iov_iter *iter)
+{
+ const struct inode *inode = file_inode(iocb->ki_filp);
+ const unsigned int blocksize = i_blocksize(inode);
+
+ /* If the file is unencrypted, no veto from us. */
+ if (!fscrypt_needs_contents_encryption(inode))
+ return false;
+
+ /* We only support DIO with inline crypto, not fs-layer crypto. */
+ if (!fscrypt_inode_uses_inline_crypto(inode))
+ return true;
+
+ /*
+ * Since the granularity of encryption is filesystem blocks, the file
+ * position and total I/O length must be aligned to the filesystem block
+ * size -- not just to the block device's logical block size as is
+ * traditionally the case for DIO on many filesystems (not including
+ * f2fs, which only allows filesystem block aligned DIO anyway).
+ *
+ * We also require that the user-provided memory buffers be block
+ * aligned too. It is simpler to have a single alignment value required
+ * for all properties of the I/O, as is normally the case for DIO.
+ * Also, allowing less aligned buffers would also imply that a data unit
+ * could cross bvecs, which would greatly complicate the I/O stack,
+ * which assumes that bios can be split at any bvec boundary.
+ */
+ if (!IS_ALIGNED(iocb->ki_pos | iov_iter_alignment(iter), blocksize))
+ return true;
+
+ return false;
+}
+EXPORT_SYMBOL_GPL(fscrypt_dio_unsupported);
+
+/**
+ * fscrypt_limit_io_blocks() - limit I/O blocks to avoid discontiguous DUNs
+ * @inode: the file on which I/O is being done
+ * @lblk: the block at which the I/O is being started from
+ * @nr_blocks: the number of blocks we want to submit starting at @lblk
+ *
+ * Determine the limit to the number of blocks that can be submitted in a bio
+ * targeting @lblk without causing a data unit number (DUN) discontiguity.
+ *
+ * This is normally just @nr_blocks, as normally the DUNs just increment along
+ * with the logical blocks. (Or the file is not encrypted.)
+ *
+ * In rare cases, fscrypt can be using an IV generation method that allows the
+ * DUN to wrap around within logically contiguous blocks, and that wraparound
+ * will occur. If this happens, a value less than @nr_blocks will be returned
+ * so that the wraparound doesn't occur in the middle of a bio, which would
+ * cause encryption/decryption to produce the wrong results.
+ *
+ * Return: the actual number of blocks that can be submitted
+ */
+u64 fscrypt_limit_io_blocks(const struct inode *inode, u64 lblk, u64 nr_blocks)
+{
+ const struct fscrypt_info *ci = inode->i_crypt_info;
+ u32 dun;
+
+ if (!fscrypt_inode_uses_inline_crypto(inode))
+ return nr_blocks;
+
+ if (nr_blocks <= 1)
+ return nr_blocks;
+
+ if (!(fscrypt_policy_flags(&ci->ci_policy) &
+ FSCRYPT_POLICY_FLAG_IV_INO_LBLK_32))
+ return nr_blocks;
+
+ /* With IV_INO_LBLK_32, the DUN can wrap around from U32_MAX to 0. */
+
+ dun = ci->ci_hashed_ino + lblk;
+
+ return min_t(u64, nr_blocks, (u64)U32_MAX + 1 - dun);
+}
+EXPORT_SYMBOL_GPL(fscrypt_limit_io_blocks);
diff --git a/include/linux/fscrypt.h b/include/linux/fscrypt.h
index 91ea9477e9bd2..87ec5f63b0a82 100644
--- a/include/linux/fscrypt.h
+++ b/include/linux/fscrypt.h
@@ -714,6 +714,10 @@ bool fscrypt_mergeable_bio(struct bio *bio, const struct inode *inode,
bool fscrypt_mergeable_bio_bh(struct bio *bio,
const struct buffer_head *next_bh);

+bool fscrypt_dio_unsupported(struct kiocb *iocb, struct iov_iter *iter);
+
+u64 fscrypt_limit_io_blocks(const struct inode *inode, u64 lblk, u64 nr_blocks);
+
#else /* CONFIG_FS_ENCRYPTION_INLINE_CRYPT */

static inline bool __fscrypt_inode_uses_inline_crypto(const struct inode *inode)
@@ -742,6 +746,20 @@ static inline bool fscrypt_mergeable_bio_bh(struct bio *bio,
{
return true;
}
+
+static inline bool fscrypt_dio_unsupported(struct kiocb *iocb,
+ struct iov_iter *iter)
+{
+ const struct inode *inode = file_inode(iocb->ki_filp);
+
+ return fscrypt_needs_contents_encryption(inode);
+}
+
+static inline u64 fscrypt_limit_io_blocks(const struct inode *inode, u64 lblk,
+ u64 nr_blocks)
+{
+ return nr_blocks;
+}
#endif /* !CONFIG_FS_ENCRYPTION_INLINE_CRYPT */

/**
--
2.34.1