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SubjectRe: [PATCH] fscrypt: Restore modular support
On Sat, Dec 21, 2019 at 10:30:20PM +0800, Herbert Xu wrote:
> The commit 643fa9612bf1 ("fscrypt: remove filesystem specific
> build config option") removed modular support for fs/crypto. This
> causes the Crypto API to be built-in whenever fscrypt is enabled.
> This makes it very difficult for me to test modular builds of
> the Crypto API without disabling fscrypt which is a pain.
>
> AFAICS there is no reason why fscrypt has to be built-in. The
> commit above appears to have done this way purely for the sake of
> simplicity. In fact some simple Kconfig tweaking is sufficient
> to retain a single FS_ENCRYPTION option while maintaining modularity.
>
> This patch restores modular support to fscrypt by adding a new
> hidden FS_ENCRYPTION_TRI tristate option that is selected by all
> the FS_ENCRYPTION users.
>
> Subsequent to the above commit, some core code has been introduced
> to fs/buffer.c that makes restoring modular support non-trivial.
> This patch deals with this by adding a function pointer that defaults
> to end_buffer_async_read function until fscrypt is loaded. When
> fscrypt is loaded it modifies the function pointer to its own
> function which used to be end_buffer_async_read_io but now resides
> in fscrypt. When it is unloaded the function pointer is restored.
>
> In order for this to be safe with respect to module removal, the
> check for whether the host inode is encrypted has been moved into
> mark_buffer_async_read. The assumption is that as long as the
> bh is alive the calling filesystem module will be resident. The
> calling filesystem would then guarantee that fscrypt is loaded.
>
> Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

I'm not sure this is a good idea, since there will probably need to be more
places where built-in code calls into fs/crypto/ too.

There's actually already fscrypt_sb_free(), which this patch forgets to handle:

void fscrypt_sb_free(struct super_block *sb)
{
key_put(sb->s_master_keys);
sb->s_master_keys = NULL;
}

Though we could make that an inline function.

But there's also a patch under review which adds inline encryption support to
ext4 and f2fs, which requires calling into fs/crypto/ from fs/buffer.c in order
to set an encryption context on bios:

https://lkml.kernel.org/linux-fscrypt/20191218145136.172774-10-satyat@google.com/

If we also add direct I/O support for inline encryption (which is planned), it
would require calling into fs/crypto/ from fs/direct-io.c and
fs/iomap/direct-io.c as well.

Another thing I've been thinking about is adding decryption support to
__block_write_begin(), which would allow us to delete ext4_block_write_begin(),
which was copied from __block_write_begin() to add decryption support.

So more broadly, the issue is that a lot of the filesystem I/O helpers
(fs/buffer.c, fs/iomap/, fs/direct-io.c, fs/mpage.c) are already built-in, and
it can be necessary to call into fs/crypto/ from such places.

Is it really much of an issue to just disable CONFIG_FS_ENCRYPTION when you want
to test building the crypto API as a module?

> diff --git a/fs/crypto/crypto.c b/fs/crypto/crypto.c
> index 3719efa546c6..6bf7f05120bd 100644
> --- a/fs/crypto/crypto.c
> +++ b/fs/crypto/crypto.c
> @@ -20,6 +20,7 @@
> * Special Publication 800-38E and IEEE P1619/D16.
> */
>
> +#include <linux/buffer_head.h>
> #include <linux/pagemap.h>
> #include <linux/mempool.h>
> #include <linux/module.h>
> @@ -286,6 +287,41 @@ int fscrypt_decrypt_block_inplace(const struct inode *inode, struct page *page,
> }
> EXPORT_SYMBOL(fscrypt_decrypt_block_inplace);
>
> +struct decrypt_bh_ctx {
> + struct work_struct work;
> + struct buffer_head *bh;
> +};
> +
> +static void decrypt_bh(struct work_struct *work)
> +{
> + struct decrypt_bh_ctx *ctx =
> + container_of(work, struct decrypt_bh_ctx, work);
> + struct buffer_head *bh = ctx->bh;
> + int err;
> +
> + err = fscrypt_decrypt_pagecache_blocks(bh->b_page, bh->b_size,
> + bh_offset(bh));
> + end_buffer_async_read(bh, err == 0);
> + kfree(ctx);
> +}
> +
> +static void fscrypt_end_buffer_async_read(struct buffer_head *bh, int uptodate)
> +{
> + /* Decrypt if needed */
> + if (uptodate) {
> + struct decrypt_bh_ctx *ctx = kmalloc(sizeof(*ctx), GFP_ATOMIC);
> +
> + if (ctx) {
> + INIT_WORK(&ctx->work, decrypt_bh);
> + ctx->bh = bh;
> + fscrypt_enqueue_decrypt_work(&ctx->work);
> + return;
> + }
> + uptodate = 0;
> + }
> + end_buffer_async_read(bh, uptodate);
> +}
> +
> /*
> * Validate dentries in encrypted directories to make sure we aren't potentially
> * caching stale dentries after a key has been added.
> @@ -418,6 +454,8 @@ static int __init fscrypt_init(void)
> if (err)
> goto fail_free_info;
>
> + end_buffer_async_read_io = fscrypt_end_buffer_async_read;
> +
> return 0;

This code would actually need to go into fs/crypto/bio.c because it depends on
CONFIG_BLOCK. UBIFS uses fs/crypto/ without CONFIG_BLOCK necessarily being set.

> +/**
> + * fscrypt_exit() - Shutdown the fs encryption system
> + */
> +static void __exit fscrypt_exit(void)
> +{
> + end_buffer_async_read_io = end_buffer_async_read;
> +
> + kmem_cache_destroy(fscrypt_info_cachep);
> + destroy_workqueue(fscrypt_read_workqueue);
> +}
> +module_exit(fscrypt_exit);

There's a bit more that would need to be done here:

mempool_destroy(fscrypt_bounce_page_pool);

and

fscrypt_exit_keyring()
=> unregister_key_type(&key_type_fscrypt);
=> unregister_key_type(&key_type_fscrypt_user);
- Eric

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