Re: [PATCH v2 10/12] ext4: add basic fs-verity support

[Chandan Rajendra]

On Friday, November 2, 2018 4:22:28 AM IST Eric Biggers wrote:
> From: Eric Biggers <ebiggers@xxxxxxxxxx>
> 
> Add basic 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 details.
> 
> This patch adds everything except the data verification hooks that will
> needed in ->readpages().
> 
> On ext4, enabling fs-verity on a file requires that the filesystem has
> the 'verity' feature, e.g. that it was formatted with
> 'mkfs.ext4 -O verity' or had 'tune2fs -O verity' run on it.
> This requires e2fsprogs 1.44.4-2 or later.
> 
> In ext4, we choose to retain the fs-verity metadata past the end of the
> file rather than trying to move it into an external inode xattr, since
> in practice keeping the metadata in-line actually results in the
> simplest and most efficient implementation.  One non-obvious advantage
> of keeping the verity metadata in-line is that when fs-verity is
> combined with fscrypt, the verity metadata naturally gets encrypted too;
> this is actually necessary because it contains hashes of the plaintext.
> 
> We also choose to keep the on-disk i_size equal to the original file
> size, in order to make the 'verity' feature a RO_COMPAT feature.  Thus,
> ext4 has to find the fsverity_footer by looking in the last extent.
> 
> Co-developed-by: Theodore Ts'o <tytso@xxxxxxx>
> Signed-off-by: Theodore Ts'o <tytso@xxxxxxx>
> Signed-off-by: Eric Biggers <ebiggers@xxxxxxxxxx>
> ---
>  fs/ext4/Kconfig | 20 +++++++++++
>  fs/ext4/ext4.h  | 20 ++++++++++-
>  fs/ext4/file.c  |  6 ++++
>  fs/ext4/inode.c |  8 +++++
>  fs/ext4/ioctl.c | 12 +++++++
>  fs/ext4/super.c | 91 +++++++++++++++++++++++++++++++++++++++++++++++++
>  fs/ext4/sysfs.c |  6 ++++
>  7 files changed, 162 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/ext4/Kconfig b/fs/ext4/Kconfig
> index a453cc87082b5..5a76125ac0f8a 100644
> --- a/fs/ext4/Kconfig
> +++ b/fs/ext4/Kconfig
> @@ -111,6 +111,26 @@ config EXT4_FS_ENCRYPTION
>  	default y
>  	depends on EXT4_ENCRYPTION
> 
> +config EXT4_FS_VERITY
> +	bool "Ext4 Verity"
> +	depends on EXT4_FS
> +	select FS_VERITY
> +	help
> +	  This option enables fs-verity for ext4.  fs-verity is the
> +	  dm-verity mechanism implemented at the file level.  Userspace
> +	  can append a Merkle tree (hash tree) to a file, then enable
> +	  fs-verity on the file.  ext4 will then transparently verify
> +	  any data read from the file against the Merkle tree.  The file
> +	  is also made read-only.
> +
> +	  This serves as an integrity check, but the availability of the
> +	  Merkle tree root hash also allows efficiently supporting
> +	  various use cases where normally the whole file would need to
> +	  be hashed at once, such as auditing and authenticity
> +	  verification (appraisal).
> +
> +	  If unsure, say N.
> +
>  config EXT4_DEBUG
>  	bool "EXT4 debugging support"
>  	depends on EXT4_FS
> diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
> index 12f90d48ba613..e5475a629ed80 100644
> --- a/fs/ext4/ext4.h
> +++ b/fs/ext4/ext4.h
> @@ -43,6 +43,9 @@
>  #define __FS_HAS_ENCRYPTION IS_ENABLED(CONFIG_EXT4_FS_ENCRYPTION)
>  #include <linux/fscrypt.h>
> 
> +#define __FS_HAS_VERITY IS_ENABLED(CONFIG_EXT4_FS_VERITY)
> +#include <linux/fsverity.h>
> +
>  #include <linux/compiler.h>
> 
>  /* Until this gets included into linux/compiler-gcc.h */
> @@ -405,6 +408,7 @@ struct flex_groups {
>  #define EXT4_TOPDIR_FL			0x00020000 /* Top of directory hierarchies*/
>  #define EXT4_HUGE_FILE_FL               0x00040000 /* Set to each huge file */
>  #define EXT4_EXTENTS_FL			0x00080000 /* Inode uses extents */
> +#define EXT4_VERITY_FL			0x00100000 /* Verity protected inode */
>  #define EXT4_EA_INODE_FL	        0x00200000 /* Inode used for large EA */
>  #define EXT4_EOFBLOCKS_FL		0x00400000 /* Blocks allocated beyond EOF */
>  #define EXT4_INLINE_DATA_FL		0x10000000 /* Inode has inline data. */
> @@ -472,6 +476,7 @@ enum {
>  	EXT4_INODE_TOPDIR	= 17,	/* Top of directory hierarchies*/
>  	EXT4_INODE_HUGE_FILE	= 18,	/* Set to each huge file */
>  	EXT4_INODE_EXTENTS	= 19,	/* Inode uses extents */
> +	EXT4_INODE_VERITY	= 20,	/* Verity protected inode */
>  	EXT4_INODE_EA_INODE	= 21,	/* Inode used for large EA */
>  	EXT4_INODE_EOFBLOCKS	= 22,	/* Blocks allocated beyond EOF */
>  	EXT4_INODE_INLINE_DATA	= 28,	/* Data in inode. */
> @@ -517,6 +522,7 @@ static inline void ext4_check_flag_values(void)
>  	CHECK_FLAG_VALUE(TOPDIR);
>  	CHECK_FLAG_VALUE(HUGE_FILE);
>  	CHECK_FLAG_VALUE(EXTENTS);
> +	CHECK_FLAG_VALUE(VERITY);
>  	CHECK_FLAG_VALUE(EA_INODE);
>  	CHECK_FLAG_VALUE(EOFBLOCKS);
>  	CHECK_FLAG_VALUE(INLINE_DATA);
> @@ -1654,6 +1660,7 @@ static inline void ext4_clear_state_flags(struct ext4_inode_info *ei)
>  #define EXT4_FEATURE_RO_COMPAT_METADATA_CSUM	0x0400
>  #define EXT4_FEATURE_RO_COMPAT_READONLY		0x1000
>  #define EXT4_FEATURE_RO_COMPAT_PROJECT		0x2000
> +#define EXT4_FEATURE_RO_COMPAT_VERITY		0x8000
> 
>  #define EXT4_FEATURE_INCOMPAT_COMPRESSION	0x0001
>  #define EXT4_FEATURE_INCOMPAT_FILETYPE		0x0002
> @@ -1742,6 +1749,7 @@ EXT4_FEATURE_RO_COMPAT_FUNCS(bigalloc,		BIGALLOC)
>  EXT4_FEATURE_RO_COMPAT_FUNCS(metadata_csum,	METADATA_CSUM)
>  EXT4_FEATURE_RO_COMPAT_FUNCS(readonly,		READONLY)
>  EXT4_FEATURE_RO_COMPAT_FUNCS(project,		PROJECT)
> +EXT4_FEATURE_RO_COMPAT_FUNCS(verity,		VERITY)
> 
>  EXT4_FEATURE_INCOMPAT_FUNCS(compression,	COMPRESSION)
>  EXT4_FEATURE_INCOMPAT_FUNCS(filetype,		FILETYPE)
> @@ -1797,7 +1805,8 @@ EXT4_FEATURE_INCOMPAT_FUNCS(encrypt,		ENCRYPT)
>  					 EXT4_FEATURE_RO_COMPAT_BIGALLOC |\
>  					 EXT4_FEATURE_RO_COMPAT_METADATA_CSUM|\
>  					 EXT4_FEATURE_RO_COMPAT_QUOTA |\
> -					 EXT4_FEATURE_RO_COMPAT_PROJECT)
> +					 EXT4_FEATURE_RO_COMPAT_PROJECT |\
> +					 EXT4_FEATURE_RO_COMPAT_VERITY)
> 
>  #define EXTN_FEATURE_FUNCS(ver) \
>  static inline bool ext4_has_unknown_ext##ver##_compat_features(struct super_block *sb) \
> @@ -2293,6 +2302,15 @@ static inline bool ext4_encrypted_inode(struct inode *inode)
>  	return ext4_test_inode_flag(inode, EXT4_INODE_ENCRYPT);
>  }
> 
> +static inline bool ext4_verity_inode(struct inode *inode)
> +{
> +#ifdef CONFIG_EXT4_FS_VERITY
> +	return ext4_test_inode_flag(inode, EXT4_INODE_VERITY);
> +#else
> +	return false;
> +#endif
> +}
> +

Hi Eric,

Can you please explain as to why we check for the presence of
EXT4_INODE_VERITY flag only when fsverity is enabled during kernel build?

-- 
chandan