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

Eric Biggers
2018-11-06T01:25:09Z

Hi Chandan,

On Fri, Nov 02, 2018 at 03:13:14PM +0530, Chandan Rajendra wrote:
> On Friday, November 2, 2018 4:22:28 AM IST Eric Biggers wrote:
> > From: Eric Biggers <ebiggers@google.com>
> >
> > 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@mit.edu>
> > Signed-off-by: Theodore Ts'o <tytso@mit.edu>
> > Signed-off-by: Eric Biggers <ebiggers@google.com>
> > ---
> > 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?
>

Good question, this might not be the best approach actually; I think this was
originally copied from the f2fs version. It does reduce the overhead introduced
by the fs-verity changes in the !CONFIG_EXT4_FS_VERITY case. But it will allow
opening verity files, even for writing which will corrupt them.

Probably we should make ext4_verity_inode() work regardless of
CONFIG_EXT4_FS_VERITY, so open(), truncate(), etc. will fail with EOPNOTSUPP on
verity files when !CONFIG_EXT4_FS_VERITY, like how ext4 encryption works.

Thanks,

- Eric