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

Chandan Rajendra
2018-11-06T06:50:16Z

On Tuesday, November 6, 2018 6:55:03 AM IST Eric Biggers wrote:
> 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.
>

Yes, I agree with what you say. I have followed the above explained logic when
implementing S_VERITY and IS_VERITY() for Ext4 and will extend that to F2FS as
well.

--
chandan