Re: [PATCH v5 3/8] fs-verity: define a function to return the integrity protected file digest

[Mimi Zohar]

On Wed, 2022-02-23 at 15:59 -0800, Eric Biggers wrote:
> On Fri, Feb 11, 2022 at 04:43:05PM -0500, Mimi Zohar wrote:
> > +/**
> > + * fsverity_get_digest() - get a verity file's digest
> > + * @inode: inode to get digest of
> > + * @digest: (out) pointer to the digest
> > + * @alg: (out) pointer to the hash algorithm enumeration
> > + *
> > + * Return the file hash algorithm and digest of an fsverity protected file.
> > + *
> > + * Return: 0 on success, -errno on failure
> > + */
> > +int fsverity_get_digest(struct inode *inode,
> > +			u8 digest[FS_VERITY_MAX_DIGEST_SIZE],
> > +			enum hash_algo *alg)
> > +{
> > +	const struct fsverity_info *vi;
> > +	const struct fsverity_hash_alg *hash_alg;
> > +	int i;
> > +
> > +	vi = fsverity_get_info(inode);
> > +	if (!vi)
> > +		return -ENODATA; /* not a verity file */
> 
> Sorry for the slow reviews; I'm taking a look again now.  One question about
> something I missed earlier: is the file guaranteed to have been opened before
> this is called?  fsverity_get_info() only returns a non-NULL value if the file
> has been opened at least once since the inode has been loaded into memory.  If
> the inode has just been loaded into memory without being opened, for example due
> to a call to stat(), then fsverity_get_info() will return NULL.
> 
> If the file is guaranteed to have been opened, then the code is fine, but the
> comment for fsverity_get_digest() perhaps should be updated to mention this
> assumption, given that it takes a struct inode rather than a struct file.
> 
> If the file is *not* guaranteed to have been opened, then it would be necessary
> to make fsverity_get_digest() call ensure_verity_info() to set up the
> fsverity_info.

Yes, fsverity_get_digest() is called as a result of a syscall - open,
execve, mmap, etc.   
Refer to the LSM hooks security_bprm_check() and security_mmap_file().
ima_file_check() is called directly in do_open().

Mimi