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

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Message-ID: <YhbeuQpGuDxEmi9o@sol.localdomain>
Date:   Wed, 23 Feb 2022 17:26:17 -0800
From:   Eric Biggers <ebiggers@...nel.org>
To:     Mimi Zohar <zohar@...ux.ibm.com>
Cc:     linux-integrity@...r.kernel.org,
        Stefan Berger <stefanb@...ux.ibm.com>,
        linux-fscrypt@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 3/8] fs-verity: define a function to return the
 integrity protected file digest

On Wed, Feb 23, 2022 at 08:21:01PM -0500, Mimi Zohar wrote:
> 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().

stat() is a syscall too, so the question is not whether this is being called as
a result of a syscall, but rather whether it's only being called while the file
is open (or at least previously opened).  Is the answer to that "yes"?

- Eric