linux-kernel - Re: [PATCH] ima: add the ability to query ima for the hash of a given file.

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Date:   Mon, 06 Jan 2020 17:15:23 +0100
From:   Florent Revest <revest@...omium.org>
To:     Mimi Zohar <zohar@...ux.ibm.com>,
        Lakshmi Ramasubramanian <nramas@...ux.microsoft.com>,
        linux-integrity@...r.kernel.org
Cc:     kpsingh@...omium.org, mjg59@...gle.com,
        linux-kernel@...r.kernel.org,
        linux-security-module@...r.kernel.org,
        Florent Revest <revest@...gle.com>
Subject: Re: [PATCH] ima: add the ability to query ima for the hash of a
 given file.

On Mon, 2019-12-23 at 12:39 -0500, Mimi Zohar wrote:
> On Fri, 2019-12-20 at 08:48 -0800, Lakshmi Ramasubramanian wrote:
> > On 12/20/2019 8:31 AM, Florent Revest wrote:
> > 
> > >   
> > > +/**
> > > + * ima_file_hash - return the stored measurement if a file has
> > > been hashed.
> > > + * @file: pointer to the file
> > > + * @buf: buffer in which to store the hash
> > > + * @buf_size: length of the buffer
> > > + *
> > > + * On success, output the hash into buf and return the hash
> > > algorithm (as
> > > + * defined in the enum hash_algo).
> > > + * If the hash is larger than buf, then only size bytes will be
> > > copied. It
> > > + * generally just makes sense to pass a buffer capable of
> > > holding the largest
> > > + * possible hash: IMA_MAX_DIGEST_SIZE
> > 
> > If the given buffer is smaller than the hash length, wouldn't it
> > be 
> > better to return the required size and a status indicating the
> > buffer is 
> > not enough. The caller can then call back with the required buffer.
> > 
> > If the hash is truncated the caller may not know if the hash is
> > partial 
> > or not.
> 
> Based on the hash algorithm, the caller would know if the buffer
> provided was too small and was truncated.
> 
> > > + *
> > > + * If IMA is disabled or if no measurement is available, return
> > > -EOPNOTSUPP.
> > > + * If the parameters are incorrect, return -EINVAL.
> > > + */
> > > +int ima_file_hash(struct file *file, char *buf, size_t buf_size)
> > > +{
> > > +	struct inode *inode;
> > > +	struct integrity_iint_cache *iint;
> > > +	size_t copied_size;
> > > +
> > > +	if (!file || !buf)
> > > +		return -EINVAL;
> > > +
> 
> Other kernel functions provide a means of determining the needed
> buffer size by passing a NULL field.  Instead of failing here, if buf
> is NULL, how about returning the hash algorithm?

I wasn't aware of that. This is nice to have indeed! I'll send a v2.
Thanks!