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Message-ID: <a8d344e7d9c4683ad87ec0aaf32eabcbfb0f48fb.camel@chromium.org>
Date: Mon, 13 Jan 2020 10:42:18 +0100
From: Florent Revest <revest@...omium.org>
To: Mimi Zohar <zohar@...ux.ibm.com>, linux-integrity@...r.kernel.org
Cc: kpsingh@...omium.org, mjg59@...gle.com, nramas@...ux.microsoft.com,
linux-kernel@...r.kernel.org,
linux-security-module@...r.kernel.org,
Florent Revest <revest@...gle.com>
Subject: Re: [PATCH v2] ima: add the ability to query the hash of a given
file.
On Wed, 2020-01-08 at 11:05 -0500, Mimi Zohar wrote:
> On Mon, 2020-01-06 at 17:25 +0100, Florent Revest wrote:
> > From: Florent Revest <revest@...gle.com>
> >
> > This allows other parts of the kernel (perhaps a stacked LSM
> > allowing
> > system monitoring, eg. the proposed KRSI LSM [1]) to retrieve the
> > hash
> > of a given file from IMA if it's present in the iint cache.
> >
> > It's true that the existence of the hash means that it's also in
> > the
> > audit logs or in
> > /sys/kernel/security/ima/ascii_runtime_measurements,
> > but it can be difficult to pull that information out for every
> > subsequent exec. This is especially true if a given host has been
> > up
> > for a long time and the file was first measured a long time ago.
> >
> > This is based on Peter Moody's patch:
> > https://sourceforge.net/p/linux-ima/mailman/message/33036180/
>
> FYI, but unlike the audit log/IMA measurement list, the iint cache
> entries can be removed. Refer to security_inode_free(). Perhaps
> mention of this difference should be included, here, in the patch
> description.
Sure, I added a comment about this in a v3.
> > [1] https://lkml.org/lkml/2019/9/10/393
> >
> > Signed-off-by: Florent Revest <revest@...gle.com>
>
> Assuming, with the above difference, you're still interested in
> having this feature upstreamed and addressing the comments above and
> below:
>
> Reviewed-by: Mimi Zohar <zohar@...ux.ibm.com>
Thank you. Yes we are still interested in this feature!
> > ---
> > include/linux/ima.h | 6 ++++
> > security/integrity/ima/ima_main.c | 46
> > +++++++++++++++++++++++++++++++
> > 2 files changed, 52 insertions(+)
> >
> > diff --git a/include/linux/ima.h b/include/linux/ima.h
> > index 6d904754d858..d621c65ba9a5 100644
> > --- a/include/linux/ima.h
> > +++ b/include/linux/ima.h
> > @@ -23,6 +23,7 @@ extern int ima_read_file(struct file *file, enum
> > kernel_read_file_id id);
> > extern int ima_post_read_file(struct file *file, void *buf, loff_t
> > size,
> > enum kernel_read_file_id id);
> > extern void ima_post_path_mknod(struct dentry *dentry);
> > +extern int ima_file_hash(struct file *file, char *buf, size_t
> > buf_size);
> > extern void ima_kexec_cmdline(const void *buf, int size);
> >
> > #ifdef CONFIG_IMA_KEXEC
> > @@ -91,6 +92,11 @@ static inline void ima_post_path_mknod(struct
> > dentry *dentry)
> > return;
> > }
> >
> > +static inline int ima_file_hash(struct file *file, char *buf,
> > size_t buf_size)
> > +{
> > + return -EOPNOTSUPP;
> > +}
> > +
> > static inline void ima_kexec_cmdline(const void *buf, int size) {}
> > #endif /* CONFIG_IMA */
> >
> > diff --git a/security/integrity/ima/ima_main.c
> > b/security/integrity/ima/ima_main.c
> > index d7e987baf127..3799b6c6c3b8 100644
> > --- a/security/integrity/ima/ima_main.c
> > +++ b/security/integrity/ima/ima_main.c
> > @@ -445,6 +445,52 @@ int ima_file_check(struct file *file, int
> > mask)
> > }
> > EXPORT_SYMBOL_GPL(ima_file_check);
> >
> > +/**
> > + * 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, return the hash algorithm (as defined in the enum
> > hash_algo).
> > + * If buf is not NULL, this function also outputs the hash into
> > buf.
>
> As of Linux 5.4.y, IMA support for appended file signatures was
> added. Should we indicate that the file hash returned is based on the
> entire file, including the appended signature?
>
> Mimi
Of course it never hurts to add a comment. :) I'll send a v3 with this
added.
>
> > + * If the hash is larger than buf_size, then only buf_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 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;
> > + int hash_algo;
> > +
> > + if (!file)
> > + return -EINVAL;
> > +
> > + if (!ima_policy_flag)
> > + return -EOPNOTSUPP;
> > +
> > + inode = file_inode(file);
> > + iint = integrity_iint_find(inode);
> > + if (!iint)
> > + return -EOPNOTSUPP;
> > +
> > + mutex_lock(&iint->mutex);
> > + if (buf) {
> > + size_t copied_size;
> > +
> > + copied_size = min_t(size_t, iint->ima_hash->length,
> > buf_size);
> > + memcpy(buf, iint->ima_hash->digest, copied_size);
> > + }
> > + hash_algo = iint->ima_hash->algo;
> > + mutex_unlock(&iint->mutex);
> > +
> > + return hash_algo;
> > +}
> > +EXPORT_SYMBOL_GPL(ima_file_hash);
> > +
> > /**
> > * ima_post_create_tmpfile - mark newly created tmpfile as new
> > * @file : newly created tmpfile
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