linux-integrity.vger.kernel.org archive mirror
 help / color / mirror / Atom feed
From: Mimi Zohar <zohar@linux.ibm.com>
To: THOBY Simon <Simon.THOBY@viveris.fr>,
	"dmitry.kasatkin@gmail.com" <dmitry.kasatkin@gmail.com>,
	"linux-integrity@vger.kernel.org"
	<linux-integrity@vger.kernel.org>,
	BARVAUX Didier <Didier.BARVAUX@viveris.fr>
Cc: Paul Moore <paul@paul-moore.com>
Subject: Re: [PATCH v4 2/5] IMA: block writes of the security.ima xattr with unsupported algorithms
Date: Tue, 27 Jul 2021 16:32:39 -0400	[thread overview]
Message-ID: <5a3a35b5f20fb17f7430046b0378e05f1dffa098.camel@linux.ibm.com> (raw)
In-Reply-To: <20210727163330.790010-3-simon.thoby@viveris.fr>

[Cc'ing Paul Moore]

Hi Simon,

On Tue, 2021-07-27 at 16:33 +0000, THOBY Simon wrote:
> By default, any write to the extended attributes security.ima will be
> accepted, even if the xattr value uses a hash algorithm not compiled in
> the kernel (which doesn't make sense, because the kernel wouldn't be able
> to appraise that file, as it lacks support for validating the hash).
> 
> Prevent such writes: only writes using hash algorithms
> available in the current kernel are now allowed. Any attempt to
> perform these writes will be denied with an audit message.
> 
> Note however that CONFIG_IMA depends on CONFIG_CRYPTO_SHA1, which
> somewhat hampers the security benefits of this measure (but MD4 and
> MD5 can be disabled, which is already a significant improvement).
> 
> Signed-off-by: Simon Thoby <simon.thoby@viveris.fr>
> ---
>  security/integrity/ima/ima_appraise.c | 51 ++++++++++++++++++++++++++-
>  1 file changed, 50 insertions(+), 1 deletion(-)
> 
> diff --git a/security/integrity/ima/ima_appraise.c b/security/integrity/ima/ima_appraise.c
> index ef9dcfce45d4..989da2fbf496 100644
> --- a/security/integrity/ima/ima_appraise.c
> +++ b/security/integrity/ima/ima_appraise.c
> @@ -575,12 +575,55 @@ static void ima_reset_appraise_flags(struct inode *inode, int digsig)
>  		clear_bit(IMA_DIGSIG, &iint->atomic_flags);
>  }
>  
> +/**
> + * ima_setxattr_validate_hash_alg() - Block setxattr with invalid digests
> + * @dentry: file being altered

There isn't a one to one mapping betwen dentry and "file".   Perhaps
something like used in  __vfs_setxattr_locked:

*  @dentry: object to perform setxattr on

> + * @xattr_value: value supplied by userland for the xattr
> + * @xattr_value_len: length of xattr_value
> + *
> + * Context: called when the user tries to write the security.ima xattr.
> + * The xattr value is mapped to some hash algorithm, and this algorithm
> + * must be built in the kernel for the setxattr to be allowed.
> + *
> + * Emit an audit message when the algorithm is invalid.
> + *
> + * Return: 0 on success, else an error.
> + */
> +int ima_setxattr_validate_hash_alg(struct dentry *dentry,
> +				   const void *xattr_value,
> +				   size_t xattr_value_len)

Should this be static?  If it is a local function, then it doesn't
really need to be prefixed with "ima_".  It could even be trimmed to
validate_hash_algo().

> +{
> +	int res = -EACCES;

I know there isn't any variable naming consistency.  The original code
used rc.  Subsequently it was replaced with result or ret.  Let's not
introduce yet another variable name here.

> +	char *path = NULL, *pathbuf = NULL;
> +	enum hash_algo dentry_hash;
> +
> +	dentry_hash = ima_get_hash_algo((struct evm_ima_xattr_data *)xattr_value,
> +				     xattr_value_len);

The hash algorithm is extracted from the xattr_value.  Perhaps rename
the variable to xattr_hash, xattr_hash_algo, or simply hash_algo?

> +
> +	if (likely(dentry_hash == ima_hash_algo
> +	    || crypto_has_alg(hash_algo_name[dentry_hash], 0, 0)))
> +		return 0;
> +
> +	pathbuf = kmalloc(PATH_MAX, GFP_KERNEL);
> +	/* no memory available ? no file path for you */

The comment here is unnecessary.  Avoid or limit comments inside a
function.  Refer to the section "8) Commenting" in
Documentation/process/coding-style.rst

> +	if (pathbuf)
> +		path = dentry_path(dentry, pathbuf, PATH_MAX);
> +
> +	/* disallow xattr writes with algorithms not built in the kernel */
> +	integrity_audit_msg(AUDIT_INTEGRITY_DATA, d_inode(dentry),
> +		path, "collect_data", "unavailable-hash-algorithm", res, 0);

This will emit an audit message without the filename when !path.  Is
this what you intended?

Now that this patch set is getting closer, examples of each new type of
audit message needs to be provided to the audit mailing list.  Paul,
any suggestions as to how/when to provide them?

> +
> +	kfree(pathbuf);
> +
> +	return res;
> +}
> +
>  int ima_inode_setxattr(struct dentry *dentry, const char *xattr_name,
>  		       const void *xattr_value, size_t xattr_value_len)
>  {
>  	const struct evm_ima_xattr_data *xvalue = xattr_value;
>  	int digsig = 0;
> -	int result;
> +	int result, rc;
>  
>  	result = ima_protect_xattr(dentry, xattr_name, xattr_value,
>  				   xattr_value_len);
> @@ -592,6 +635,12 @@ int ima_inode_setxattr(struct dentry *dentry, const char *xattr_name,
>  		digsig = (xvalue->type == EVM_XATTR_PORTABLE_DIGSIG);
>  	}
>  	if (result == 1 || evm_revalidate_status(xattr_name)) {
> +		/* the user-supplied xattr must use an allowed hash algo */

With a function name containing "validate_hash_alg", this comment is
unnecessary.

thanks,

Mimi

> +		rc = ima_setxattr_validate_hash_alg(dentry, xattr_value,
> +							xattr_value_len);
> +		if (!rc)
> +			return rc;
> +
>  		ima_reset_appraise_flags(d_backing_inode(dentry), digsig);
>  		if (result == 1)
>  			result = 0;



  reply	other threads:[~2021-07-27 20:32 UTC|newest]

Thread overview: 17+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-07-27 16:33 [PATCH v4 0/5] IMA: restrict the accepted digest algorithms for THOBY Simon
2021-07-27 16:33 ` [PATCH v4 1/5] IMA: remove the dependency on CRYPTO_MD5 THOBY Simon
2021-07-27 17:57   ` Mimi Zohar
2021-07-27 16:33 ` [PATCH v4 2/5] IMA: block writes of the security.ima xattr with unsupported algorithms THOBY Simon
2021-07-27 20:32   ` Mimi Zohar [this message]
2021-07-28  7:00     ` THOBY Simon
2021-07-28 12:43       ` Mimi Zohar
2021-07-28 12:53         ` THOBY Simon
2021-07-28 13:09           ` Mimi Zohar
2021-07-27 16:33 ` [PATCH v4 3/5] IMA: add support to restrict the hash algorithms used for file appraisal THOBY Simon
2021-07-27 20:38   ` Mimi Zohar
2021-07-27 16:33 ` [PATCH v4 4/5] IMA: add a policy option to restrict xattr hash algorithms on appraisal THOBY Simon
2021-07-27 21:07   ` Mimi Zohar
2021-07-27 16:33 ` [PATCH v4 5/5] IMA: introduce a new policy option func=SETXATTR_CHECK THOBY Simon
2021-07-27 17:25   ` Mimi Zohar
2021-07-27 17:58     ` THOBY Simon
2021-07-27 17:47 ` [PATCH v4 0/5] IMA: restrict the accepted digest algorithms for Mimi Zohar

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=5a3a35b5f20fb17f7430046b0378e05f1dffa098.camel@linux.ibm.com \
    --to=zohar@linux.ibm.com \
    --cc=Didier.BARVAUX@viveris.fr \
    --cc=Simon.THOBY@viveris.fr \
    --cc=dmitry.kasatkin@gmail.com \
    --cc=linux-integrity@vger.kernel.org \
    --cc=paul@paul-moore.com \
    --subject='Re: [PATCH v4 2/5] IMA: block writes of the security.ima xattr with unsupported algorithms' \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).