From: Mimi Zohar <zohar@linux.ibm.com>
To: Roberto Sassu <roberto.sassu@huawei.com>,
paul@paul-moore.com, stephen.smalley.work@gmail.com,
casey@schaufler-ca.com, stefanb@linux.ibm.com
Cc: linux-integrity@vger.kernel.org,
linux-security-module@vger.kernel.org,
linux-kernel@vger.kernel.org, selinux@vger.kernel.org
Subject: Re: [PATCH] evm: Check xattr size misalignment between kernel and user
Date: Mon, 21 Jun 2021 08:22:17 -0400 [thread overview]
Message-ID: <073e829709dcbfb19244b1b1df2c39461297f337.camel@linux.ibm.com> (raw)
In-Reply-To: <20210621093923.1456675-1-roberto.sassu@huawei.com>
On Mon, 2021-06-21 at 11:39 +0200, Roberto Sassu wrote:
> The kernel and the user obtain an xattr value in two different ways:
>
> kernel (EVM): uses vfs_getxattr_alloc() which obtains the xattr value from
> the filesystem handler (raw value);
>
> user (ima-evm-utils): uses vfs_getxattr() which obtains the xattr value
> from the LSMs (normalized value).
>
> Normally, this does not have an impact unless security.selinux is set with
> setfattr, with a value not terminated by '\0' (this is not the recommended
> way, security.selinux should be set with the appropriate tools such as
> chcon and restorecon).
>
> In this case, the kernel and the user see two different xattr values: the
> former sees the xattr value without '\0' (raw value), the latter sees the
> value with '\0' (value normalized by SELinux).
>
> This could result in two different verification outcomes from EVM and
> ima-evm-utils, if a signature was calculated with a security.selinux value
> terminated by '\0' and the value set in the filesystem is not terminated by
> '\0'. The former would report verification failure due to the missing '\0',
> while the latter would report verification success (because it gets the
> normalized value with '\0').
>
> This patch mitigates this issue by comparing in evm_calc_hmac_or_hash() the
> size of the xattr returned by the two xattr functions and by warning the
> user if there is a misalignment.
Instead of "misalignment" how about using the word "discrepancy" here
and in the Subject line?
> Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
> Suggested-by: Mimi Zohar <zohar@linux.ibm.com>
Otherwise,
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>
prev parent reply other threads:[~2021-06-21 12:22 UTC|newest]
Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-06-21 9:39 [PATCH] evm: Check xattr size misalignment between kernel and user Roberto Sassu
2021-06-21 12:22 ` Mimi Zohar [this message]
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