linux-kernel - [RFC v1 07/14] krsi: Check for premissions on eBPF attachment

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Date:   Tue, 10 Sep 2019 13:55:20 +0200
From:   KP Singh <kpsingh@...omium.org>
To:     linux-kernel@...r.kernel.org, bpf@...r.kernel.org,
        linux-security-module@...r.kernel.org
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        James Morris <jmorris@...ei.org>,
        Kees Cook <keescook@...omium.org>,
        Thomas Garnier <thgarnie@...omium.org>,
        Michael Halcrow <mhalcrow@...gle.com>,
        Paul Turner <pjt@...gle.com>,
        Brendan Gregg <brendan.d.gregg@...il.com>,
        Jann Horn <jannh@...gle.com>,
        Matthew Garrett <mjg59@...gle.com>,
        Christian Brauner <christian@...uner.io>,
        Mickaël Salaün <mic@...ikod.net>,
        Florent Revest <revest@...omium.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        Mauro Carvalho Chehab <mchehab+samsung@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Nicolas Ferre <nicolas.ferre@...rochip.com>,
        Stanislav Fomichev <sdf@...gle.com>,
        Quentin Monnet <quentin.monnet@...ronome.com>,
        Andrey Ignatov <rdna@...com>, Joe Stringer <joe@...d.net.nz>
Subject: [RFC v1 07/14] krsi: Check for premissions on eBPF attachment

From: KP Singh <kpsingh@...gle.com>

Add validation checks for the attachment of eBPF programs.

The following permissions are required:

- CAP_SYS_ADMIN to load eBPF programs
- CAP_MAC_ADMIN (to update the policy of an LSM)
- The securityfs file being a KRSI hook and writable (O_RDWR)

Signed-off-by: KP Singh <kpsingh@...gle.com>
---
 security/krsi/ops.c | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/security/krsi/ops.c b/security/krsi/ops.c
index cf4d06189aa1..a61508b7018f 100644
--- a/security/krsi/ops.c
+++ b/security/krsi/ops.c
@@ -23,11 +23,31 @@ static struct krsi_hook *get_hook_from_fd(int fd)
 		goto error;
 	}
 
+	/*
+	 * Only CAP_MAC_ADMIN users are allowed to make
+	 * changes to LSM hooks
+	 */
+	if (!capable(CAP_MAC_ADMIN)) {
+		ret = -EPERM;
+		goto error;
+	}
+
 	if (!is_krsi_hook_file(f.file)) {
 		ret = -EINVAL;
 		goto error;
 	}
 
+	/*
+	 * It's wrong to attach the program to the hook
+	 * if the file is not opened for a write. Note that,
+	 * this is an EBADF and not an EPERM because the file
+	 * has been opened with an incorrect mode.
+	 */
+	if (!(f.file->f_mode & FMODE_WRITE)) {
+		ret = -EBADF;
+		goto error;
+	}
+
 	/*
 	 * The securityfs dentry never disappears, so we don't need to take a
 	 * reference to it.
-- 
2.20.1