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

[KP Singh <kpsingh@chromium.org>]

From: KP Singh <kpsingh@google.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@google.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