linux-kernel - [PATCH] debugfs: allow access relay files in lockdown mode

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Message-Id: <20230412001030.88441-1-junxiao.bi@oracle.com>
Date:   Tue, 11 Apr 2023 17:10:30 -0700
From:   Junxiao Bi <junxiao.bi@...cle.com>
To:     linux-kernel@...r.kernel.org, linux-security-module@...r.kernel.org
Cc:     paul@...l-moore.com, jmorris@...ei.org, serge@...lyn.com,
        nathanl@...ux.ibm.com, axboe@...nel.dk, konrad.wilk@...cle.com,
        joe.jin@...cle.com
Subject: [PATCH] debugfs: allow access relay files in lockdown mode

Relay files are used by kernel to transfer information to userspace, these
files have permission 0400, but mmap is supported, so they are blocked by
lockdown. But since kernel just generates the contents of those files while
not reading it, it is saft to access relay files in lockdown mode.

With this, blktrace can work well in lockdown mode.

Signed-off-by: Junxiao Bi <junxiao.bi@...cle.com>
---
 fs/debugfs/file.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/fs/debugfs/file.c b/fs/debugfs/file.c
index 1f971c880dde..04fa813a227e 100644
--- a/fs/debugfs/file.c
+++ b/fs/debugfs/file.c
@@ -21,6 +21,7 @@
 #include <linux/pm_runtime.h>
 #include <linux/poll.h>
 #include <linux/security.h>
+#include <linux/relay.h>
 
 #include "internal.h"
 
@@ -142,6 +143,11 @@ EXPORT_SYMBOL_GPL(debugfs_file_put);
  * Only permit access to world-readable files when the kernel is locked down.
  * We also need to exclude any file that has ways to write or alter it as root
  * can bypass the permissions check.
+ * Exception:
+ * Relay files are used by kernel to transfer information to userspace, these
+ * files have permission 0400, but mmap is supported, so they are blocked by
+ * lockdown. But since kernel just generates the contents of those files while
+ * not reading it, it is saft to access relay files in lockdown mode.
  */
 static int debugfs_locked_down(struct inode *inode,
 			       struct file *filp,
@@ -154,6 +160,9 @@ static int debugfs_locked_down(struct inode *inode,
 	    !real_fops->mmap)
 		return 0;
 
+	if (real_fops == &relay_file_operations)
+		return 0;
+
 	if (security_locked_down(LOCKDOWN_DEBUGFS))
 		return -EPERM;
 
-- 
2.24.3 (Apple Git-128)