linux-kernel - [PATCH next-lockdown 1/1] debugfs: avoid EPERM when no open file operation defined

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Date:   Wed, 21 Nov 2018 13:05:10 +0100
From:   Vasily Gorbik <gor@...ux.ibm.com>
To:     David Howells <dhowells@...hat.com>,
        James Morris <jmorris@...ei.org>
Cc:     Martin Schwidefsky <schwidefsky@...ibm.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org, linux-security-module@...r.kernel.org
Subject: [PATCH next-lockdown 1/1] debugfs: avoid EPERM when no open file
 operation defined

With "debugfs: Restrict debugfs when the kernel is locked down"
return code "r" is unconditionally set to -EPERM, which stays like that
until function return if no "open" file operation defined, effectivelly
resulting in "Operation not permitted" for all such files despite kernel
lock down status or CONFIG_LOCK_DOWN_KERNEL being enabled.

In particular this breaks 2 debugfs files on s390:
/sys/kernel/debug/s390_hypfs/diag_304
/sys/kernel/debug/s390_hypfs/diag_204

To address that set EPERM return code only when debugfs_is_locked_down
returns true.

Fixes: 3fc322605158 ("debugfs: Restrict debugfs when the kernel is locked down")
Signed-off-by: Vasily Gorbik <gor@...ux.ibm.com>
---
 fs/debugfs/file.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/fs/debugfs/file.c b/fs/debugfs/file.c
index 51cb894c21f2..89c86faaa02a 100644
--- a/fs/debugfs/file.c
+++ b/fs/debugfs/file.c
@@ -167,9 +167,10 @@ static int open_proxy_open(struct inode *inode, struct file *filp)
 
 	real_fops = debugfs_real_fops(filp);
 
-	r = -EPERM;
-	if (debugfs_is_locked_down(inode, filp, real_fops))
+	if (debugfs_is_locked_down(inode, filp, real_fops)) {
+		r = -EPERM;
 		goto out;
+	}
 
 	real_fops = fops_get(real_fops);
 	if (!real_fops) {
@@ -296,9 +297,10 @@ static int full_proxy_open(struct inode *inode, struct file *filp)
 		return r == -EIO ? -ENOENT : r;
 
 	real_fops = debugfs_real_fops(filp);
-	r = -EPERM;
-	if (debugfs_is_locked_down(inode, filp, real_fops))
+	if (debugfs_is_locked_down(inode, filp, real_fops)) {
+		r = -EPERM;
 		goto out;
+	}
 
 	real_fops = fops_get(real_fops);
 	if (!real_fops) {
-- 
2.18.0.13.gd42ae10