[PATCH v2] LSM: general protection fault in legacy_parse_param

From: Casey Schaufler <casey@schaufler-ca.com>
To: Paul Moore <paul@paul-moore.com>
Cc: Christian Brauner <christian.brauner@ubuntu.com>,
	Christian Brauner <christian@brauner.io>,
	James Morris <jmorris@namei.org>,
	Linux Security Module list 
	<linux-security-module@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	syzbot <syzbot+d1e3b1d92d25abf97943@syzkaller.appspotmail.com>,
	David Howells <dhowells@redhat.com>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	selinux@vger.kernel.org, Casey Schaufler <casey@schaufler-ca.com>
Subject: [PATCH v2] LSM: general protection fault in legacy_parse_param
Date: Thu, 27 Jan 2022 08:51:44 -0800	[thread overview]
Message-ID: <a19e0338-5240-4a6d-aecf-145539aecbce@schaufler-ca.com> (raw)
In-Reply-To: <CAHC9VhT=dZbWzhst0hMLo0n7=UzWC5OYTMY=0x=LZ97HwG0UsA@mail.gmail.com>

The usual LSM hook "bail on fail" scheme doesn't work for cases where
a security module may return an error code indicating that it does not
recognize an input.  In this particular case Smack sees a mount option
that it recognizes, and returns 0. A call to a BPF hook follows, which
returns -ENOPARAM, which confuses the caller because Smack has processed
its data.

The SELinux hook incorrectly returns 1 on success. There was a time
when this was correct, however the current expectation is that it
return 0 on success. This is repaired.

Reported-by: syzbot+d1e3b1d92d25abf97943@syzkaller.appspotmail.com
Signed-off-by: Casey Schaufler <casey@schaufler-ca.com>
---
  security/security.c      | 17 +++++++++++++++--
  security/selinux/hooks.c |  5 ++---
  2 files changed, 17 insertions(+), 5 deletions(-)

diff --git a/security/security.c b/security/security.c
index 3d4eb474f35b..e649c8691be2 100644
--- a/security/security.c
+++ b/security/security.c
@@ -884,9 +884,22 @@ int security_fs_context_dup(struct fs_context *fc, struct fs_context *src_fc)
  	return call_int_hook(fs_context_dup, 0, fc, src_fc);
  }
  
-int security_fs_context_parse_param(struct fs_context *fc, struct fs_parameter *param)
+int security_fs_context_parse_param(struct fs_context *fc,
+				    struct fs_parameter *param)
  {
-	return call_int_hook(fs_context_parse_param, -ENOPARAM, fc, param);
+	struct security_hook_list *hp;
+	int trc;
+	int rc = -ENOPARAM;
+
+	hlist_for_each_entry(hp, &security_hook_heads.fs_context_parse_param,
+			     list) {
+		trc = hp->hook.fs_context_parse_param(fc, param);
+		if (trc == 0)
+			rc = 0;
+		else if (trc != -ENOPARAM)
+			return trc;
+	}
+	return rc;
  }
  
  int security_sb_alloc(struct super_block *sb)
diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index 5b6895e4fc29..371f67a37f9a 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -2860,10 +2860,9 @@ static int selinux_fs_context_parse_param(struct fs_context *fc,
  		return opt;
  
  	rc = selinux_add_opt(opt, param->string, &fc->security);
-	if (!rc) {
+	if (!rc)
  		param->string = NULL;
-		rc = 1;
-	}
+
  	return rc;
  }
  

