linux-kernel - [PATCH] LSM: general protection fault in legacy_parse

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Message-ID: <018a9bb4-accb-c19a-5b0a-fde22f4bc822@schaufler-ca.com>
Date:   Mon, 11 Oct 2021 15:40:22 -0700
From:   Casey Schaufler <casey@...aufler-ca.com>
To:     Christian Brauner <christian@...uner.io>,
        Paul Moore <paul@...l-moore.com>,
        James Morris <jmorris@...ei.org>,
        Linux Security Module list 
        <linux-security-module@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        syzbot <syzbot+d1e3b1d92d25abf97943@...kaller.appspotmail.com>,
        David Howells <dhowells@...hat.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Cc:     Casey Schaufler <casey@...aufler-ca.com>
Subject: [PATCH] LSM: general protection fault in legacy_parse_param

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.

Reported-by: syzbot+d1e3b1d92d25abf97943@...kaller.appspotmail.com
Signed-off-by: Casey Schaufler <casey@...aufler-ca.com>
---
 security/security.c | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/security/security.c b/security/security.c
index 09533cbb7221..3cf0faaf1c5b 100644
--- a/security/security.c
+++ b/security/security.c
@@ -885,7 +885,19 @@ int security_fs_context_dup(struct fs_context *fc, struct fs_context *src_fc)
 
 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)