[PATCH] SELinux: allow other LSMs to use custom mount args

[mortonm@chromium.org (Micah Morton)]

The security_sb_copy_data LSM hook allows LSMs to copy custom string
name/value args passed to mount_fs() into a temporary buffer (called
"secdata") that will be accessible to LSM code during the
security_sb_kern_mount hook further down in mount_fs(). Currently,
SELinux effectively prevents any other LSMs from copying custom mount
args into the temporary buffer (and being able to access them during
security_sb_kern_mount), as it will fail with -EINVAL and print
"SELinux:  unknown mount option" to the kernel message buffer if args it
doesn't recognize are present in the temporary buffer when
selinux_sb_kern_mount is called. This change adds an arg to the list of
those accepted by SELinux during security_sb_kern_mount. SELinux won't
do anything with this arg besides allow the name/value pair to be passed
along to any other LSM that is stacked after SELinux.

Developed on v4.18.

Signed-off-by: Micah Morton <mortonm@chromium.org>
---
 security/selinux/hooks.c            |  7 ++++++-
 security/selinux/include/security.h | 11 ++++++-----
 2 files changed, 12 insertions(+), 6 deletions(-)

diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index 2b5ee5fbd652..e70ccc701eb8 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -445,6 +445,7 @@ enum {
 	Opt_rootcontext = 4,
 	Opt_labelsupport = 5,
 	Opt_nextmntopt = 6,
+	Opt_lsm_custom_arg = 7,
 };
 
 #define NUM_SEL_MNT_OPTS	(Opt_nextmntopt - 1)
@@ -455,6 +456,7 @@ static const match_table_t tokens = {
 	{Opt_defcontext, DEFCONTEXT_STR "%s"},
 	{Opt_rootcontext, ROOTCONTEXT_STR "%s"},
 	{Opt_labelsupport, LABELSUPP_STR},
+	{Opt_lsm_custom_arg, LSM_CUSTOM_ARG_STR "%s"},
 	{Opt_error, NULL},
 };
 
@@ -1156,6 +1158,8 @@ static int selinux_parse_opts_str(char *options,
 			break;
 		case Opt_labelsupport:
 			break;
+		case Opt_lsm_custom_arg:
+			break;
 		default:
 			rc = -EINVAL;
 			printk(KERN_WARNING "SELinux:  unknown mount option\n");
@@ -2758,7 +2762,8 @@ static inline int selinux_option(char *option, int len)
 		match_prefix(FSCONTEXT_STR, sizeof(FSCONTEXT_STR)-1, option, len) ||
 		match_prefix(DEFCONTEXT_STR, sizeof(DEFCONTEXT_STR)-1, option, len) ||
 		match_prefix(ROOTCONTEXT_STR, sizeof(ROOTCONTEXT_STR)-1, option, len) ||
-		match_prefix(LABELSUPP_STR, sizeof(LABELSUPP_STR)-1, option, len));
+		match_prefix(LABELSUPP_STR, sizeof(LABELSUPP_STR)-1, option, len) ||
+		match_prefix(LSM_CUSTOM_ARG_STR, sizeof(LSM_CUSTOM_ARG_STR)-1, option, len));
 }
 
 static inline void take_option(char **to, char *from, int *first, int len)
diff --git a/security/selinux/include/security.h b/security/selinux/include/security.h
index 23e762d529fa..0ead836a0625 100644
--- a/security/selinux/include/security.h
+++ b/security/selinux/include/security.h
@@ -59,11 +59,12 @@
 #define SE_SBPROC		0x0200
 #define SE_SBGENFS		0x0400
 
-#define CONTEXT_STR	"context="
-#define FSCONTEXT_STR	"fscontext="
-#define ROOTCONTEXT_STR	"rootcontext="
-#define DEFCONTEXT_STR	"defcontext="
-#define LABELSUPP_STR "seclabel"
+#define CONTEXT_STR         "context="
+#define FSCONTEXT_STR       "fscontext="
+#define ROOTCONTEXT_STR     "rootcontext="
+#define DEFCONTEXT_STR      "defcontext="
+#define LABELSUPP_STR       "seclabel"
+#define LSM_CUSTOM_ARG_STR  "lsm_custom_arg="
 
 struct netlbl_lsm_secattr;
 
-- 
2.19.0.rc0.228.g281dcd1b4d0-goog