[PATCH RFC v2 18/42] selinux: use fc_init_pseudo() for selinuxfs

[Christian Brauner <brauner@kernel.org>]

Initialize selinuxfs as a pseudo filesystem. This raises SB_I_PSEUDO and
SB_I_NOEXEC. selinuxfs never holds executables. SB_I_NOEXEC also
refuses PROT_EXEC mappings but the status and policy files are only
ever mapped for reading.

But selinuxfs needs device nodes. The kernel replaces file descriptors
with /sys/fs/selinux/null on exec via dentry_open() which doesn't care
about SB_I_NODEV. Android init however opens /sys/fs/selinux/null by
path as /dev/null doesn't exist yet at that point. So clear SB_I_NODEV
again in sel_fill_super().

Reviewed-by: Alexander Mikhalitsyn <alex@amutable.com>
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
---
 security/selinux/selinuxfs.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/security/selinux/selinuxfs.c b/security/selinux/selinuxfs.c
index 310e19efff1d0..d7ae853b6d685 100644
--- a/security/selinux/selinuxfs.c
+++ b/security/selinux/selinuxfs.c
@@ -18,6 +18,7 @@
 #include <linux/vmalloc.h>
 #include <linux/fs.h>
 #include <linux/fs_context.h>
+#include <linux/pseudo_fs.h>
 #include <linux/hex.h>
 #include <linux/mount.h>
 #include <linux/mutex.h>
@@ -1852,6 +1853,9 @@ static int sel_fill_super(struct super_block *sb, struct fs_context *fc)
 		/* last one */ {"", NULL, 0}
 	};
 
+	/* Android init opens the null device below by path. */
+	sb->s_iflags &= ~SB_I_NODEV;
+
 	ret = selinux_fs_info_create(sb);
 	if (ret)
 		goto err;
@@ -1955,6 +1959,7 @@ static const struct fs_context_operations sel_context_ops = {
 
 static int sel_init_fs_context(struct fs_context *fc)
 {
+	fc_init_pseudo(fc);
 	fc->ops = &sel_context_ops;
 	return 0;
 }

-- 
2.53.0