[PATCH v2] selinux: fall back to SECURITY_FS_USE_GENFS if no xattr support

* [PATCH v2] selinux: fall back to SECURITY_FS_USE_GENFS if no xattr support
@ 2021-01-13 12:38 Ondrej Mosnacek
  2021-01-13 13:58 ` Paul Moore
  0 siblings, 1 reply; 2+ messages in thread
From: Ondrej Mosnacek @ 2021-01-13 12:38 UTC (permalink / raw)
  To: selinux, Paul Moore; +Cc: Stephen Smalley, Vivek Goyal

When a superblock is assigned the SECURITY_FS_USE_XATTR behavior by the
policy yet it lacks xattr support, try to fall back to genfs rather than
rejecting the mount. If a genfscon rule is found for the filesystem,
then change the behavior to SECURITY_FS_USE_GENFS, otherwise reject the
mount as before. A similar fallback is already done in security_fs_use()
if no behavior specification is found for the given filesystem.

This is needed e.g. for virtiofs, which may or may not support xattrs
depending on the backing host filesystem.

Example:
    # seinfo --genfs | grep ' ramfs'
       genfscon ramfs /  system_u:object_r:ramfs_t:s0
    # echo '(fsuse xattr ramfs (system_u object_r fs_t ((s0) (s0))))' >ramfs_xattr.cil
    # semodule -i ramfs_xattr.cil
    # mount -t ramfs none /mnt

Before:
    mount: /mnt: mount(2) system call failed: Operation not supported.

After:
    (mount succeeds)
    # ls -Zd /mnt
    system_u:object_r:ramfs_t:s0 /mnt

See also:
https://lore.kernel.org/selinux/20210105142148.GA3200@redhat.com/T/
https://github.com/fedora-selinux/selinux-policy/pull/478

Cc: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Ondrej Mosnacek <omosnace@redhat.com>
---

v2:
 - incorporated Paul's suggestions
 - corrected the `ls` command in reproducer

 security/selinux/hooks.c | 77 +++++++++++++++++++++++++++-------------
 1 file changed, 52 insertions(+), 25 deletions(-)

diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index 6b1826fc3658e..f9e52b622f467 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -484,6 +484,55 @@ static int selinux_is_sblabel_mnt(struct super_block *sb)
 	}
 }
 
+static int sb_check_xattr_support(struct super_block *sb)
+{
+	struct superblock_security_struct *sbsec = sb->s_security;
+	struct dentry *root = sb->s_root;
+	struct inode *root_inode = d_backing_inode(root);
+	u32 sid;
+	int rc;
+
+	/*
+	 * Make sure that the xattr handler exists and that no
+	 * error other than -ENODATA is returned by getxattr on
+	 * the root directory.  -ENODATA is ok, as this may be
+	 * the first boot of the SELinux kernel before we have
+	 * assigned xattr values to the filesystem.
+	 */
+	if (!(root_inode->i_opflags & IOP_XATTR)) {
+		pr_warn("SELinux: (dev %s, type %s) has no xattr support\n",
+			sb->s_id, sb->s_type->name);
+		goto fallback;
+	}
+
+	rc = __vfs_getxattr(root, root_inode, XATTR_NAME_SELINUX, NULL, 0);
+	if (rc < 0 && rc != -ENODATA) {
+		if (rc == -EOPNOTSUPP) {
+			pr_warn("SELinux: (dev %s, type %s) has no security xattr handler\n",
+				sb->s_id, sb->s_type->name);
+			goto fallback;
+		} else {
+			pr_warn("SELinux: (dev %s, type %s) getxattr errno %d\n",
+				sb->s_id, sb->s_type->name, -rc);
+			return rc;
+		}
+	}
+	return 0;
+
+fallback:
+	/* No xattr support - try to fallback to genfs if possible. */
+	rc = security_genfs_sid(&selinux_state, sb->s_type->name, "/",
+				SECCLASS_DIR, &sid);
+	if (rc)
+		return -EOPNOTSUPP;
+
+	pr_warn("SELinux: (dev %s, type %s) falling back to genfs\n",
+		sb->s_id, sb->s_type->name);
+	sbsec->behavior = SECURITY_FS_USE_GENFS;
+	sbsec->sid = sid;
+	return 0;
+}
+
 static int sb_finish_set_opts(struct super_block *sb)
 {
 	struct superblock_security_struct *sbsec = sb->s_security;
@@ -492,30 +541,9 @@ static int sb_finish_set_opts(struct super_block *sb)
 	int rc = 0;
 
 	if (sbsec->behavior == SECURITY_FS_USE_XATTR) {
-		/* Make sure that the xattr handler exists and that no
-		   error other than -ENODATA is returned by getxattr on
-		   the root directory.  -ENODATA is ok, as this may be
-		   the first boot of the SELinux kernel before we have
-		   assigned xattr values to the filesystem. */
-		if (!(root_inode->i_opflags & IOP_XATTR)) {
-			pr_warn("SELinux: (dev %s, type %s) has no "
-			       "xattr support\n", sb->s_id, sb->s_type->name);
-			rc = -EOPNOTSUPP;
-			goto out;
-		}
-
-		rc = __vfs_getxattr(root, root_inode, XATTR_NAME_SELINUX, NULL, 0);
-		if (rc < 0 && rc != -ENODATA) {
-			if (rc == -EOPNOTSUPP)
-				pr_warn("SELinux: (dev %s, type "
-				       "%s) has no security xattr handler\n",
-				       sb->s_id, sb->s_type->name);
-			else
-				pr_warn("SELinux: (dev %s, type "
-				       "%s) getxattr errno %d\n", sb->s_id,
-				       sb->s_type->name, -rc);
-			goto out;
-		}
+		rc = sb_check_xattr_support(sb);
+		if (rc)
+			return rc;
 	}
 
 	sbsec->flags |= SE_SBINITIALIZED;
@@ -554,7 +582,6 @@ static int sb_finish_set_opts(struct super_block *sb)
 		spin_lock(&sbsec->isec_lock);
 	}
 	spin_unlock(&sbsec->isec_lock);
-out:
 	return rc;
 }
 
-- 
2.29.2


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