anon_inode: raise SB_I_NODEV and SB_I_NOEXEC

It isn't possible to execute anonymous inodes because they cannot be
opened in any way after they have been created. This includes execution:

execveat(fd_anon_inode, "", NULL, NULL, AT_EMPTY_PATH)

Anonymous inodes have inode->f_op set to no_open_fops which sets
no_open() which returns ENXIO. That means any call to do_dentry_open()
which is the endpoint of the do_open_execat() will fail. There's no
chance to execute an anonymous inode. Unless a given subsystem overrides
it ofc.

However, we should still harden this and raise SB_I_NODEV and
SB_I_NOEXEC on the superblock itself so that no one gets any creative
ideas.

Link: https://lore.kernel.org/20250407-work-anon_inode-v1-5-53a44c20d44e@kernel.org
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Cc: stable@vger.kernel.org # all LTS kernels
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: brauner

fs/anon_inodes.c

@@ -86,6 +86,8 @@ static int anon_inodefs_init_fs_context(struct fs_context *fc)
 	struct pseudo_fs_context *ctx = init_pseudo(fc, ANON_INODE_FS_MAGIC);
 	if (!ctx)
 		return -ENOMEM;
+	fc->s_iflags |= SB_I_NOEXEC;
+	fc->s_iflags |= SB_I_NODEV;
 	ctx->dops = &anon_inodefs_dentry_operations;
 	return 0;
 }