anon_inode: explicitly block ->setattr()

It is currently possible to change the mode and owner of the single
anonymous inode in the kernel:

int main(int argc, char *argv[])
{
        int ret, sfd;
        sigset_t mask;
        struct signalfd_siginfo fdsi;

        sigemptyset(&mask);
        sigaddset(&mask, SIGINT);
        sigaddset(&mask, SIGQUIT);

        ret = sigprocmask(SIG_BLOCK, &mask, NULL);
        if (ret < 0)
                _exit(1);

        sfd = signalfd(-1, &mask, 0);
        if (sfd < 0)
                _exit(2);

        ret = fchown(sfd, 5555, 5555);
        if (ret < 0)
                _exit(3);

        ret = fchmod(sfd, 0777);
        if (ret < 0)
                _exit(3);

        _exit(4);
}

This is a bug. It's not really a meaningful one because anonymous inodes
don't really figure into path lookup and they cannot be reopened via
/proc/<pid>/fd/<nr> and can't be used for lookup itself. So they can
only ever serve as direct references.

But it is still completely bogus to allow the mode and ownership or any
of the properties of the anonymous inode to be changed. Block this!

Link: https://lore.kernel.org/20250407-work-anon_inode-v1-3-53a44c20d44e@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>

1 files changed, 7 insertions, 0 deletions

diff --git a/fs/anon_inodes.c b/fs/anon_inodes.c
index 42e4b9c34f89..cb51a90bece0 100644
--- a/fs/anon_inodes.c
+++ b/fs/anon_inodes.c
@@ -57,8 +57,15 @@ int anon_inode_getattr(struct mnt_idmap *idmap, const struct path *path,
 	return 0;
 }
 
+int anon_inode_setattr(struct mnt_idmap *idmap, struct dentry *dentry,
+		       struct iattr *attr)
+{
+	return -EOPNOTSUPP;
+}
+
 static const struct inode_operations anon_inode_operations = {
 	.getattr = anon_inode_getattr,
+	.setattr = anon_inode_setattr,
 };
 
 /*