Re: [PATCH v4] proc: Allow pid_revalidate() during LOOKUP_RCU

[Linus Torvalds <torvalds@linux-foundation.org>]

On Tue, Jan 5, 2021 at 12:00 PM Al Viro <viro@zeniv.linux.org.uk> wrote:
>
> We are not guaranteed the locking environment that would prevent
> dentry getting renamed right under us.  And it's possible for
> old long name to be freed after rename, leading to UAF here.

This whole thing isn't important enough to get the dentry lock. It's
more of a hint than anything else.

Why isn't the fix to just use READ_ONCE() of the name pointer, and do
it under RCU?

That's what dentry_name() does for the much more complex case of
actually even following parent data for a depth up to 4, much less
just a single name.

So instead of

                       spin_lock(&dentry->d_lock);
                       audit_log_untrustedstring(ab, dentry->d_name.name);
                       spin_unlock(&dentry->d_lock);

why not

                       rcu_read_lock();
                       audit_log_untrustedstring(ab,
READ_ONCE(dentry->d_name.name));
                       rcu_read_unlock();

which looks a lot more in line with the other dentry path functions.

Maybe even have this as part of fs/d_path.c and try to get rid of
magic internal dentry name knowledge from the audit code?

                  Linus