Re: [PATCH v3 2/2] vfs: avoid duplicating creds in faccessat if possible

From: Linus Torvalds <torvalds@linux-foundation.org>
To: Mateusz Guzik <mjguzik@gmail.com>,
	Kees Cook <keescook@chromium.org>,
	Eric Biggers <ebiggers@google.com>,
	Alexander Potapenko <glider@google.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>,
	Christian Brauner <brauner@kernel.org>,
	serge@hallyn.com, paul@paul-moore.com,
	linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-security-module@vger.kernel.org
Subject: Re: [PATCH v3 2/2] vfs: avoid duplicating creds in faccessat if possible
Date: Thu, 2 Mar 2023 11:10:03 -0800	[thread overview]
Message-ID: <CAHk-=wi11ZbOBdMR5hQDz0x0NNZ9gM-4SxXxK-7R3_yh7e10rQ@mail.gmail.com> (raw)
In-Reply-To: <CAHk-=wjp5fMupRwnROtC5Yn+MVLA7v=J+_QJSi1rr3qAjdsfXw@mail.gmail.com>

On Thu, Mar 2, 2023 at 11:03 AM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> It might be best if we actually exposed it as a SLAB_SKIP_ZERO thing,
> just to make it possible to say - exactly in situations like this -
> that this particular slab cache has no advantage from pre-zeroing.

Actually, maybe it's just as well to keep it per-allocation, and just
special-case getname_flags() itself.

We could replace the __getname() there with just a

        kmem_cache_alloc(names_cachep, GFP_KERNEL | __GFP_SKIP_ZERO);

we're going to overwrite the beginning of the buffer with the path we
copy from user space, and then we'd have to make people comfortable
with the fact that even with zero initialization hardening on, the
space after the filename wouldn't be initialized...

            Linus