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

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Message-ID: <ZAD21ZEiB2V9Ttto@ZenIV>
Date:   Thu, 2 Mar 2023 19:19:49 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Mateusz Guzik <mjguzik@...il.com>,
        Kees Cook <keescook@...omium.org>,
        Eric Biggers <ebiggers@...gle.com>,
        Alexander Potapenko <glider@...gle.com>,
        Christian Brauner <brauner@...nel.org>, serge@...lyn.com,
        paul@...l-moore.com, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-security-module@...r.kernel.org
Subject: Re: [PATCH v3 2/2] vfs: avoid duplicating creds in faccessat if
 possible

On Thu, Mar 02, 2023 at 11:10:03AM -0800, Linus Torvalds wrote:
> On Thu, Mar 2, 2023 at 11:03 AM Linus Torvalds
> <torvalds@...ux-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...

ACK; same in getname_kernel() and sys_getcwd(), at the very least.