linux-kernel - Re: [PATCH v2 2/2] vfs: avoid duplicating creds in faccessat if possible

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Date:   Wed, 25 Jan 2023 12:07:48 -0500
From:   Paul Moore <paul@...l-moore.com>
To:     Mateusz Guzik <mjguzik@...il.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        viro@...iv.linux.org.uk, serge@...lyn.com,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-security-module@...r.kernel.org
Subject: Re: [PATCH v2 2/2] vfs: avoid duplicating creds in faccessat if possible

On Wed, Jan 25, 2023 at 10:00 AM Mateusz Guzik <mjguzik@...il.com> wrote:
> On 1/24/23, Paul Moore <paul@...l-moore.com> wrote:
> > Although I'm looking at this again and realized that only
> > do_faccessat() calls access_override_creds(), so why not just fold the
> > new access_need_override_creds() logic into access_override_creds()?
> > Just have one function that takes the flag value, and returns an
> > old_cred/NULL pointer (or pass old_cred to the function by reference
> > and return an error code); that should still provide the performance
> > win Mateusz is looking for while providing additional safety against
> > out-of-sync changes.  I would guess the code would be smaller too.
>
> It is unclear from the description if you are arguing for moving the new
> func into access_override_creds almost as is just put prior to existing
> code *or* mixing checks with assignments.

"arguing" is a bit strong of a word for what I was thinking, it was
more of just tossing out an idea to see if it has any merit with you,
the VFS folks, and possibly Linus.

> static bool *access_override_creds(struct cred **ptr)
>         [snip]
>         if (!uid_eq(cred->fsuid, cred->uid) ||
>             !gid_eq(cred->fsgid, cred->gid))
>                 return false;
>         /* remaining checks go here as well */
>         [snip]
>
>         override_cred = prepare_creds();
>         if (!override_cred) {
>                 *ptr = NULL;
>                 return true;
>         }
>
>         override_cred->fsuid = override_cred->uid;
>         override_cred->fsgid = override_cred->gid;
>         [snip]
>
> If this is what you had in mind, I note it retains all the duplication
> except in one func body which I'm confident does not buy anything,
> provided the warning comment is added.
>
> At the same time the downside is that it uglifies error handling at the
> callsite, so I would say a net loss.

Yes, I was thinking of combining the two functions into one to better
link the cred checks with the cred adjustments.

> Addition of the warning comment makes sense, but concerns after that
> don't sound legitimate to me.

Well, as we talked about earlier, it's really up to the VFS folks to
pick what they want, and they have been suspiciously quiet thus far.

--
paul-moore.com