linux-kernel - Re: [GIT PULL] execve updates for v6.13-rc1

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Message-ID: <CAHk-=wh4k8ks_oq4v=LXxidXZ+r-_er7cORjNzySjDaD8Xc9ng@mail.gmail.com>
Date: Wed, 20 Nov 2024 14:50:39 -0800
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: Kees Cook <kees@...nel.org>
Cc: linux-kernel@...r.kernel.org, Alexander Viro <viro@...iv.linux.org.uk>, 
	Christophe JAILLET <christophe.jaillet@...adoo.fr>, Dan Carpenter <dan.carpenter@...aro.org>, 
	"Eric W. Biederman" <ebiederm@...ssion.com>, Nir Lichtman <nir@...htman.org>, 
	syzbot+03e1af5c332f7e0eb84b@...kaller.appspotmail.com, 
	Tycho Andersen <tandersen@...flix.com>, Vegard Nossum <vegard.nossum@...cle.com>, 
	Zbigniew Jędrzejewski-Szmek <zbyszek@...waw.pl>
Subject: Re: [GIT PULL] execve updates for v6.13-rc1

On Wed, 20 Nov 2024 at 14:33, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> On Wed, 20 Nov 2024 at 14:32, Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
> >
> > People: we *have* a filename. It's right there in the dentry. Which is
> > right there as bprm->file->f_dentry.dentry.
>
> ... that should obviously be '...->f_path.dentry'.

One thing to look out for is that dentry->name can be switched around
by renames etc

So you probably want to do something like

        const char *name = smp_load_acquire(&dentry->d_name.name);

under the RCU read lock before then copying it with strscpy(). It
should always be NULL-terminated.

If you want to be extra careful, you might surround it with a

        read_seqbegin_or_lock(&rename_lock, &seq);

        ..

        if (need_seqretry(&rename_lock, seq)) {
                seq = 1;
                goto restart;
        }
        done_seqretry(&rename_lock, seq);

but I seriously doubt we even care that much. If people want to mess
with the executable filename, they can just use links or whatever, so
this is a "politeness" thing rather than anything else.

(And yes, our d_path() code tries to be extra smart and also uses
'name->len' to avoid having to search for the end of the string etc,
but that then causes huge complexities with ->len and ->name not
matching, so it has to compensate for that with being extra careful.
So don't do that)

                     Linus