linux-kernel - Re: [PATCH v3 10/16] exec: Remove do_execve

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Date:   Wed, 8 Jul 2020 12:41:48 +0000
From:   Luis Chamberlain <mcgrof@...nel.org>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     linux-kernel@...r.kernel.org, David Miller <davem@...emloft.net>,
        Greg Kroah-Hartman <greg@...ah.com>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Kees Cook <keescook@...omium.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Al Viro <viro@...iv.linux.org.uk>, bpf <bpf@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Gary Lin <GLin@...e.com>, Bruno Meneguele <bmeneg@...hat.com>,
        LSM List <linux-security-module@...r.kernel.org>,
        Casey Schaufler <casey@...aufler-ca.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH v3 10/16] exec: Remove do_execve_file

On Wed, Jul 08, 2020 at 06:35:25AM +0000, Luis Chamberlain wrote:
> On Thu, Jul 02, 2020 at 11:41:34AM -0500, Eric W. Biederman wrote:
> > Now that the last callser has been removed remove this code from exec.
> > 
> > For anyone thinking of resurrecing do_execve_file please note that
> > the code was buggy in several fundamental ways.
> > 
> > - It did not ensure the file it was passed was read-only and that
> >   deny_write_access had been called on it.  Which subtlely breaks
> >   invaniants in exec.
> > 
> > - The caller of do_execve_file was expected to hold and put a
> >   reference to the file, but an extra reference for use by exec was
> >   not taken so that when exec put it's reference to the file an
> >   underflow occured on the file reference count.
> 
> Maybe its my growing love with testing, but I'm going to have to partly
> blame here that we added a new API without any respective testing.
> Granted, I recall this this patch set could have used more wider review
> and a bit more patience... but just mentioning this so we try to avoid
> new api-without-testing with more reason in the future.
> 
> But more importantly, *how* could we have caught this? Or how can we
> catch this sort of stuff better in the future?

Of all the issues you pointed out with do_execve_file(), since upon
review the assumption *by design* was that LSMs/etc would pick up issues
with the file *prior* to processing, I think that this file reference
count issue comes to my attention as the more serious issue which I
wish we could address *first* before this crusade.

So I have to ask, has anyone *really tried* to give a crack at fixing
this refcount issue in a smaller way first? Alexei?

I'm not opposed to the removal of do_execve_file(), however if there
is a reproducible crash / issue with the existing user, this sledge
hammer seems a bit overkill for older kernels.

  Luis