linux-kernel - Re: [PATCH 3/5] exec: Remove recursion from search_binary

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Date:   Mon, 11 May 2020 09:33:21 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Oleg Nesterov <oleg@...hat.com>, Jann Horn <jannh@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Greg Ungerer <gerg@...ux-m68k.org>,
        Rob Landley <rob@...dley.net>,
        Bernd Edlinger <bernd.edlinger@...mail.de>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Casey Schaufler <casey@...aufler-ca.com>,
        LSM List <linux-security-module@...r.kernel.org>,
        James Morris <jmorris@...ei.org>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        Andy Lutomirski <luto@...capital.net>
Subject: Re: [PATCH 3/5] exec: Remove recursion from search_binary_handler

Linus Torvalds <torvalds@...ux-foundation.org> writes:

> On Sat, May 9, 2020 at 9:30 PM Tetsuo Handa
> <penguin-kernel@...ove.sakura.ne.jp> wrote:
>>
>> Wouldn't this change cause
>>
>>         if (fd_binary > 0)
>>                 ksys_close(fd_binary);
>>         bprm->interp_flags = 0;
>>         bprm->interp_data = 0;
>>
>> not to be called when "Search for the interpreter" failed?
>
> Good catch. We seem to have some subtle magic wrt the fd_binary file
> descriptor, which depends on the recursive behavior.

Yes.  I Tetsuo I really appreciate you noticing this.  This is exactly
the kind of behavior I am trying to flush out and keep from being
hidden.

> I'm not seeing how to fix it cleanly with the "turn it into a loop".
> Basically, that binfmt_misc use-case isn't really a tail-call.

I have reservations about installing a new file descriptor before
we process the close on exec logic and the related security modules
closing file descriptors that your new credentials no longer give
you access to logic.

I haven't yet figured out how opening a file descriptor during exec
should fit into all of that.

What I do see is that interp_data is just a parameter that is smuggled
into the call of search binary handler.  And the next binary handler
needs to be binfmt_elf for it to make much sense, as only binfmt_elf
(and binfmt_elf_fdpic) deals with BINPRM_FLAGS_EXECFD.

So I think what needs to happen is to rename bprm->interp_data to
bprm->execfd, remove BINPRM_FLAGS_EXECFD and make closing that file
descriptor free_bprm's responsiblity.

I hope such a change will make it easier to see all of the pieces that
are intereacting during exec.

I am still asking: is the installation of that file descriptor useful if
it is not exported passed to userspace as an AT_EXECFD note?

I will dig in and see what I can come up with.

Eric