linux-kernel - Re: [PATCH v2 1/8] exec: Correct comments about "point of no return"

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Message-ID: <CAGXu5jKTaXLU+H6DnNuy6ggxcMDgo9G-wEmZ4RP=QneJaZuNDg@mail.gmail.com>
Date:   Mon, 10 Jul 2017 09:04:15 -0700
From:   Kees Cook <keescook@...omium.org>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        David Howells <dhowells@...hat.com>,
        Serge Hallyn <serge@...lyn.com>,
        John Johansen <john.johansen@...onical.com>,
        Casey Schaufler <casey@...aufler-ca.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Michal Hocko <mhocko@...nel.org>,
        Ben Hutchings <ben@...adent.org.uk>,
        Hugh Dickins <hughd@...gle.com>,
        Oleg Nesterov <oleg@...hat.com>,
        "Jason A. Donenfeld" <Jason@...c4.com>,
        Rik van Riel <riel@...hat.com>,
        James Morris <james.l.morris@...cle.com>,
        Greg Ungerer <gerg@...ux-m68k.org>,
        Ingo Molnar <mingo@...nel.org>,
        Nicolas Pitre <nicolas.pitre@...aro.org>,
        Stephen Smalley <sds@...ho.nsa.gov>,
        Paul Moore <paul@...l-moore.com>,
        Vivek Goyal <vgoyal@...hat.com>,
        Mickaël Salaün <mic@...ikod.net>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-security-module <linux-security-module@...r.kernel.org>,
        SE Linux <selinux@...ho.nsa.gov>
Subject: Re: [PATCH v2 1/8] exec: Correct comments about "point of no return"

On Mon, Jul 10, 2017 at 1:46 AM, Eric W. Biederman
<ebiederm@...ssion.com> wrote:
>
> But you miss it.
>
> The "point of no return" is the call to de_thread.  Or aguably anything in
> flush_old_exec.  Once anything in the current task is modified you can't
> return an error.
>
> It very much does not have anything to do with brpm.    It has
> everything to do with current.

Yes, but the thing that actually enforces this is the test of bprm->mm
and the SIGSEGV in search_binary_handlers().

-Kees

>
>
>> diff --git a/fs/exec.c b/fs/exec.c
>> index 904199086490..7842ae661e34 100644
>> --- a/fs/exec.c
>> +++ b/fs/exec.c
>> @@ -1285,7 +1285,14 @@ int flush_old_exec(struct linux_binprm * bprm)
>>       if (retval)
>>               goto out;
>>
>> -     bprm->mm = NULL;                /* We're using it now */
>> +     /*
>> +      * After clearing bprm->mm (to mark that current is using the
>> +      * prepared mm now), we are at the point of no return. If
>> +      * anything from here on returns an error, the check in
>> +      * search_binary_handler() will kill current (since the mm has
>> +      * been replaced).
>> +      */
>> +     bprm->mm = NULL;
>>
>>       set_fs(USER_DS);
>>       current->flags &= ~(PF_RANDOMIZE | PF_FORKNOEXEC | PF_KTHREAD |
>
> Eric



-- 
Kees Cook
Pixel Security